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Home My WebLink AboutORD 5828 CITY OF RENTON, WASHINGTON ORDINANCE NO. 5828 AN ORDINANCE OF THE CITY OF RENTON, WASHINGTON, AMENDING SECTIONS 4-2-110, 4-2-115, 4-3-040, 4-3-100, 4-3-110, 4-4-030, 4-4-040, 4-4- 060, 4-4-070, 4-4-080, 4-4-090, 4-4-110, 4-6-030, 4-6-060, 4-7-150, 4-7-200, 4-7- 220, 4-9-150, 4-9-200, 4-11-060, 4-11-090, 4-11-120, 4-11-160, 4-11-190, 9-10-2, 9-10-11, 9-15-1 OF THE RENTON MUNICIPAL CODE, ADOPTING LOW IMPACT DEVELOPMENT PRINCIPLES AND PRACTICES IN COMPLIANCE WITH THE NATIONAL POLLUTANT DISCHARGE ELIMINATION SYSTEM PHASE II MUNICIPAL STORMWATER PERMIT, INCLUDING ADDING AND AMENDING REGULATIONS AND THE DEFINITIONS OF "IMPERVIOUS SURFACE;' "FEASIBLE (WITH REGARD TO APPLICATION OF THE SURFACE WATER DESIGN MANUAL IN RMC 4-6-030);" "INFILTRATION FACILITY;" "LID BEST MANAGEMENT PRACTICES;" "LOW IMPACT DEVELOPMENT (LID);" "PERVIOUS SURFACE;" "STORMWATER FACILITY"AND "SURFACE WATER DESIGN MANUAL," AND ADOPTING THE 2016 KING COUNTY SURFACE WATER MANUAL WITH THE CITY OF RENTON'S AMENDMENTS THERETO, AND THE APRIL 2016 KING COUNTY STORMWATER POLLUTION PREVENTION MANUAL. WHEREAS, the City of Renton holds a Western Washington National Pollutant Discharge Elimination System (NPDES) Phase II Municipal Stormwater permit; and WHEREAS, this permit requires that Low Impact Development (LID) principles and practices be used as the primary means of site development, where it is feasible; and WHEREAS, this permit further required the City to review codes, rules, and standards to integrate and require LID principles and practices; and WHEREAS, the City engaged a consultant to complete the review and make recommendations for potential code amendments; and WHEREAS, those recommendations were considered by City staff and forwarded to the Planning Commission for them to study and consider the recommendations; and WHEREAS, the Planning Commission held a public hearing on the matter of the proposed regulations for LID integration into City Code on August 17, 2016. 1 ORDINANCE N0. 5828 WHEREAS, the Planning Commission deliberated and made a recommendation regarding LID integration on September 7, 2016; NOW, THEREFORE, THE CITY COUNCIL OF THE CITY OF RENTON, WASHINGTON, DOES ORDAIN AS FOLLOWS: SECTION I. The Private Street Improvements section of 4-2-110.C, Development Standards for Residential Manufactured Home Park Zoning Designation, of Chapter 2, Zoning Districts — Uses and Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as shown below. The rest of the table shall remain as it is currently codified. INDIVIDUAL MANUFACTURED HOME SPACES DETACHED NEW PARK Primary and Attached ACCESSORY Development or Redevelopment Accessory Structures STRUCTURESS PRIVATE STREET IMPROVEMENTS On-Site Asphaltic or concrete streets and NA NA Private concrete curbings shall be provided to Streets, each lot. The minimum width of streets Curbs and shall be 30 ft. Concrete sidewalks of at Sidewalks least 5 ft. in width shall be placed along at least 1 side of each street or located in the back or side of each lot so that there is sidewalk access to all lots. Sidewalks shall be made of permeable material to the extent reauired bv the Surface Water Desi�n Manual. Illumination:A street lighting plan shall be approved if it provides sufficient illumination between sunset and sunrise to illuminate adequately the roadways and walkways within a mobile home park. 2 ORDINANCE N0. 5828 SECTION II. The Lot Configuration subsection of 4-2-115.E.1, Site Design, of Chapter 2, Zoning Districts – Uses and Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as shown below. The rest of the table shall remain as it is currently codified. Guidelines: Developments shall create pedestrian oriented environments and amplify the mutual relationship between housing units, roads, open space, and pedestrian amenities, while also protecting the privacy of individuals. Lots shall be configured to encourage variety within the development. To the maximum extent qracticable as defined bv the Surface Water Desi�n Manual, retain soils with potential for infiltration. Standards: RC, R-1, n/a a n d R-4 One of the following is required: 1. Lot width variation of 10 feet (10') minimum of one per four (4) abutting R-6 and street-fronting lots, or R-8 2. Minimum of four (4) lot sizes (minimum of four hundred (400) gross square feet size difference), or 3. A front yard setback variation of at least five feet (5') minimum for at least every four (4) abutting street fronting lots. Lots shall be confi�ured to achieve both of the followin�: 1. The location of stormwater infiltratin� LID facilities is optimized, All consistent with the Surface Water Desi�n Manual. Buildin� and propertv — line setbacks are sqecified in the Surface Water Desi�n Manual for zones infiltration facilities. 2. Soils with �ood infiltration potential for stormwater mana�ement are preserved to the maximum extent practicable as defined bv the Surface Water Desi�n Manual. 3 ORDINANCE N0. 5828 SECTION II1. Subsection 4-2-115.E.2, Open Space, of Chapter 2, Zoning Districts — Uses and Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 2. Open Space: t��P.E =r ;.... �� - ' �rirme��h�g � � �:�g�� r.,,�;�qw$;;.. � �.� � �3. I . ,U[1F�tGS � ,�v��. �`s �r�� a c �� . e. n�,r 1 �'Ce��ar�t f an�ncrea� e o c� itt�s�t� �' ��_�� :� `��;��.=,t �'-� � .x.�. _ .. _ _Pi? �-.- . .;�..�>_,- a e�itt�..:€��..p�a.�� ::�. `��..8�,�. �, y.�, r �=f. _ _ . ? . _ _ . ;=�_._�`�.�:�_.. .�: . .. .>� . Guidelines: All open space shall be designed to preserve existing trees particularlv native conifers, native deciduous trees, and other native ve�etation consistent with RMC 4-4-070. Landscapin�. Except for Native Growth Protection Areas, all common open space areas shall be designed to accommodate both active and passive recreational opportunities and be visible and open to the street. Pocket parks shall be designed to serve four (4)to ten (10) homes. Private yards are located at the rear or side of homes and can include trees, planting beds, and privacy fences. Reciprocal use easements can provide greater usability of private yards. Landscaping: R-10 and See RMC 4-4-070, Landscaping. R-14 IStandards for Parks: R-10 For developments that are less than ten (10) net acres: No park is required, but is and allowed. R-14 For developments that are greater than ten (10) net acres: A minimum of one one- half(.5) acre park, in addition to the common open space requirement, is required. �Standards for Common Open Space: R-10 Developments of three (3) or fewer dwelling units: No requirement to provide and common open space. R-14 Developments of four(4) or more units: Required to provide common open space as follows: 1. For each unit in the development, three hundred fifty (350) square feet of common open space shall be provided. 2. Open space shall be designed as a park, common green, pea-patch, pocket park, or pedestrian entry easement in the development and shall include picnic areas, space for small recreational activities, and other activities as appropriate. 3. Open space shall be located in a highly visible area and be easily accessible to the neighborhood. 4 ORDINANCE NC}. 5828 4. Open space(s) shali be contiguous to the majority of the dweliings in the develapment and accessible to alf dwellings, and shall be at least twenty feet (20') wide. 5. A pedestrian entry easement can be used to meet the access requirements if it has a minimum width of twenty feet (20'�with a minimum five feet (5') of sidewaik. 6. Pea-patches shall be at least ane thousand (1,OOQ} square feet in size with individual piots that measure at least ten feet by ten feet (10' x 10'). Additionally, the pea-patch shall include a tool shed and a common area with space for compost bins. Water shall be provided to the pea-patch. Fencing that meets the standards for frant yard fencing shall surround the pea-patch with a one foat (1') landscape area on the outside of the fence.This area is to be landscaped with flowers, plants, and/or shrubs. 7. Grass-crete or other perviaus surfaces may be used in the cammon open space for the purpose of ineeting the one hundred fifty feet (150') distance requirement for emergency vehicle access but shali not be used far personal vehicle access ar to meet off-street parking requirements. 8. Storm ponds may be used to meet the common open space requirement if designed ta accommadate a fifty{50)year storm and ta be dry ninety percent (90°!0} of the year. �Standards for Private Yards: R-10 Developments of three (3) ar fewer dwelling units: Each individual dwelling shall and have a private yard that is at minimum six hundred (600) square feet in size. Backyard R-14 patios and reciprocal use easements may be included in the calcuiation of private yard. Developments of faur(4) or more dwelling units: Each ground-refated dwelling shall have a private yard that is at least two hundred fifty (250) square feet in size with no dimension (ess than eight feet (8'} in width. An additional two hundred fifty (25Q) square feet of open space per unit shall be added to the required amount of common open space far each unit that is not ground related. �Common Open Space or Park Substitutions: R-10 See RMC A�-1-240. and R-14 �Sidewalks, Pathways,and Pedestrian Easements: R-10 All af the fallowing are required: and R-14 5 ORDINANCE N0. 5828 1. Sidewalks shall be provided throughout the neighborhood. The sidewalk may disconnect from the road, provided it continues in a logicat route throughout the development. Permeable pavement sidewalks shall be used where feasible, consistent with the Surface Water Desi�n Manual. 2. Front yards shall have entry walks that are a minimum width of three feet (3') and a maximum width of four feet (4'). 3. Pathways shall be used to connect common parks, green areas, and pocket parks to residential access streets, limited residential access streets, or other pedestrian connections. They may be used to provide access to homes and common open space. They shall be a minimum three feet (3') in width and made of paved asphalt, concrete, or porous material such as: porous paving stones, crushed gravel with soil stabilizers, or paving blocks with planted joints. Sidewalks or pathways for parks and green spaces shall be located at the edge of the common space to allow a larger usable green and easy access to homes. 4. Pedestrian Easement Plantings: Shall be planted with plants and trees. Trees are required along all pedestrian easements to provide shade and spaced twenty feet (20') on center. Shrubs shall be planted in at least fifteen percent (15%) of the easement and shall be spaced no further than thirty_six inches (36") on center. 5. For all homes that do not front on a residential access street, limited residential access street, a park, or a common green: Pedestrian entry easements that are at least fifteen feet (15')wide plus a five-foot (5') sidewalk shall be provided. SECTION IV. Subsection 4-3-040.D, Development Standards for Uses Located Within the Renton Automall — Areas A and B, of Chapter 3, Environmental Regulations and Overlay Districts, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: D. DEVELOPMENT STANDARDS FOR USES LOCATED WITHIN THE RENTON AUTOMALL—AREAS A AND B: All permitted uses in Area A and all auto sales and related uses in Area B of the Renton Automall shall comply with the following development standards: 6 ORDINANCE NO. 5828 ALL USES IN AREA A, NON-DEALERSHIPS AND RELATED USES DEALERSHIPS AND RELATED USES IN AREA B IN AREA B SERVICE AREA Service areas shall not face public Service areas shall not face public street ORIENTATION street frontage. frontage. LANDSCAPING — A 15-foot-wide (andscape strip Pursuant to landscaping requirements STREET along these street frontages. This listed in chapter 4-2 RMC (requirements FRONTAGE frontage requirement is in lieu of for the underlying zone) and RMC 4-4- LANDSCAPING the frontage requirement listed 070. REQUIREMENTS for the zone in chapter 4-2 RMC. for lots which Unimproved portions of the right- abut Lind of-way may be used in Avenue S.W., combination with abutting private S.W. Grady Way, property to meet the required 15- Talbot Road 5. foot landscape strip width. (SR-515) and The landscaping shall include a Rainier Avenue minimum 30-inch-high berm and 5. red maples (Acer rubrum or other equivalent tree species required or approved bv the Administrator on the Citv's Approved Tree List per RMC 4-4- 070.L, planted 25 feet on center. LANDSCAPING — Minimum 2.5% of the gross site Pursuant to landscaping requirements MINIMUM area shall be provided as on-site listed in chapter 4-2 RMC (requirements AMOUNT AND landscaping. Landscaping shall be for the underlying zone) and RMC 4-4- LOCATION consolidated and located at site 070. entries, building fronts, or other visually prominent locations as approved through the site plan development review process. Minimum landscaping may be reduced to 2% of the gross site area where bioretention, permeable paving, or other low impact development techniques consistent with the Surface Water Desi�n Manual are inte�rated. LANDSCAPE MAINTENANCE All landscapin�is subiect to maintenance pursuant to RMC 4-4-070.P. AND TREE 7 ORDINANCE N0. 5828 ALL USES IN AREA A, NON-DEAIERSHIPS AND RELATED USES DEALERSHIPS AND RELATED USES �N AREA B IN AREA B REMOVAL WHEEL STOPS If frontage landscaping is If frontage landscaping is relocated, relocated,then permanent then permanent wheel stops or wheel stops or continuous continuous curbs must be installed a curbs must be insta►led a minimum of 2.5 feet from sidewalks to minimum of 2.5 feet from prevent bumper overhang of sidewalks to prevent bumper sidewalks. Where these requirements overhang of sidewalks. Where differ from the requirements of the these requirements differ from parking, loading and driveway the requirements of the regulations of chapter 4-4 RMC, these parking, loading and driveway requirements shall govern. regulations of chapter 4-4 RMC, these requirements shall govern. CUSTOMER Customer parking shall be Customer parking shall be designated PARKING designated and striped near and striped near entry drives and entry drives and visible from visible from public streets. Where public streets. Where possible, possible, customer parking shall be customer parking shall be combined with abutting dealership combined with abutting customer parking and shared access. dealership customer parking Where these requirements differ from and shared access. Where these the requirements of the parking, requirements differ from the loading and driveway regulations of requirements of the parking, chapter 4-4 RMC, these requirements loading and driveway shall govern. regulations of chapter 4-4 RMC, these requirements shall govern. AUTOMALL Development shall be Development shall be coordinated with RIGHT-OF-WAY coordinated with the adopted the adopted right-of-way improvement IMPROVEMENT right-of-way improvement plan plan which addresses gateways, PLAN which addresses gateways, signage, landscaping, and shared COORDINATION signage, landscaping, and access. shared access. AUTOMALL All development shall All development shall coordinate with IMPROVEMENT coordinate with the Automall the Automall Improvement Plan PLAN Improvement Plan adopted by adopted by Resolution No. 3457. The COMPLIANCE Resolution No. 3457. The plan plan addresses potential street 8 ORDINANCE N0. 5828 ALL USES IN AREA A, NON-DEALERSHIPS AND RELATED USES DEALERSHIPS AND RELATED USES IN AREA B IN AREA B addresses potential street vacations, right-of-way improvements, vacations, right-of-way area gateways, signage, landscaping, improvements, area gateways, circulation, and shared access. signage, landscaping, circulation, and shared access. MODIFICATIONS Where full compliance with these provisions would create a hardship for existing uses undergoing major modifications, the Zoning Administrator may modify them. Hardship for existing uses may result from existing lot coverage, existing siting of buildings, etc., which preclude full compliance. SECTION V. The Service Element Location and Design subsection of 4-3-100.E.1, Site Design and Building Location, of Chapter 3, Environmental Regulations and Overlay Districts, of Title tV (Development Regulations) of the Renton Municipal Code, is amended as shown below. The rest of the table shall remain as currently codified. SERVICE ELEMENT IOCATION AND DESIGN lntent:To reduce the potentiat negative impacts of service elements (i.e.,waste receptacles, laading docks} by(ocating service and loading areas away from pedestrian areas, and screening them from view in high visibility areas. Guidelines: Service elements shall be concentrated and located so that impacts to pedestrians and other abutting uses are minimized. The impacts of service elements shall be mitigated with landscaping and an enclosure with fencing that is rrtade of quality materials. Service areas not adiacent to streets, pathwavs, or pedestrian-oriented snaces are encoura�ed to implement ve�etative screening in addition to or as part of service enclosures. Standards: All of the following are required: All Districts 1. Service elements shall be located and designed to minimize the impacts on the pedestrian environment and adjacent and/or abutting uses. Service elements 9 4RDINANCE N�. 5828 shali be concentrated and located where they are accessible ta service vehides and eonvenient for tenant use. 2. In addition to standard enclosure requirements, garbage, recycling collection, and utility areas shall be enclased on all sides, include a roof and be screened around their perimeter by a wall or fence and have self-clasing doars (illustratian belaw�. Ra�f enclasure L�ndscapin� \ „�'''� ''""� to keep birds out � �'f'' � `�. � �i '"` S�?If-GIt�Slli� r- do�r� Trellis "'#'' t�onc�te pad 3. Service enclosures shall be made of masonry, ornamental metal or waod, or some cornbination of the three (3�. 4. If the service area is adjacent to a street, pathway, or pedestrian-ariented space, a landscaped planting strip, minimum#hree feet (3'}wide, shaN be located on three (3} sides of such facility. SECTION VI. The Vehicular Access subsection of 4-3-100.E.2, Parking and Vehicular Access, of Chapter 3, Environmental Regulations and Overlay Districts, of Title IV {Development Regulations} af the Rentan Municipal Code, is amended as shown below. 7he rest af the table shal) remain as currently codified. 10 ORDINANCE N�. 5828 Guidelines:Vehicular access to parking garages and parking lots shall not impede or interrupt pedes#rian mobility.The impacts af curb cuts to pedestrian access on sidewalks shall be minimized. 5tandards: The following is required: 1. Access to parking iots and garages shall be from alleys, when available. If nat Districts available, access shall occur at side streets. A, B, and D 2, The number of driveways and curb cuts shall be minimized for vehicular access purposes, so that pedestrian circulation along the sidewalk is minimally impeded. Both of the following are required: 1. Parking garages shall be accessed at the rear of buildings. District C 2 parking lot entrances, driveways, and other vehicular access points shall be restricted to one entrance and exit lane per five hundred (500) linear feet as measured horizontally along the street. SECTION VII. The Pedestrian Circulation subsection of 4-3-100.E.3, Pedestrian Environment, of Chapter 3, Environmental Regulations and Overlay Districts, of Title !V (Development Regufations) of the Renton IVlunicipal Code, is amer�ded as shown below. The rest of the table shall remain as currently codified. Guidelines:The pedestrian environment shall be given priarity and importance in the design of prajects. Sidewatks andjor pathways shal{ be provided and shall provide safe access ta buiidings fram parking areas. Praviding pedestrian connections to abutting praperties is an important aspect of connectivity and encourages pedestrian activity and shali be cansidered. Pathways shall be easily identifiable to pedestrians and drivers. 11 ORDINANCE N0. 5828 IStandards: Districts All of the following are required: A, C, and � A pedestrian circulatian system af pathways that are ciearly delineated a and connect buildings, open space, and parking areas with the sidewalk system and abutting properties shall be provided. a. Pathways shall be located so that there are clear sight lines,to increase safety b. Pathways shall be an all-weather or appropriate permeable walking surface materiai, uniess the applicant can demonstrate that the propased surface is appropriate for the anticipated number af users and camplementary to the design of the develapment. 2. Pathways within parking areas shall be provided and differentiated by material or texture (i.e., raised walkway, stamped concrete, or pavers) from abutting paving materials (illustration below). Permeable materials are encouraged. The pathways shall be perpendicular to the applicable building facade and no greater than one hundred fifty feet (150'} apart. , ��"" �� .��-�� •x-�. ,�„_�. ��. '— � _ �-�-,.�.�.:��.�- � �I LI �I .��: w �- �� - - �, .�.,�.��v`� � �� ^�T+�� f � � . .,� � �� .,.,_._: � i - : �� - - _ ,ti ��. ~� . � � � 3. Sidewalks and pathways along the facades of buildings shall be of 12 ORDINANCE NO. 5828 sufficient width to accommodate anticipated numbers of users. Specifically: a. Sidewalks and pathways along the facades of mixed use and retail buildings one hundred (100) or more feet in width (measured along the facade) shall provide sidewalks at least twelve feet (12') in width. The pathway shall include an eight-foot (8') minimum unobstructed walking surface. b. Interior pathways shall be provided and shall vary in width to establish a hierarchy. The widths shall be based on the intended number of users; to be no smaller than five feet (5') and no greater than twelve feet (12'j. 4. Mid-block connections between buildings shall be provided. All 5. Permeable pavement pedestrian circulation features shall be used where Districts feasible, consistent with the Surface Water Desi�n Manual. SECTION VIII. Subsection 4-3-100.E.4, Recreation Areas and Common Open Space, of Chapter 3, Environmental Regulations and Overlay Districts, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as shown below. The rest of the table shall remain as currently codified. 4. Recreation Areas and Common Open Space: Guidelines: Developments located at street intersections should provide pedestrian-oriented space at the street corner to emphasize pedestrian activity (illustration below). Recreation and common open space areas are integral aspects of quality development that encourage pedestrians and users. These areas shall be provided in an amount that is adequate to be functional and usable;they shall also be landscaped and located so that they are appealing to users and pedestrians. 13 ORDINANCE N0. 5828 + ��` �t3C't'l�" �ul�t�IClt� + � w - - r � r ♦�ry�r � ��f�i .� •�� '`r`f•I`r;J•f•t J+i+r •+/ �w •! +�r�r r�r•r� �e.tl +�r��r r� r+��♦ f`��f�r` f"r'r'f` - � r�1� ��r��1� ' r.r.l .`.f. Ca�nerentr� ,,. .-�-,.,. �-f-,- �,►ith ina-e�sed ,r-----.� �„-�. setb►ack � P�destn�an-cxtierr�ed s��e .��. � Standards: All of the following are required: 1. All mixed use residential and attached housing developments of ten (10) or more dwelling units shall provide common open space and/or recreation areas. a. At minimum, fifty (50) square feet per unit shall be provided. b. The location, layout, and proposed type of common space or recreation area shall be subject to approval by the Administrator. Districts c. Open space or recreation areas shall be located to provide sun and light A, C, exposure to the area and located so that they are aggregated to provide usable and D area(s) for residents. d. For proiects with more than one hundred (100) dwellin� units, ve�etated low impact development facilities mav be used in reauired or provided open space where feasible and desi�ned consistent with the Surface Water Desi�n Manual. Such facilities shall be counted towards no more than fiftv percent (50%) of the required oqen space. �e. At least one of the following shall be provided in each open space and/or recreation area (the Administrator may require more than one of the following 14 ORDINANCE N4. 5828 elements for developments having more than one hundred {200) units): i. Courtyards, plazas, pea patches, or multi-purpose apen spaces; ii. Upper level common decks, patios,terraces, ar raof gardens. Such spaces above the street level must feature views or amenities that are unique to the site and are pravided as an asset to the development; iii. Pedestrian corridors dedicated to passive recreation and separate from the public street system; iv. Recreation facilities incfuding, but not limited to, tennisJsparts caurts, swimming poals, exercise areas, game rooms, or other similar faciiities; or v. Children's play spaces that are centrally located near a majority of dwelling units and visible from surraunding units. They shail also be iocated away from haza�-dous areas such as garbage dumpsters, drainage fac+lities, and parking areas. e:f. The fallawing shall not be counted taward the cammon open space or recreation area requirement: i. Required iandscaping, driveways, parking, ar ather vehicular use areas. ii. Required yard setback areas. Except for areas that are developed as private or semi-private (fram abutting or adjacent properties) courtyards, plazas or passive use areas containing landscaping and fencing sufficient to create a fully usabie area accessib{e to all residents of the development (illustration belaw). 15 ORDINANCE Nq. 5828 � � � � I �� , � � G E' 1 � ""'�� ' � "._.- �� ..� �.- �..�,,., C� �' -�� .� ;-- � ��. ��-. -�� __ ��-- r . �� 1� r � � � � � ��., , - � - � � � �-= � .� .: . ,w , . ��.. . ;� -�-�--.�- � . ... _. __---.�..-.�..- _ . � � � . _ .r_ __ � \ , _._-. ��r N ` . -�.� � �.._. .,, _�— � �_��'a� �--..�-�� 1�...-�----- --�„� .,.._--�-�;-�:.,��,,��;. . . �� , . � ��..�_� �"" ` .-- -�., � ;= x: �.�-.. - ,� ,.,4 � ,,,�,�`.-��,`" � `� `--..��---�"`-.. � ,�:r-��.,. '`^y, J'�,�r" �. �.�^'' r"' i, X ��,-�^. " !,• f iii. Private decks, balconies, and private ground floar open space. iv. �ther required landscaping and sensitive area buffers without common access links, such as pedes#rian trails. 2. All buildings and developments with over thirty thousand (30,Q00) square feet of nonresidential uses (excludes parking garage flaorpiate areas� shall provide pedestrian-oriented space, a. The pedestrian-oriented space shali be provided according to the fallowing formula: 1% of the site area + 1% af the gross building area, at minimum. b. The pedestrian-oriented space shall include all of the fallowing: i. Visual and pedestrian access (inciuding barrier-free access) to the abutting structures from the public right-of-way or a nonvehicuiar courtyard; and ii. Paved walking surfaces of either concrete or approved unit paving; and iii. C3n-site ar building-mounted iighting praviding at ieast four (4} foot- candles (average) on the ground; and iv. At least three (3) Nneal€eet of seating area (bench, ledge, etc.) ar one individual seat per sixty (60) square feet of plaza area or open space. c. The following areas shall not count as pedestrian-oriented space: i. The minimum required walkway. Nowever, where walkways are widened or enhanced beyond minimum requirements,the area may count as 16 ORDINANCE N0. 5828 pedestrian-oriented space if the Administrator determines such space meets the definition of pedestrian-oriented space. ii. Areas that abut landscaped parking lots, chain link fences, blank walls, and/or dumpsters or service areas. d. Outdoor storage (shopping carts, potting soil bags, firewood, etc.) is prohibited within pedestrian-oriented space. 3. Open space substitution: see RMC 4-1-240. The following is required: All attached housing developments shall provide at least one hundred fifty (150) square District feet of private usable space per unit. B 1. At least one hundred (100) square feet of the private space shall abut each unit. 2. Private space may include porches, balconies, yards, and decks. All of the following are required: 1. Public plazas shall be provided at intersections identified in the Commercial Arterial Zone Public Plaza Locations Map and as listed below. 2. The plaza shall measure no less than one thousand (1,000) square feet with a minimum dimension of twenty feet (20') on one side abutting the sidewalk. 3. The public plaza must be landscaped consistent with RMC 4-4-070, including at minimum street trees, decorative paving, pedestrian-scaled lighting, and seating. Ve�etated low impact development facilities mav be used in the plaza where feasible and desi�ned consistent with the Surface Water Desi�n Manual. Such facilities shall count towards no more than fiftv percent (50%) of the plaza reauirement. 4. Public plazas are to be provided at the following intersections: All a. Benson Area: Benson Drive 5./108th Avenue S.E. and S.E. 176th. Districts b. Bronson Area: Intersections with Bronson Way North at: i. Factory Avenue N./Houser Way S.; ii. Garden Avenue N.; and iii. Park Avenue N. and N. First Street. c. Cascade Area: Intersection of 116th Avenue S.E. and S.E. 168th Street. d. Northeast Fourth Area: Intersections with N.E. Fourth at: i. Duvall Avenue N.E.; ii. Monroe Avenue N.E.; and iii. Union Avenue N.E. e. Grady Area: Intersections with Grady Way at: 17 ORDINANCE N0. 5828 i. �ind Avenue S.W.; ii. Rainier Avenue S.; iii. Shattuck Avenue �.; and iv. Talbot Road 5. f. Puget Area: Intersection of S. Puget Drive and Benson Raad S. g. Rainier Avenue Area: Intersections with Rainier Avenue 5. at: i. Airport WayJRenton Avenue S.; ii. S. Secand Street; iii. 5.Third StreetJS.W. Sunset Baulevard; iv. S. Fourth Street; and v. 5. Seventh Street. h. North Rentan Area: Intersections with Park Avenue N. at: i. N. Fourth Street; and ii. N. Fifth Street. i. Northeast Sunset Area: intersections with N.E. Sunset Baulevard at: i. Duvall Avenue N.E.; and ii. Unian Avenue N.E. SECTION !X. Subsection 4-3-110.E.3, Uses AIlowed in Contiguous Open Space, of Chapter 3, Environmental Regulations and Overlay Districts, of Title IV (Development Regulations} of the Renton Municipal Code, is amended as follows: 3. Uses Allowed in Contiguous Ctpen Space. a. Passive recreatian with no development of active recrea#ian facilities except within a municipal park. b. Pla#-��t Soft surface pedestrian trails, provided the adiacent area is lar�e enou�h to infiltrate runoff�enerated from the pathway consistent with the Surface Water Desi�n Manual. 18 QRDINANCE N0. 5828 c. Anima! husbandry (small, medium and large�; provided, that fencing is subject to the conditions in ��+�t RMC 4-3-110,E 3.g e#�k�+s �e��. d. Existing residences and accessary uses and structures. e. Smail and medium utilities and large underground utilities. SECTION X. Subsection 4-4-030.C.6, Hydroseeding Required, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations� of the Renton Municipal Code, is amended as follaws: 6. Hydraseeding Required: a. Within thirty (30) days of campletion of grading wark, #he applicant shall hydroseed ar plant an appropriate ground cover over any portion of the site that is graded or cieared of vegetation and where no further construction work will accur within ninety (90) days. b. Alternative measures to hvdraseedin� such as mulch, sodding, or plastic covering as specified in the Surface Water Design Manual may be proposed between the dates of October 1st and Apri) 30th of each year. Plastic coverin�mav be used for soil qrotection for soils tnat will be utilized as structurai or fill materials onlv, provided that sails#hat are to be reused for other purgases shail be covered for no more than faurteen f 14} davs, c_The Development Services Division's approval of this work is required prior to final inspectian and approval of the permit. 19 ORDINANCE N0. 5828 SECTION XI. Subsection 4-4-040.C.1.b, Hedges, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: b. Hedges: The height shall be measured from the topmost portion of vegetation to the ground. Hed�es desi�ned as part of stormwater low impact development best mana�ement practices shall be desi�ned consistent with the Surface Water Desi�n Manual, and the Clear Vision Area defined in RMC 4-11- 030, and avoid blockin� public views to public places. SECTION XII. Subsection 4-4-040.C.2.e.i, Terrace Width, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: i.Terrace Width: No portion of a retaining wall shall be measured as part of the terrace width. The width of a terrace shall be equal to the height of the tallest abutting retaining wall; however, the minimum terrace width shall be two feet (2') ^��-;�;w;;::�:�;;-; :�e�.aire�-avid�h�k�all ��-#'wc�*�7. Terrace width shall be measured from the back edge of a lower retaining wall to the foremost edge of the immediately succeeding and higher retaining wall. SECTION XIII. Subsection 4-4-060.H, Engineering Grading Requirements, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: H. ENGINEERING GRADING REQUIREMENTS: 20 ORDINANCE NQ. 5828 1. Reparts Required: Soil engineering and e�►g+r�ee�i^^ ^^^'^^., �eotechnical reports shall be required as defined in RMC 4-11-19Q€ and 4-11- 190�. During grading ali necessary reports, compactian data and soii engineering and engineering geology recammendatians shall be submitted to the civil engineer and the Public Warks Department by the soil engineer and the engineering gealogist. The Pubiic Works Department may waive reports for minor grading operatians. 2. Civil Engineer Responsibilities: For purposes of preparing and/or approving engineered grading plans, the engineer shall be a professiona) engineer registered in the State to practice in the field af civil works. The civil engineer shall be responsible for the plans, any special soil engineering and testing reports, design of drainage facilities and structures, and be competent to recommend and obtain special tests, survey data, and geologicai or hydrauiic reports shauld they be necessary. The civil engineer shall pravide an acceptable plan and report based on good engineering practices and the requirements designated by the Public Works Department. He shall, upon return of his plans, provide any corrections necessary and corrected copies for use of the City in reviewing the grading wark. The civi# engineer shal! be responsible for reporting monthly or more frequently on farms provided by the Public Works Department: a. Extent and locatian of grading. b. All tests made or taken in canjunction with the grading operation. 21 ORDINANCE N0. 5828 c. Extent of drainage, structure, and safety activity report on the project. d. Any special testing, as-built plans or revised requests necessary. In addition, he shall certify to the safety and stability of the slopes, safety earthwork operation, and special problems which might occur. 3. Soil Engineer Responsibilities: The soil engineer's area of responsibility shall include but need not be limited to the professional inspection and certification concerning the preparation of ground to receive fills, testing for required compaction, stability of all finish slopes and the design of buttress fills, where required, incorporating data supplied by the engineering geologist. 4. Engineering Geologist Responsibilities: The engineering geologist's area of responsibility shall include but need not be limited to professional inspection and certification of the adequacy of natural ground for receiving fills and the stability of cut slopes with respect to geological matters, and the need for subdrains or other groundwater drainage devices. He shall report his findings to the soil engineer and the civil engineer for engineering analysis. 5. Building Division Responsibilities: The Building Division shall inspect the project at frequent intervals to determine that adequate control is being exercised by the operator and the civil engineer. Should hazardous conditions occur, the Building Department inspector shall have the responsibility and authority to issue a partial or total stop work order. 22 ORDINANCE NO. 5828 6. Specifications: A fence six feet (6') in height with openings no larger than two inches (2") (other than gates) may be required for safety reasons completely around any area worked upon for which a permit is issued for engineered grading prior to commencing any other work. All gates shall be locked when not in use and shall bear a sign denoting danger. 7. Setbacks: Engineered grading sites may be required to have a peripheral area a maximum of seventy_five feet (75') in width which shall be retained in its natural topographic condition. The setback area shall be used for, but is not limited to, access roads, planting, fencing, landscaped berms for screening purposes, employee and visitor parking, offices, directional signs and business signs identifying the occupant. 8. Contractors: Durin� construction, flow control best mana�ement practices shall be protected consistent with the Surface Water Desi�n Manual. SECTION XIV. Subsection 4-4-060.J.3, Clearing and Rounding Tops of Slopes, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 3. Clearing and Rounding Tops of Slopes: All trees, timber, stumps, brush or debris shall be cleared to a point at least ten feet (10') back from the top of any slope involving cuts greater than ten feet (10'); provided that exceptions mav be �ranted in areas where trees and brush have a si�nificant role in maintainin� slope stabilitv. After excavation, the top of all slopes shall be rounded to prevent a sheer breaking point. 23 ORDINANCE N0. 5828 SECTION XV. Subsection 4-4-060.J. Work in Progress, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 1. WORK IN PROGRESS: 1. Maximum Slopes — Work in Progress: No slopes greater than one horizontal to one vertical will be permitted for cuts, fills, or during excavations that exceed ten feet (10') in height without physical restraint by timbering or approval by the �i�gCommunitv and Economic Development Department of an engineering or geologist report assuring slope will maintain its shape without undue risk of failing. 2. Safety: Workmen shall be allowed in the vicinity of the toe or top of slope only after close visual inspection of slope to assure safety against breakage . or sliding. 3. Clearing and Rounding Tops of Slopes: All trees, timber, stumps, brush or debris shall be cleared to a point at least ten feet (10') back from the top of any slope involving cuts greater than ten feet (10'). After excavation, the top of all slopes shall be rounded to prevent a sheer breaking point. 4. Property and Setback Location: Property location and approved setbacks must be established and stakes set under the supervision of a registered land surveyor. These stakes must be maintained in place until final inspection of work so that the inspector can determine at any time if the excavation is properly located as related to the property lines. 24 �RDENANCE N0. 5828 5. Maximum Noise l.evels: Noise leveis at al! operatians shail be contralled to prevent undue nuisance to the public. Maximum allowable daytime sound pressure as measured in any residentiaE zot�e shall not exceed the following at least ninety percent j90°lo} of the time between the hours af seven o'clock (7:00) a.m. and eight o'clack (8:00} p.m. SOUND PRES5URE LEVELS FREQUENCY SOUND PRESSURE LEVEL BAND IN IN DECIBEIS re. O.QQ02 CYCLES/SECOND MICROBAR 25—300 8Q 3Q0—2,400 70 Abave 2,400 60 6. Permitted Work Wours: All mining, excavation and grading wark done in residential areas or within three hundred feet (300') of residential areas shall be between the haurs of seven o'clock (7:00) a.m. and eight o'clock (8.OQ� p.m., Monday through Friday, except repairs to machinery. Work may be permitted on Saturdays and Sundays only if approved in writing in advance by the �eue�i:�:..::*' ��rvises '��•��^^ "��^`'+^' Administratar. The �-e�e� Administrator is authorized to grant an extensian of working time during an emergency. An emergency shail include but is not limited to natural and manmade disasters. 7. Compliance with Pollution Control Regulations. Discharge af materials into the air ar water shall be subject to the requiremer►ts of the appropriate gaverning agency. 25 ORDINANCE NO. 5828 8. Control of Dust and Mud: Activities shall be operated so as to reduce dust and mud to a minimum. Unless otherwise specified by the Public Works and �i+�g Communitv and Economic Development Departments, operations shall be conducted in accordance with the following standards: a. Access Roads: Access roads shall be maintained in a condition that confines the mud and dust to the site. Such roads shall be improved to a width sufficient to permit the unhindered movement of emergency vehicles. One-way roads shall have by-pass routes to permit the movement of emergency vehicles. b. Dozing and Digging: Dozing, digging, scraping and loading of excavated materials shall be done in a manner which reduces to the minimum level possible the producing of dust and mud. 9. Soil Erosion and Sedimentation: Soil erosion and sedimentation shall be confined to the site by such means as a temporary cover of vegetation, mulches, diversions, sedimentation ponds or other acceptable methods. No toxic materials shall be allowed to wash from the site or be discharged into receiving watercourses. 10. Appearance: All activities under the jurisdiction of this Section shall be operated and maintained in a neat and orderly manner, free from junk, trash, or unnecessary debris. Buildings shall be maintained in a sound conditions, in good repair and appearance. Salvageable equipment stored in a nonoperating condition shall be suitably screened or garaged. Landscaping adjacent to and 26 ORDENANCE N4. 5828 around the main entrance�s} and office shaU be sufficiently watered and cared for to insure its health and weEi-being. 11. Sail Campaction: After �radin�, permeabilitv af soils shal! be reestabiished in areas intended for stormwater mana�ement and infiitration or areas not required to have a structural capacit� such as future lawn and open space areas. Techniaues mav include deep-tillin� and laosenin� soils compacted durin�site �rading in arder to restore their natural infiltration capacit��. SECTIQN XVI. Subsection 4-4-06Q.N.3, Preparation of Ground, af Chapter 4, City- Wide Praperty Develapment Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 3. Preparatian of Graund:The ground surface shall be prepared to receive fill by removing vegetation, noncamplying fill, topsoil and other unsuitable materials as determined by the soil engineer, and where the slopes are five to one (5:1� ar steeper, by benching into sound bedrock or ather campetent material, pravided native ve�etation and si�nificant trees are protected pursuant ta RMC 4-4-130. SECTION XVI1. Subsection 4-4-060.N.5, Minimum Compaction, of Chapter 4, City- Wide Property Deveiopment Standards, of Title IV (pevelapment Regulations} of the Renton Municipai Code, is amended as foilows: 5. Minimum Compaction: All fills shall be campacted to a minimum of ninety_five percent (95%) of maximum density as determined by American Public Works Assaciation (APWA) specifications. Field density shall be determined in 27 QRDINANCE N0. 5828 accordance with APWA standards. Exceptions to the compactian reauirement incl�de soils below areas set aside far law impact development best mana�ement practices desi�ned consistent with RMC 4-6-030. SECTION XVII1. Subsection 4-4-060.R, Final Reports, of Chapter 4, City-Wide Praperty Development Standards, of Title IV (Development Reguiations} of the Renton Municipa) Code, is amended as follows: R. FINAL REPORT5: 1. Construction Timin�and Final Approval: a. No work related ta nermanent or temporaru storm draina�e contraf for a �ermitted development maw proceed without the appraval of the Administrator. b. Erosian and sediment control measures associated with both the interim and permanent svstems shall be: i. Canstructed in accordance with the a�proved plan prior to any �radin� or land clearin� other than assaciated with an approved erosion and sediment control t�lan: and ii. Satisfactorilv seauenced and maintained until al! improvements. restoratian and landscapin� associated with the permit and approvals for the proiect are completed and the potential for onsite erosion has passed. 2$ ORDINANCE N0. 5828 �2. Plans and Reports: Upon completion of the rough grading work and at the final completion of the work, the Development Services Division may require the following reports and drawings and supplements thereto: a. As-Graded Grading Plan: An as-graded grading plan prepared by the civil engineer including original ground surface elevations, as-graded ground surface installations, lot drainage patterns and locations and elevations of all surface and subsurface drainage facilities. The civil engineer shall provide certification that the work was done in accordance with the final approved grading plan. b. Soil Grading Report: A soil grading report prepared by the soil engineer including locations and elevations by field density tests, summaries of field and laboratory tests and other substantiating data and comments on any changes made during grading and their effect on the recommendation made in the soil engineering investigation report. The soil engineer shall provide certification as to the adequacy of the site for the intended use. c. Geologic Grading Report: A geologic grading report prepared by the engineering geologist including a final description of the geology of the site including any new information disclosed during the grading and the effect of same on recommendations incorporated in the approved grading plan. The engineering geologist shall provide certification as to the adequacy of the site for the intended use as affected by geologic factors. 29 4RDINANCE N�. 5828 �3. Notification of Completion: The permittee or his agent shall notify the Development Services Division when the grading operation is ready for finai inspection. ' ai�i-�raiiia�c—fa�iliEies c�fl� ��i�Ii'-�F8�26cNd2 ,'1.,.,�.,.,� .-.rl Y11 `•-t}SI$fi—�E3F�FF$� 4. Final Approval: The applicant shall construct and have in o�eration those portions of the draina�e facilities necessarv to accommadate the control af surface and stormwater runofF dischar�ir�� from the site al before the canstruction of anv ather improvements or buildin�s on the site. and bl prior #o final recordin� of a plat ar short plat. However, upon written reauest and provision for securitv acceptable to the Citv, the Administrator mav authorize a deiav in the construction of draina�e facilities bevond other construction and/or recordin� if such delav would minimize adverse draina�e impacts related to weather andlor soil conditions. Final aopraval shall nat be �iven until all work, includin� installation of a!I draina�e facilities and their qrotective devices and all erosion control measures have been completed in accordance with the final appraved �radin�qfan and the required reports have been submitted. SECTiON XIX. Subsection 4-4-070.F.6.b, Minimum Amaunts of Interior Parking Lat Landscaping, of Chapter 4, City-Wide Property Deveiopment Standards, of Title IV (Develapment Regulatians) of the Renton Municipa) Cade, is amended as follaws: 30 ORDINANCE N0. 5828 b. Minimum Amounts of Interior Parking Lot Landscaping:Surface parking lots with more than fourteen (14} stalis shall be landscaped with plantin�s and trees as identified in RMC 4-4-Q70. Pianters shaii be sized to dimensians of at least nine feet t9'1 bv thirteen feet (13`1 to accommodate trees as thev mature. Minimum landscape area shall be provided as faliows: Tatal Number of Parking Minimum Landscape StaUs A+'�� 15#a 50 :—I 1S sfjparking space I 51 to 99 ( 25 sf/parking space � 100 or more � 35 sf/parking space SECTION XX. Subsectian 4-4-070.F.8, Storm Drainage Facilities, of Chapter 4, City- Wide Praperty Development Standards, of Title IV (Development Regulations) of the Rentan Municipal Code, is amended as fallows: 8. Storm Drainage Facilities: a. Flaw Control and�or Water Quality Treatment Facilities; The perimeter of al) new flow control andJor water quality treatment starmwater facilities shall be landscaped in accordance with the provisions af this Section and the Surface Water Design Manual, unless otherwise determined thraugh the site plan review or subdivision review process. b. Low Impact Development Facilities: Biaretention, infiltratian, or other low im�act develoament starmwater facilities shall be located to avoid, onsite clearin� and �radin�, to the extent feasible. Such facilities shall be 31 o�oi�vaNc� No. sa2a desi�ned to incorporate piant s�ecies consistent with the Surface Water Desi�n Manual, with a preference far native trees and shrubs., SECTIQN XXi. Subsection 4-4-07Q.G.3, Retention of Existing landscaping and Existing Trees Encauraged, of Chap#er 4, City-Wide Property Develapment Standards, af Title iV (Development Regulations) of the Renton Municipa) Code, is amended as follows: 3. Retention of Existing Landscaping and Existing Trees Encouraged: Where possible, existing native trees and shrubs, rock outcroppings, and mature arnamenta! landscaping shall be preserved and incorporated in the landscape layout and can be caunted towards required landscaping. Develapment or redevelopment of properties shall retain existing trees when possible and minimize the impact af tree lass during development, Landscape pians are subiect to RMC 4-4-130's reauirements to protect si�nificant trees. and ve�etation with habitat value �^� °^"r �_n_,�n�Tr�^ SECTION XXI1. Subsection 4-4-07Q.G, General Landscaping Requirements, of Chapter 4, City-Wide Praperty Development Standards, of Title IV (Development Regulations} of the Rentan Municipal �ode, is amended as follows: G. GENERAL LANDSCAPE REQUIREMENTS: 1. Compliance Required: landscaping and screening required by this Section must compiy with ali of the provisions of this Sectian. The iandscaping standards are minimums; higher standards can be substituted as long as fencing 32 ORDINANCE N0. 5828 and vegetation do not exceed height limits specified in RMC 4-4-040. Crime prevention and safety should be considered in landscape design. 2. Protection of Street Trees: It shall be unlawful for any person, without prior written approval of the City to remove, destroy, cut, break, or injure any street tree that is planted or growing in or upon any street right-of-way. 3. Retention of Existing Landscaping and Existing Trees Encouraged: Where possible, existing trees, rock outcroppings, and mature ornamental landscaping shall be preserved and incorporated in the landscape layout and can be counted towards required landscaping. Development or redevelopment of properties shall retain existing trees when possible and minimize the impact of tree loss during development per RMC 4-4-130, Tree Retention and Land Clearing Regulations. 4. Calculation of Required Plantings: Some required (andscaping areas require a minimum amount of plantings per square feet of area. If the calculation of the number of plantings results in a fraction of 0.5 or greater, the applicant shall round up to the next whole number. If the calculation of the number of plantings results in a fraction of 0.4 or less, the applicant shall round down to the next whole number. 5. Avoidance of Hazards: All landscaping shall be planned in consideration of the public health, safety, and welfare. a. Landscaping shall not intrude within the clear vision areas at driveways and street intersections; 33 C?RDINANCE NO. 5828 b. 7rees planted near overhead power lines shall be species that wiil comply with utility purveyor dearar►ce requirements; c. �andscaping shali not obscure fire hydrants or access for emergency response vehicies; and d. landscaping in a parking lat shali not canflict with the safety of those using a parking lot, abutting sidewalks, or with traffic safety. 6. Ve�etation Preference: Ve�etation within reguired setbacks or screenin�areas shall be retained or nlanted in this order of preference: al native coniferous trees. b1 native deciduous trees; cl other native ve�etation. Ve�etated low impact develot�ment starmwater facilities mau be incorporated as part of landscaaed setbacks or screening reauirement. SECTION XXII1. Subsectian 4-4-070.H.4.a of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipai Code, is amended as follows: a.Trees shall be two inche� (2"� • �• "�'�+nr w* "r^��* �""^ht _/�.,lhh\ �ic�c��c aAUT caliper for multi-family, commercial, and industrial uses at an average minimum rate of one �tree per thirty {30) iineal feet of street frontage. Trees shall be one and a half inch (2.5"1 caliper €or law impact development starmwater mana�ement facilities associated with anu land use. SECTiON XXIV. Subsectians 4-4-070.L.2, Ground Cover is Required, 4-4-070.L.3, Shrubs, 4-4-070.L.4, Trees and 4-4-070.L.5, Prohibited Plant Materials, of Chapter 4, City-Wide 34 QRDINANCE N0. 5828 Property Development Standards, of Title IV (Develapment Regulations� of the Renton Municipal Code, are amended as fallows: 2. Ground Cover ts Required: a. AN af the landscaped area that is not planted with trees and shrubs or covered with a tree grate must be planted in ground caver piants, which may include grasses. Mulch must be confined to areas underneath plants and is not a substitute for graund cover plants. b, Size and 5pacing: Ground cover plants, other than grasses, must be at least the faur-inch (4"} pot size, provided such plants have well-develaped raots and are not raot baund or 1-roated; afternative standards mav be applied pursuant ta 4-4-070.C. Area planted in graund cover plants, other than grass seed or sod, must be planted in triangular spacing as depicted below. Ground cover plants must be planted at a density that will cover the entire area within three (3) years. .•�+. r �.r� c. Plu�s or Bareraot Plants: In (ieu of four-inch (4"1 pots. the Administrator mav allow or re�uire incor�oration of ten-inch t10") landsca�e plu�s or bareraot plants provided the roots are well-developed, can be pfanted 35 ORDINANCE N0. 5828 durin� the appropriate season, and can meet the covera�e requirements in subsection b above. d. Supplementar� Seedin�: Where feasible, the Administrator ma�r require supplementarv seedin� to promote �enetic diversity of �roundcovers and plant material. e. Turf-Limited: The Administrator mav condition development permits to limit the extent of turf to promote species that are drou�ht-tolerant and to maximize application of native ve�etation or ve�etation associated with low impact development best mana�ement practices. 3. Shrubs: All shrubs must be of sufficient size and number to meet the required standards within three (3) years of planting. Shrubs must be at least a two (2) gallon container size at planting. Shrubs shall be in beds that include a layer of mulch at least two inches (2") in depth. The Administrator mav allow smaller size shrubs provided the applicant demonstrates to the Administrator's satisfaction that the plants can meet the covera�e reauirements in time. 4.Trees: a.Approved Tree Species: The Department of Community and Economic Development can provide an Approved Tree List. The list is available on the City website. b. Planting Size: Broadleaf trees at the time of planting must be fully branched and no smaller than one and one-half inches (1.5") �^ ^'��m^+^� ,* h.^^�+ "^;,�"+ ��,""` caliper. Broadleaf trees planted in residential zones must be 36 ORDINANCE NCJ. 5828 a minimum of one and one-half inches (1.5"j �^ �'�~^�^+a~ ��}caliper. Broadleaf trees planted in all other zones must be a minimum of two inches (2") +� �{��j caliper. Conifer trees at the time of pianting must be fuily branched and a minimum of six feet (6') in height. c. Mulch: Except far trees with a tree grate, trees shall include a mulch ring that has a depth of at least �we-three inches (�3") and is at least �e three feet (�3'� in radius araund the tree. 5. Prohibited Plant Materials: Plants listed as a nuisance or prohibited by Washin�tan State Noxious Weed Control Board or listed bv King County on the Countv's invasive species list are prohibited in reqtaired landscaped areas. SECTION XXV. Su6sectian 4-4-080.E.3.b, Agreement Required, of Chapter 4, City- Wide Property Development Standards, of Title IV (Development Reguiations} of the Renton Municipal Cade, is amended as follaws: b.Agreement Required:A parking agreement ensuring that joint use parking is available for the duration of the uses shal! be approved by the Community and Economic Develapment Administrator, folfowing review by the City Attorney. Notice of terminatian of the a�reement shafl be provided to the Administrator and additional parkin� must be provided if the a�reement is terminated, consistent with RMC 4-4-0$O.F.10. SECTION XXVt. 5ubsectian 4-4-080.F.4, Linkages, of Chapter 4, City-Wide Property pevelopment Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 37 C?RDINANCE N�. 5828 4. Linkages:The Department of Community and Economic Development shall have the authority to establish, or cause to be established, bicyde, high occupancy uehiclet and pedestrian linkages within pubiic and private developments. Conditions mav inc{ude but are not limited to: a. Reservin� parkin� spaces for rideshare ar other hi�h occupancv vehicies. b. Ensurin� adeauate on-site non-motarized paths connectin� to aublic non-motorized facilities. c. Providin� transit shelters. bus turnout lanes or other trar►sit improvements. Enfarcement shall be administered through the normal site design review and/or permitting process. SECTION XXVII. The General subsection af 4-4-080.F.10.d, Parking Spaces Based an Land Use, of Chapter 4, City-Wide Property Qevelopment 5tandards, af Title IV (Development Regulations) of the RentQn Municipal Code, is amended to add a new row labeled "Parking in Excess of Maximum Star�dards," to read as shown belaw. The rest af the table shall remain as currently codified. USE I NUMBER tJF REQUIRED SPACES GENERAL: Mixed occupancies: The total requirements far off-street parking facilities shalf be (2 ar 3 different uses in the the sum of the requirements far the several uses computed same building or sharing a lot. separately, uniess the building is ciassified as a "shapping For 4 or more uses, see center" as defined in RMC 4-11-190. "shopping center" requirements) 38 oRo�NaNCE �vo. sszs U�� NUMBER OF REQUIRED SPACES Uses not specifically Department af Community and Economic Develapment staff identified in this Section: shali determine which of the beiow uses is most similar based upon staff experience with various uses and information provided by the applicant. The amount of required parking for uses not listed above shall be the same as for the most similar use listed below. Bicycle parking: See minimum requirements in ��{"�^r+�^^ ��' �$:�;� `�� RMC 4-4-0$O.F.11. Parkin�in Excess of Maximum Maximum ratios for off-street parkin,�facNities mav be Standards: exceeded bv u�a to 10°10 if the at�plicant implements low imr�act development techniaues that reduce starmwater runaff and, mana�es stormwater on-site in a waa that exceeds the requirements af surface water mana�ement in RMC 4-6-030. SECTION XXVIII. Subsections 4-4-080.G.1, Surfacing Requirements for Parking Areas, and 4-4-080.G.5, Drainage, of Chapter 4, City-Wide Property Development Standards, of Title IV {Development Regulations) of the Rentan IVlunicipal Cade, are amended as follows: 1. Surfacing Requirements for Parking Areas: All o#f-street parking areas shall be paved with asphaltic cancrete, cement ar equivalent alternative material of a permanent nature as approved by the Public Works Department. Surfacin� treatments that provide increased infiltration ap�ortunities, such as permeable pavements, shall be used where feasible and to the extent reauired b�� the Surface Water Desi�n Manual. 5. Drainage: Drainage shaN meet City requirements, including the 4ocation of the drains and the disposal of water and shall be in com�liance with the Surface Water Desi�n Manual. 39 ORDINANCE N4. 5828 SECTION XXIX. Subsectian 4-4-080.1, Driveway Design Standards, of Chapter 4, City- Wicle Property Development Standards, of Title IV (Development Regulations} of the Rer�ton Municipal Code, is amended as€ailows: 1. DRIVEWAY DEStGN STANDARDS: 1. Driveway Loca#ion — Hazard Prohibited: No driveway sha11 be constructed in such a manner as ta be a hazard to any existing street lighting standard, utility pole, traffic regulating device, fire hydrant, abutting street traffic, ar similar devices or canditions. The cost of relocating any such street structure when necessary to do so shall be borne by the abutting property awner. Said relocation of any street structure shall be perfarmed only through the department and person holding authority for the particular structure involved. 2. Driveway Spacing Based Upan land Use: a, Industrial,Warehouse and Shapping Center Uses: i. The location of ingress and egress driveways shaU be subject to approval of the #he City under c�arb c�t permit procedures. ii. There shafl be a minimum af forty feet (40') between driveway curb returns where there is more than one driveway on property under unified ownership or controi and used as one premises. 40 C?RDINANCE N0. 5828 iii. Driveways shall nat be closer than five feet (5'� ta any praperty line (except as allowed under ��^^+:,,,, n „�+h:� cr,.+;,.., RMC 4-408Q.1.9, Joint Use C}riveways). b.All Other Uses: i. The locatian of ingress and egress driveways shall be subject to approval of the Department of Community and Economic Deveiapment under curb cut permit procedures. ii. Driveway width (aggregate width if more than one driveway exists) shaN not exceed forty percent (4Q%} of the street frontage. iii. Driveways shal! not be doser than five feet {5') to any property line (except as allowed under h �*•^ " ^�+";` `""+'^^ RMC 4-6-080.1.9, laint Use Driveways}. iv. There shall be a minimum of eighteen feet (18') between driveway curb returns where there is more than one driveway on property under single awnership ar control and used as one premises. 3. �riveway Width Maximums Based Upon Land Use: a. Industrial,Warehotase and Shopping Center Uses. i. Driveway width {aggregate width if more than one driveway exists} shail not exeeed forty percent (40%} of the street frantage. ii. The width af any driveway shail nat exceed fifty feet �50') exclusive of the radii of the returns ar taper sectian, the measurement being made parallel to the centerline of the street roadway. 41 ORDINANCE NO. 5828 iii. The Administrator may grant an exception upon proper application in writing and for good cause shown, which shall include, but not be limited to, the absence of any reasonable alternative. b. Single Family and Duplex Uses: The maximum width of single loaded garage driveways shall not exceed nine feet (9') and double loaded garage driveways shall not exceed sixteen feet (16'). If a garage is not present on the subject property the maximum width of a driveway shall be sixteen feet (16'). c. All Other Uses: The width of any driveway shall not exceed thirty feet (30') exclusive of the radii of the returns or the taper section, the measurement being made parallel to the centerline of the street roadway. 4. Maximum Number of Driveways Based Upon Land Use: a. Industrial, Warehouse and Shopping Center Uses: There shall be no more than two (2) driveways for each three hundred thirty feet (330') of street frontage serving any one property or among abutting properties under unified ownership or control. For each additional one hundred sixty five feet (165') of street frontage an additional driveway may be allowed. b. All Other Uses: There shall be no more than one driveway for each one hundred sixty five feet (165') of street frontage serving any one property or among properties under unified ownership or control; for each one hundred sixty five feet (165') of additional street frontage another driveway may be permitted subject to the other requirements of this Section. 42 QRp1NANCE N0. 582$ 5. Driveway Angle— Minimum: The angle between any driveway and the street roadway or curb line shall not be less than forty five degrees (45°). 6. Driveway Grades—Maximum Based Upon larid Use: a. Single Family and Twa (2) Famify Uses: Maximum driveway slapes shall not exceed fifteen percent (15%); provided, that driveways exceeding eight percent (8%) shall provide slotted drains at the lower end with positive drainage discharge to restrict runoff from entering the garage/residence or crossing any public sidewalk. To exceed fifteen percent {15%Q), a variance from the Administratar is required. b. All Qther Uses: Maximum driveway sfape shall nat exceed eight percent (8°fo}. The Administratar may allow a driveway ta exceed eight percent (8%) slape but not more than fifteen percent (15%) slope, upon proper application in writing and far good cause shown, which shall include, but not be limited to, the absence of any reasanable alternative. To exceed fifteen percent (15%), a variance from the Administrator is required. ?. Surfacin� Reauirements for Driveways and Drivewav Encroachments: Drivewavs and drivewav ap�raaches in the pt�blic ri�ht-of-wav shall be paved with asphaltic cancrete, cement or eauivalent alternative materials af a, permanent nature as approved bv the Public Warks Department. Surfacin� treatments that provide increased infiltration a�portunities, such as permeable pavements. shall be used where feasible and consistent with the Surface Water Desi�n Manual. 43 ORDtNANCE N(J. 5828 8. Two-track DrivewaY Desi�n: For sin�le-famiiv (ots, drivewavs mav pravide a pervious strip in the center in order ta reduce the impervious surface, af drivewavs. A two-track drivewav does nat aliow far an increase in aliowed, drivewav width. �9. loint Use Driveways: a. Benefits-e#: Joint use driveways reduce the number of curb cuts along individual streets and thereby improve safety and reduce congestian while providing for additional on-street parking opportunities. Joint use driveways should be encouraged when feasible and appropriate, particularlv when there is existin�underutilized parkin�proximate ta a subiect site. b. Where Permitted: Adjoining commercial ar industrial uses may utilize a jaint use driveway where such joint use driveway reduces the total number of driveways entering the street network, subject to the approval of the Department of Community and Economic Development. Joint use driveways must be created upon the common praperty line of the proper#ies served or through the granting of a permanent access easement when said driveway does not exist upon a comman praperty iine. loint use access to the driveway shall be assured by easement or other legal form acceptable ta the City. 8T10. Driveways Praviding Access or Connection Ta and Fram the State Highway System: Any driveway providing access or cannection to or from the state highway system shall be designed and installed pursuant ta RMC 4-6- 060F.10. 44 �RDINANCE N0. 5828 SECTION XXX. Subsection 4-4-090.C, General Requirements Applicable ta All Uses (Except Single Family and 7wo {2) Attached Dwelling Units}, of Chapter 4, City-Wide Praperty Develapment Standards, of Title IV (Development Regulations) af#he Renton Municipal Cade, is amended to add a new subsection 4-4-090.C.7, Containment in Zane 1 Wellhead Pratection Areas, to read as shown below. The current subsections 4-4-090.C.7 through 4-4-090.C.11 shall be renumbered as 4-4-090.C.8 through 4-4-090.C.12, respectively. 7. Containment in Zone 1 Welihead Protectian Areas: Containment shal! be provided if the site infiltrates to a Zone 1 Wellhead Protection Area. SECTION XXXI. Subsection 4-4-110.D.3.b, Screening Required for Recognized Higher Risk Storage, of Chapter 4, City-Wide Praperty Development Standards, af Title IV f Developrnent Regulatians) of the Renton Municipal Code, is amended as fallaws: b. Screening Required for Recognized Higher Risk Storage: Thase bulk storage uses which are considered as having a recognized higher risk shall have a barrier as specified in ��"�^^+;,,., n,� „f ��,� c,,,.*;,,� RMC 4-4-110.D.12, Hazardaus Materials, with a screen that is at least eighty percent (8Q%) apaque on tap af the barrier and set back at least twenty feet (20'} from the praperty line. i. Height af Screen: The barrier shall have a maximum height of four feet (4') when measured as in ��^^+�^„ r„',� ^� +";� �^^+;^^ RMC 4-4- 110.D.12.d, The combined height of the four foot (4') (maximum) barrier and screen shall be at least twenty_five percent (25%� of the height of the bulk 45 ORDINANCE N0. 5828 storage provided such combination is at least eight feet (8') high. An optional security fence shall have at least a twenty foot (20') setback. ii. Landscaping Required: All areas between the property lines and the screen shall be landscaped except for ingress and egress areas and except when a second bulk storage facility has a contiguous side or rear property line with an existing bulk storage facility constructed to the standards specified in this Section. The landscape plan shall be prepared by a licensed landscape architect and approved by the Administrator. iii. Tree Retention and Land Clearin�: Bulk Stora�e Facilities and accessory uses shall comply with Tree Retention and Land Clearin� Re�ulations in RMC 4-4-130. SECTION XXXII. Subsection 4-4-110.D.3.c, Screening Required for Other Bulk Storage, of Chapter 4, City-Wide Property Development Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: c. Screening Required for Other Bulk Storage:Those bulk storage uses not included in ��"�^^+;^^ ^�" ^{+�,;� c^�*;� RMC 4-4-110.D.3.b shall have a screen including gates and shall be at least eighty percent (80%) opaque surrounding the property area. Said screen shall be at least twenty_five percent (25%) as high as the bulk storage containers or stock piles provided said screen is at least eight feet (8') high. The screen and optional security fence shall be set back at least twenty feet (20') from all property lines except that for those bulk storage facilities whose total ownership is less than two and one-half (2-1/2) 46 u ORDINANCE N0. 5828 cantiguous acres in area, the Hearing Examiner may reduce this setback up to fifty percent (5Q°l4} for good cause and upon proper written application. AN areas between the property lines ar�d the screen shaii be landscaped except for ingress and egress areas and except when a secand bulk storage facility has a cantiguous side or rear property line that abuts an existing bulk starage facility constructed to the standards specified in this Sectian provided there is at least a twenty_foot (2q') landscaped strip. The landscape plan is to be approved by the Administrator. A landscaped berm may be used by itself or in combination with a screen pravided the required height is met. The slopes of said berm shall be a� � a maximum of two feet (2') horizontal to one faot {1') vertical. There shal! be a flat area an tap of the berm with a minimum width af twa feet (2`}. A retaining wall may be substituted for the internal side of the berm provided the retaining wali is approved by a licensed engineer. SECTION XXXIII. 5ubsection 4-4-110.D.5, Surface Drainage, of Chapter 4, City-Wide Property Development Standards, of Title IV (Develapment Regulations) of the Renton Municipal Code, is amended as follows: 5. Surface Drainage: a. In#ent: The intent of this standard is to pro#ect property from damage and lass due to floading, erosion and depositian caused by the adverse aiteration of natural drainage flow patterns and rates, and to promote development practices which enhance the quality, benefits and enjoyment of the natural watercaurses. 47 ORDINANCE N4. 5828 b. Standards: Surface drainage shaU be approved by the Development Services Division and shall comply with the design specifications set forth in the latest editions or revisions of Standard Specificatians far Municipal P�rbiic Works Canstructian by the WasF�ingtan State Chapter American Public Warks Association, and Highway Hydraulics Manual by the Washingtan State Highway Cammission, Department of Highways. c. Desi�n Criteria: The desi�n of bulk stora�e facilities and accessory uses shall minimize the size of impervious areas: avoid sails with infiltration potential, and reduce soi! comaaction ta the extent feasible, and be consistent with the Surface Water�esi�n Manual. SECTiON XXXIV. Subsection 4-4-110.D.7.d, Traffic F1ow, Setbacks fram Access Routes and Curb Cuts, of Chapter 4, City-Wide Praperty Develapment Standards, of Trtle IV (Develaprnent Regulations) of the Renton Municipai Cade, is amended as follows: d.Traffic Flow, Setbacks from Access Routes and Curb Cuts:A definitive traffic f4ow pattern shall be provided an the property for all traffic, both truck and automobHe, such that all trafFic shall crass lot lines traveling in a forward directian. Necessary transpartation between different parts of the same building ar complex of buildings when lacated on ane continuaus lot shall be by private access rautes, canfined to the property so as to not cause unnecessary congestion or hazards on pubiic streets. Such on-site access routes shafl be located at a distance of at least ten feet (10'), or on the property side of any required planting strip, from a!I pedestrian sidewalks or edge af public right-of- 4$ ORDINANCE N0. 5828 way. Curb cuts for purposes of vehicular access shall be kept to a minimum on both number and width consistent with the property traffic flow pattern. Curb cuts for purposes of stormwater mana�ement shall be desi�ned to allow runoff to enter low impact development best mana�ement practices such as bioretention. SECTION XXXV. Subsections 4-6-030.C, Adoption of Surface Water Design Manual, 4- 6-030.D, When Required, and 4-6-030.E, Drainage Review, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, are amended as follows: C.ADOPTION OF SURFACE WATER DESIGN MANUAL: The �A�3-2016 King County, Washin�ton Surface Water Design Manual {IN3�FB�P�4�, -„ � .. "^.^-,�+^.- ,�„ , "^ as amended by �:^^ �^„�+„ ^r the City of Renton Amendments to the Kin� Countv Surface Water Desi�n Manual, dated December 12, 2016, a�is #e�e�yF-adopted bv reference and referred to hereafter as the Surface Water Design Manual, �� a^^*^� "•� �^�^-^^r^ 8r�e Ee�-e#�The Surface Water Design Manual shall be filed with the City Clerk and available for viewin�on the Citv's website. n ��iurni ncn�uorn. nii N„ .,i.,�.,., f.,. ., „� +ti„ �„ii,...,:..,. � .....:+,. ..�,�i.... �!� �"�!4 �-M i n i n�e�Esa v��i e r� v i�i�a�-rirrspcnrrrc-vrrrccn vc� �. �L,., .,1'., .,,,�+• � 49 ORDINANCE N0. 5828 ��i,,,,,� ,. .,+..,,i ",.,,, .,,,.-.,.,;+. . , 4-�+�r+sfe�-� �.-��.-+ ..i.,�. r....., C c., -,I .. ...i+• v. � '+ 1� ' I I-. rl rl'r+��rl�-.h��• 7. T�m�erar��rrt�;��;��g,��z„��; 4 Q��I.Ji.,., D.,.-.Y.�+• i �.,..., w,�.,+• . � YtY-9Tt: p.��. .M ArcT�T� �� r.,.,��.-�,�+;.,� o,,.-.,.,�+. . ��err��a�e�-�e�+� 1� Q�...J�r,r. Ci+., Dl�,n• • ' �+ ..v. ' +�.,.� � .,.,+ +4.., • ...�.� �r, ...1.. �L......:�: ,,. ..::) wa�^. N ,... .,� .... .. M:..... m»�I�csu_I�t nau o nfl�i_e��h w.��+...J '+L. ��.+h .� � c�c.v�c-qc.cnT��ar�ii��ccca vvit�� iai c e�-r�a�ire��,pplisa�iens The—plan +.,.J '+4. ,J,J'+' 1 ' f +�., + +6, �+ f +l, D 6,►' 5 a e s��I erx►�r,��a Yd i cr� 'a a a i c i v i i'a r i�ircn�rircrvirvac—cm.�cq�a�c�r-vrcrn.�c �n�,,.i,.- n�.,-„-+.,.,,,..+ � ^�^'"'"�� D. REVIEW REQUIRED: �� Wbe��e�i�ed��Drainage review is required when any proposed project is subject to a City of Renton development permit or approval as ,��F, .,,�,,.�„��,,,.+:,,., � „c+�,� �,,,.+;,,., and: 50 ORDINANCE NO. 5828 �1. Would result in two thousand (2,000) square feet or more of new impervious surface, replaced impervious surface or new plus replaced impervious surface; or �2. Would involve seven thousand (7,000) square feet of land disturbing activity; or E3. Would construct or modify a drainage pipe or ditch that is twelve inches (12") or more in size or depth or receives surface or stormwater runoff from a drainage pipe or ditch that is twelve inches (12") or more in size or depth; or �4. Contains or is abutting a critical area designation, defined and regulated in RMC 4-3-050; or �5. Is a single family residential development that would result in new impervious surface, replaced impervious surface or new plus replaced impervious surface. E. DRAINAGE REVIEW: �1. Scope of Review: The drainage review for any proposed project shall be scaled to the scope of the project's size, type of development and potential for stormwater impacts to the �eg+e+�a4-surface water and �roundwater s�st� to facilitate the preparation and review of project applications. If drainage review for a proposed project is required�^��"�^�*;^„ r� „�+�;.- � +• , the R�en�e��evels�mer��e�--�r+ee��e� Ci�t r_shall determine which of the 51 ORDINANCE N0. 5828 following drainage reviews apply as specified in the Surface Water Design Manual: a. �:::�!! ��Simplified drainage review (also known as �^��,�� ""�'�'�^^ ^^�m�+ small proiect drainage review); b. Targeted drainage review; c. Directed draina�e review; Ed. Full drainage review; �e. Large project drainage review. �2. Core Requirements: A proposed project required to have drainage review h., �,."�^�+;^^ �� ^� +h;� c^�+;^^ must meet each of the following core requirements which are described in detail in the Surface Water Design Manual. Projects subject only to small project drainage review (also known as residential building permit drainage review) that meet the small project drainage requirements specified in the Surface Water Design Manual, including flow control best management practices, erosion and sediment control measures, and drainage plan submittal requirements are deemed to comply with the following core requirements: a. Core Requirement #1: �Discharge at the Natural LocationL�A�4 ra�rf'l�ri�nrl �}�..,,,...�+.,. .- ..F�F.,,.,,, -. M.-.,�.,.-+ .-M-.II L, .J' h .J + +M + I 1.,.--,+�.,.�, � „+ +„ h., .J:..,,.+�„d .,Y,+., � ,.I + +' i � . �� :'.'.....,^,�" :., .•M:��'' .'.....�FF' .J' 4. .J F +L.., ' + '+ L, II ..+ .. + ... .. .. ., i� a i�c��a iscm�=vrrrcrr�.-�S�=vJc cnTsc �irm-rr„� ,,.,.,;�L�.'� 52 ORDINANCE NO. 5828 §ISFIi#iEs"H��F2f3� .:�r��e �ewn h i I I��e�^�s��ra+�ge�+�w:� ..., s�eFifie� in �he �isEl�arge-�e��iremen�s e#�he �prfa�� `�«;��s„ "�� b. Core Requirement #2: �Offsite Analysis`• T"^ �^�*"' ,^^��^^+'^^ �+,. �"..0 inEl��e a��r�e .,�.,�,� ,,.+ +�.,+ `.. .�.:+�..�.`�nn +�. +I�i. ..`� TL.� ��.� .. .....�b.. � .. ,.. .., �.�. . 7d'kitFF::�.�.� ,�.�:�� .::�Y'd'�t. :� ..��+��" .. �C�/�� .�I� �+r....��. ....:��..rZr;s � '� ') �esEri�e��t#c.��rf�se �"�,*^. n��^^ ^",^;:w!. :r ;�;M�sTs�-i�e�t+�ied, �k�e �1�,� • ,.+.. .-�.,i� ,�„,,,,+ ., .,i„--,�,i„ ., „�i,,.�... .. ,.:r:,. .,,,,��,-,,,�,,,,,,+.- ., 'F,=o7�..... ..,.... �.,.,��.,.J �., +M., c��rf-.�., �ni�+�. n.,�;.,., nn...,��-,I c. Core Requirement#3:�Flow Control Facilities:T 7 D..,.,.,.-.,.J ., .,,.+.- � �I��,di.,.. r.,.d.,...,1.,...,-,�,.,+ ., .,.,+.- +h-,+ . ��I.,I ��!: ... .:ve--�he�san��,�^�r��#ee� ,.� ��...- .�� r.�� ;m.,.,.-.�i.,�� .-,�.-F-,��, .,.- +6,7r+.. ��..., +h.,��-,.,.J i�c nnn� �..��.,.-., �� + f � ' . . . i � � �h@ c�„-�-,,.., �ni-,+,,. n,,.-�.,., nn.,..��-.i �i. Rre7es�s sa�7eE�� .....,. �Nes+f;�f�e�--ser��a�f�c�ITt-� re��iremerr�s as shewn ir� R�e#e�c�Ee �� 4�# taac.��;rfase `"',* ^ •^ ^" � ��w!! ;;�;aat_cnc cf�h�.fg;;��{;AiN sen�rc�l fasili��erfer�n��se '� �iree�e� in �he c,..�.,,.��,..+„r ��,,.� ��...,,,.,�. 53 C}RQINANCE N0. 5828 ,,. �wvu ...�*�f-i--�-k�e , � ; �. ri,Mt� ,..+,. ' �f�191��. .. ., .,,.,. 0 ; , 0 ##�� ,.w +ti •^a� �F�+'d�� ,........... ....., .,�+M: ' � A� d. Core Requirement#4:—Conveyance SystemL , . 54 ORDINANCE N0. 5828 �xaaaifiad 6a�thc �o;,o�"a��E2-F2 �iremenn--fAf ne�r�,n� @�El��fia + '� r........... � Y' � �anuca�c�ncc irt�M�c�i�Tiii'aTivii reaaireme��s �e��ri�c� �R t�c,��rftiEe' �ir�ef n„ r nn� -,I e. Core Requirement#5:—��^�;^^ ^^^' ��^�^^+ r,,.,+.-„� Construction Stormwater Pollution Prevention; • "" ^ ^� ^ ^�^^:��,",w� �.�� ^,� ,�^ ^ , ��i E�Fwti se '��*'a"��-�*,�-r� �:.� ��t a I�-�re v i�e ere�rlon—m�,--�e�;,;TefilrEe,Tt-f-0;--*� nr.,.,.,�+� +., +L,� �m e�E�en��ras�is��E?, �hA cioi��p^vTi��ca;mcnc-{Tvrrr , .....,.,... i • � r+� .,+.,, < < „i;��l ; ,,.-,�.,„,.,, ,.,;+1, �nnr n n ncn -, .J onnr 4__/l_'12l1 -, .-;�;�,.J h.. +4.., +.,.�,r,.,.--..-.. .-;.,., -.Y,.J .-.,.J�....., + + I an��erfer;,,�TEe��:-:� ..::�' ;w,.,�„w,,,,,,..,+,,, `.,+.. ... +"�� +�,- n„�c.,� nn�.,��-,i f. Core Requirement #6: �Maintenance and Operations�T Main��n�nse ef all �r�i�age f�c�it-+es��:::�!:v::se rviih�ic�--ef�n Main�en�i�se c+.,.,,�-,.,�� ;� ��„ r ,,,.�:�.:�:+., „� +h� -, ��;,....,+ „ ..r„�„r+., „ ,..,.�, �e—deccr_i6od_ 'n +Mr� G�.-f-. �/ +�/� �-, 'I'+' �' Cl� GG'Gl IAGP ffl CIIC �p��QEe—Wa�er Besisn Man�al �]�ri��TTf�T� � � �� . ��•,.� ��,�.,,� M 6 � i s�e s�i en a r�c�t h e v a i i'a cc�-rc'rcc��c�ig-rrrrraytavr. g. Core Requirement #7: �Financial Guarantees and LiabilityL� d_r_�.i_n.�s f_��L�tte� +.-�.�+.,a �' ^� fer�re}e��5, e�Esep� � `p c. c. v c �vc��c�co cvi�ociaccca vi iiiv Iflc vdVt'1� 9E1� ir+F�l+r�+7�n -.nrl rl��r�r�-i.�.n r�i�+�w�.- F.�.. �7nR1� f•.w�il.. .- �irl�n+�•.l I�+� ��+ I � � :;T" �he liabilit�re�airemen#��#'RME�-�b-A�9� 55 ORDINANCE N0. 5828 h. Core Requirement #8: �Water Quality Facilities; and �epesed ��,.+� +t,.,+ .,,.,��I.� .- ��I+ �., f�,,,, *ti.,,��-,.,.J I[ nnnl � ., F.,�+ F .. .. .,.w� . � n.,ll��+�„� �anar�tin2imnar�rin" �r{�� . +1,7r+„ F�. „ +4, �t I2C nnnl r o o r 7 i i fee� er mere ttl �}��iittViTs��•.'••"�•b Ncr�as�c:� ..� ��..� ..�t� r�+�-li+..i+l�.�w�.+..+ .. ' ..+.- +L.-.+ rccrcocrvpi�-r..�.� p..":j 2.,�.. �..... . , �civa_r_c fnot—o r m o r_ f +' �a � «< o, ,,,o,� o� new �n� rC�It7cc�—p6llac1911�e1'�2futiia�p��., ��rfase, sh�ll-�revi�e w�� •*��,+ + �.,,.•�•+• + +r + n + ,� � c i a'a�i[ �ca cm cn c-racnTcrc r�v crccr c�vrrcrccc+�arrtrc2` and��ermwa�er raneff�ene�ra�e� ��new��e�;aEe��at�eT,-ge+��„s ,w,r„r.,;,,,,< < ,r�-,,.,,� ..., ��sifie� in-t��;rfaEe ��-.+„r r,,,�,,,� ��.,.,...,� :A�a�e� c�t+ali��rea�r�en� faEili�fes shall m�e� �he lan��3e �* �'+ + + „ + r�+r.��ir� n+r �+r�r) }4�� i-.+.�.r n��-��i+�i r�l� n+-++i�.� rn .++,- -....-.1:^-�h� �.aqa...�..r" ..�.�., ..r.r.�� he�refec�-siTe as sPesi#ied-i�t-k��ar�ase �"��*^. ^��;�-� ""��..,� T�� speEifie� �hese �e�e��::�� ,.:-1��sig�ed�e re�Ee��!!w�:~� �e�s aEEerd+p��a �k�� w��!:�a�-G::::��! ,.�.•er�ge—;�erfe�r�anEe��w!� !;,�� ",h�u�cic�� 3`:;E�an��ii�-ef �his �e��ien fer ninet-�--f+a�� ^^��^^* �oc°" � +" C.,. h-,��. .�+.,.„ -.I�+..• .- „1,+.. ., �+ lono/1 ,.i+L.,, +,.+-.1 c��.-.......�J...J �..I�.J.-• 0 +1�.,+..+�I � i. Core Requirement#9: Flow Control BMPs. 56 ORDINANCE N0. 5828 4=3. Special Requirements: A proposed project required "., �,�"�^�+�^^ � ^{ +";< <^^*�^^ to have drainage review shall meet any of the following special requirements which apply to the site and which are described in detail in the Surface Water Design Manual. The City^� °^^*^^ ^^„^�^^^^^�+ c,,..,;,.,,.- n;.,;�;,,., shall verify if a proposed project is subject to and must meet any of the following special requirements: a. Special Requirement #1: �Other Area Specific Requirements�l�e �efase .,�., -, ,,.-�.• +• + �„ ,. n• d�-,�n-,.,,, „ ,.��,. �...,-:.. p+e.o. -„�„�+,,.� -, ,-cf�,. .- ,.i.,+�.,�.- .... ., .,,.�.�. ,,..., ...u'�• ��,.i,,,�� . ,,.,,,,,..�,. �w.,+ �...,�-, ,�„-,,,.+ �„-,.-��., ,,.. +ti„ .�..,:....� ,�,,.-;.,� „f .. „�,,,. �..., �....� ��... .. ..�.....,.a r.-.,.,r.�.,.J ., .-+ • L b. Special Requirement #2: �Flood Hazard Delineation: '��a�e�e� ... .., ww;:�en��e�-s�ream, lake, , ,^+'^��, ^ ��^�^a ,� '^ , ;{ „+h„r �;+„ .��la�ien� re�air^ �+,,,�,• ^� F� ,� �,., ,� � �• „ + +� ,� „.-,,;��+ +ti., ., w. �,�r , ,.. ..�..,.. �e�erm�r�e�l a�� elinea�� en�h^ ��*^ � ^� ^ * � � ^{'� � F;^,' w.�� :-�e�,ared fer�he�repe3e��reTes�. T�^�ow "�„_a �+,,,�., .-�.,�� „ �re�a�e� fer as�pesifie� E�r-�he�a�{a�� ^-r�c'rcc^,-��rb-r, ^�.;�L c. Special Requirement #3: --Flood Protection Facilities: '��-^�-^��e�e�e� n,-„c,,,.+ ,. r+.,;.,� „ .,,�1;-, .,+ +„ -, �+,-„�.... +I,-,+ 1,., • +• �i ,J � +• r � faEiJF��-s�+�!': ..., .. �:: � � +r. '+I,.,. +.....+ � ee—r�ve�r�en� er ��rn��T�� RLTTCI �„��rt.C�z, n.nw—or mot�_i�ic-�n—m�irtinn f� � r.+�.r+' + + + o�. �—a„ ��,��,��Roa�,ro«�ien fasili��hen �ht flee�--��e�eEt-+e� 57 ORDINANCE N0. 5828 f""'I'�"'` `�," "e�r�al�t� an� ��igned a� spe�ifie� in �he3a�rc�z ��cr�8,�.,Tb-q Man�al �e senfer� w��he �e��:-w! "�;ergens�EVlanagemen� �ge�c� ��,1-,+0,,.,� -,� �„��.,.J �n nn r � o . . . .� d. Special Requirement #4: �Source Control:-��Ee,;,-�-re�c-+a� ;"'�"`'+�"' ^^' ��'+a-f,m��;pre}ec�s-rirre�pes�iv�-e� si�e}-�n�ersein�,� r-ev+ew a�e-re�Nire� �e im�leme�� a��liEa�le + � ' � •+►, +1,., v;.,.. r„�,.,+., c+.,.,�,,.,�+�.- o„ii� +• � +• n .,.,,.-.i -.�d t� c��.-F��„ �ni.,+,,. .._ ._. .� .._...� .......,...��..., . �,....�,..,, , ,...�..-„�.o„--�.,...,..., ....r.� e n„�•.,,, nn-,.,��-,i . .� e. Special Requirement #5: �Oil Control: '� � ^�^^^�^� �-�;��+ :� , �'11Sf'1-c;� SFt�, +1,.,., .,�I � .,+.-..I �4..,11 b.o ., .,Ii.,.J +., -,II .- .,�F �.-.,.., +1,., L,�„h �e�-�ien ef�he-si�e�s�esifie� in �h^ � {� �^� + n n^ � • and , f. Special Requirement #6: --Aquifer Protection Area. �r",°A-r,T: '�a , � e-�refeF� ma�� Fempl�—dvizh drainag�e-�e��c::�� ,,, �.".� ��-�w�".-P.�«T i -.,,,� onnr n_� n�n SECTION XXXVI. Subsection 4-6-030.F.1.a, Covenants, Conditions and Restrictions, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: a. Covenants, Conditions and Restrictions: Covenants, conditions and restrictions, which are approved by the Administrator, shall be recorded with the King County Recorder's Office prior to or concurrent with recording the plat. The 58 ORDINANCE N0. 5828 applicant shall provide a copy of the recorded document. These covenants shall specify, at a minimum, the following: i. Ownership, maintenance, and repair for the commonly owned tract, landscaping, and facilities (excluding maintenance of the drainage facilities); and ii. No modification of the tract or landscaping within the tract shall be allowed without the City's prior written approval. iii. These covenants shall be irrevocable and binding on all the property owners, including their assigns, heirs, and successors. SECTION XXXVII. Subsection 4-6-030.G.2, Fencing Around New or Expanded Storm Drainage Ponds and Signage Required, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 2. Fencing Around New or Expanded Storm Drainage Ponds and Signage Required: All flow control and water quality treatment ponds and similar facilities, as determined by City ^^.,^�^^m^^+ c^�,,;�^�, shall be fenced with a six- foot (6') tall chain link fence and access gatej�. Fencing is required immediately outside each new stormwater flow control and/or water quality treatment pond and other similar facilities, as determined by City oevelep„Te�r�e�:;Ees. For stormwater ponds, the fence shall be placed at the top of the berm with the maintenance access road on the inside of the fence; or five feet (5') minimum from top of berm if there is no maintenance access road to allow access for proper maintenance of the facility. 59 ORDINANCE N0. 5828 The chain link fence shall be coated with black or green bonded vinyl and installed as determined by the City between the facility and the required landscaping. Unless otherwise determined by the City, the fence gate must be posted with a twelve inch (12") by eighteen inch (18") "No Trespassing" sign. Cedar or other fencing materials may be installed only if the stormwater facility is a privately maintained facility that is owned and operated by the owner(s) of the tract. SECTION XXXVIII. Subsection 4-6-030.1, Adoption of Stormwater Pollution Prevention Manual (SPPM), of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: I. ADOPTION OF STORMWATER POLLUTION PREVENTION MANUAL�`�: The �AA�-April 2016 King County Stormwater Pollution Prevention Manual� /fD� �r nrnnr �r �� hara�ftr.ar m •• �� .� h v• „ r + +l, r•+ f � ', � b r ,��r-Q* , hereby referred to as the Stormwater Pollution Prevention Manual, is hereby adopted by reference. One copy of the manual shall be filed with the CityClerk ,�,.',,,�;.,,, -, w.,,.,,�w,,,.,��+�„r„+„ SECTION XXXIX. Subsection 4-6-030.J.1.a.xvi of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: xvi. Swimming pool a� or spa cleaning wastewater and filter backwash containing water disinfectants (chlorine, bromine, or other chemicals); SECTION XL. Subsection 4-6-030.J.1.b of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 60 ORDINANCE NO. 5828 b. The following are examples of allowed discharges by this Section if the discharges do not contain pollutants and unless the Administrator evaluates and determines that they are causing an adverse impact: i. Diverted stream flows; ii. Spring water; iii. Rising groundwater; iv. Uncontaminated groundwater infiltration; v. Uncontaminated pumped groundwater; vi. Foundation or footing drains; vii. Water from crawl space pumps; viii. Air conditioning condensation; ix. Flows from riparian habitat and wetland; x. Discharges from emergency fire fighting activities; xi. Discharges specified in writing by the authorized enforcement agency as being necessary to protect public health and safety; xii. Irrigation water from an agricultural source that is commingled with stormwater runoff; xiii. Storm system dye testing is allowable by the City, and any dye testing by others requires verbal notification to the Public Works Department at least one day prior to the date of the test. xiv. Nonstormwater dischar�es authorized bv another NPDES or State Waste Dischar�e Permit. 61 ORDINANCE N0. 5828 SECTION XLI. Subsection 4-6-030.J.6, Inspections, Investigation and Sampling, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is repealed in its entirety and the remaining subsection 4-6-030.J.6.7 shall be renumbered as 4-6-030.J.6. SECTION XLII. Subsection 4-6-030.K.1, Process, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 1. Process: All en�ineerin� storm drainage plans and supportive calculations shall be prepared in accordance with the Surface Water Desi�n Manual, and ;� ,. ,,,.+,,,., ..,;+�, ., ,.� +�� .,...�:�� u~��: :F.f.�a�a�s J+s�e� �e��;�� of �#is �eE�i�:: �.".w!l�--�e submitted to the Citv for review and approval +„ +�„ ►,,,,,,,�„�„.,,,�+ c,,.-.,;,.,,� �;,,;..:..., in connection with the associated development permits. SECTION XLIII. Section 4-6-030, Drainage (Surface Water) Standards, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended to add a new subsection 4-6-030.0, entitled "Inspection, Investigation and Sampling," to read as shown below. The remaining subsections 4-6-030.0 through 4-6-030.R, shall be re-lettered as subsections 4-6-030.P through 4-6-030.5, respectively. O. INSPECTION. INVESTIGATION AND SAMPLING: The Administrator mav lawfullv enter propertv to inspect draina�e facilities. 62 ORDINANCE NO. 5828 1. The Administrator shall be permitted to lawfullv enter and inspect sites subiect to re�ulation under this Chapter as often as mav be necessarv to determine compliance with the requirements of the re�ulations, at all reasonable hours for the purpose of inspections, samplin�s or records examination. The Administrator shall have the ri�ht to set ua on the propertv necessarv devices to conduct samplin�, inspection, compliance monitorin�, and/or meterin� actions. 2. If the premises or portion thereof is occupied, the Administrator shall first make a reasonable effort to locate the owner or other person havin� char�e or control of the premises or portion thereof and seek entrv. Proper in�ress and e�ress shall be provided to the Administrator. 3. The Administrator shall notifv the responsible partv in writin� of failure to complv with this access requirement. The responsible partv shall respond and complv with a lawful request for entrv within seven (7) davs from the receipt of notification. The Administrator mav use all le�al means to order the work required completed or otherwise address the cause of improper access includin�, but not limited to, performin� emer�encv work or obtainin� a warrant of abatement. The obli�ation for the pavment of all costs that mav be incurred or expended bv the Citv in causin�the work to be done shall therebv be imposed on the person holdin�title to the subiect propertv. 63 ORDINANCE N0. 5828 4. If the responsible partv fails to maintain the facilities, the Administrator mav take such actions as mav be reauired to enforce the provisions of this section. 5. Within thirtv (301 davs of receivin� an inspection report from the Citv, the propertv owner or operator shall file with the Citv a plan and time schedule to implement anv reauired modifications to the site or to the monitorin� plan needed to achieve compliance with the intent of this section. This plan and time schedule shall also implement all of the recommendations of the Citv. SECTION XLIV. Subsection 4-6-030.Q Variance, currently codified as 4-6-030.P, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: P. VARIANCE: 1. If complying with u��^^+;^^ r� ^� +";� c^�*•^ the Surface Water Desi�n Manual will deny all reasonable use of a property, a variance to the requirements in the Surface Water Design Manual may be requested from the Community and Economic Development Administrator or designee in accordance with the variance process defined in the Surface Water Design Manual and RMC 4-9-250. 2. A request for a variance shall be processed in accordance with RMC 4- 9-250. 64 ORDINANCE NO. 5828 SECTION XLV. Subsection 4-6-060.F.2, Minimum Design Standards for Public Streets and Alleys, of Chapter 6, Street and Utitity Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 2. Minimum Design Standards for Public Streets and Alleys: All such improvements shall be constructed to the City Standards for Municipal Public Works Construction. Standards for construction shall be as specified in the following table, and by the Administrator. MINIMUM DESIGN STANDARDS TABLE FOR PUBLIC STREETS AND ALLEYS: Minimum Design 1 Functional Classifications: Public Streets and Alleys Standards (see notes) Commercial- Mixed Use, Industrial, & Commercial- Neighborhood Mixed Use Limited Principal Minor Collector & Industriat Residential Residential Arterial Arterial Arterial Access Access Access Alleys Structural See Standard Drawing or Pavement section and may be designed using procedures Design described in the WSDOT Design Manual, latest edition. Average 14,000— 3,000— 3,000— 0—3,000 0—3,000 0—250 N/A Daily 40,000 20,000 14,000 Vehicle Trips (ADT) Right-of- 4 lanes— 4 Ianes— 2 lanes—83' 2 lanes— 2 lanes— 1 lane— Res. Way 91' 91' 3 lanes—94' 69' S3' 45' — 16' (R-O-W) S lanes— 5 lanes— 3 lanes— Com. 103' 103' 80' — 16' 6 lanes— 6 lanes— 113' 113' 7 lanes— 7 lanes— 125' 125' Sidewalksz 8' both 8' both 8' both sides3 6' both 5' both 5' both None 65 ORDINANCE NO. 5828 sides3 sides3 sides sides12 sideslz Planting 8' between 8' between 8' between 8' between 8' 8' None Strips4 curb &walk curb & walk curb & walk curb &walk between between both sides both sides both sides both sides curb & curb & walk both walk both sides sides Tree grates and hardscape may be substituted for May be reduced if planting strip area if approved by Administrator. approved by Administrators Street Required, see Street Trees Standards RMC 4-4-070. N/A Trees Curbs Curb both Curb both Curb both Curb both Curb both Curb both None sides sides sides sides sides sides Parking Allowed at Allowed at 8' both sides 8' both 6' one 6' one Lanes 8' 8' sides side6 side6 Bicycle All classifications of Arterials will have None None None N/A Facilities� Class I, or Class II, or Class III bicycle facility. Paved 4 lanes— 4 lanes— 2 lanes—30' 2 lanes— 2 lanes— 1 lane— Res. Roadway 54' S4' 3 lanes—41' 20' 20' 12'$ — 12' Width, not 5 lanes— 5 lanes— 3 lanes— Com. including 66' 66' 31' — 16' parking 6 lanes— 6 lanes— 76' 76' 7 lanes— 7 lanes— 88' 88' Lane 11' travel lanes, 5' bike 10' travel 10' travel lanes 1 travel Res. Widths9 lanes, and 12' center left lanes, 5' bike lane — — 12' turn lanes. lanes, and 11' 12'8 Com. centerleft — 16' turn lanes. Center Center median allowed for boulevard N/A Median treatment and center left turn lane. Width will be width of center left turn lane minus 1'#ee�from through traffic travel lanes on both sides. Pull-outs with a minimum 25'#ee�length required for maintenance and emergency vehicles within the median 66 ORDINANCE NO. 5828 at intervals of 300—350' I I I I Pedestrian Curb bulb-outs required where on-street parking is located. N/A N/A Bulb-outs Intersection 35' turning 35' turning 35' turning 25' turning 25' 25' N/A Radii10 radius radius radius11 radius11 turning turning radius radius At the intersection of two classes of streets,the radius for the higher class street is to be used. Where larger trucks, transit and school buses are anticipated, further design will be required to determine an adequate radius. The minimum curb radius is 15'#ee�. Cul-de-sacs Limited application per RMC 4-6-060.H. Limited application. �N/A See RMC 4-6-060.H for pavement and R- 0-W widths when permitted. Maximum 0.5 —8% 0.5—8% 0.5— 10% 0.5—15%, greater than 15%only 0.5 — Grades13 allowed within approved hillside 15% subdivisions.14 Site Access Determined 125' from 125' from N/A N/A N/A N/A on a case- intersection intersection by-case basis. Street & Street (ighting required per RMC 4-6-060.1, as it exists or may be amended. N/A Pedestrian Lighting NOTES AND CONDITIONS: MINIMUM DESIGN STANDARDS TABLE FOR PUBLIC STREETS AND ALLEYS: � Minimum design standards may be altered to allow alternative stormwater management and low impact development techniques within the R-O-W by the Department. z Sidewalk width will be 12 feet on both sides in the City Center Community Planning Area. This sidewalk width includes street tree grates for locating street trees. To accomplish low impact development best mana�ement practices permeable pavement mav be allowed bv the Administrator. a Sidewalk areas may be required at a wider width to accommodate required multi-use path facilities when a Class I multi-use path is required within a street R-O-W by the Department. The width of a required 5 foot bicycle lane will be transferred to the sidewalk area to create a Class I multi-use path. To accomplish low impact development best mana�ement practices permeable 67 ORDINANCE N0. 5828 pavement mav be ailowed bv the Administrator 4 Maintenance Responsibiiities. Unless otherwise agreed upon by the City of Renton, maintenance af landscaping within the planting strip area, including but nat limited ta elements such as groundcover,turf, softscape, and hardscape, is the responsibility of the adjacent property awner. Maintenance for street trees within the public right-of-way shall be the responsibility of the City of Renton. s Planting strips may be reduced if one of the foiiowing conditions is met: (a) when R-O-W acquisition is problematic; or (b) when critical areas wauid be impacted. IF approved, a permanent alternative landscaped area should be provided equal or greater than the allowed planting strip area reduction that is in additian to any minimum existing cade requirements. 6 A second parking lane may be required by the Administrator. 7 Class II bicycle facilities (bike lanesj included in roadway width for both s+des. Bicycle#acilities that are shared travel lanes, Class I!I bicycle facilities, require less raadway width. Gass III travel lanes are a minimum of 14 feet. s Requirement: Either fire sprinklers shall be pravided as approved by the Fire Department or a clear roadway area shall be provided for emergency vehicles midblack. All of the clear area must be 20 feet in width for vehicular movement with a minimum length of 50 feet and maximum length af 100 feet, so as ta provide emergency access to homes within 150 feet. Along the clear area only,the planting strip would nat be required and the clear area will be in place of the landscaping area. 9 The City may require different lane width dimensions to address safety concerns or to meet state and federal requirements for state routes or grant funding. lo Turning radius dimensions represent the vehicle turning path.The smallest curb radius shauld be used while maintaining the specified tuming radius. Lane width and the presence af a bike lane and parking lane affect a vehicle's turning path. On streets with mare than one lane in that direction of travel, large vehicles may encroach into no more than one-half af the adjacent travel lane to complete the turn. On Arterials and Callectar Arterials, encroachment inta oncoming travel ianes is unacceptable. The minimum curb radius is 15 feet. ll Turning radius for streets which include industrial access may increase to 54 feet. 1z Sidewalks shall be pravided on both sides of the street, however the Administrator mav approve sidewalks on one side of the street pursuant to RMC 4-6-060.G. 68 ORDfNANCE NtJ. 5828 '-3 Sidewaiks mau be desi�ned to be reverse sioped awav fram the street,, provided that the sidewaiks have a maximum lon�,slope of 2°la and are designed ta drain tawards a publicla-owned law impact deve(oament facilitv alan�the raadwav instead of directiv inta the street. SfCTiON XLVI. Subsection 4-6-060.F.5, Pavement Thickness, of Chapter 6, Street and Utility Standards, of Title IV iDevelopment Regulationsj of the Renton Municipa) Code, is amended as foilows: 5. Pavement Thickness: New impermeable pavement shail be a minimum of four inches (4"j of asphalt over six inches (6"� of crushed rock. Permeable pavement desi�n is �overned bv the Surface Water Desi�n Manual. Pavement thickness for new or repaired arterial or caHector streets or widening of arteriafs or caElectar streets must be approved by the Department of Cammunity and Economic Development. Pavement thickness design shall be based an standard engineering pracedures and wei�ht/loadin� reauirements for emer�ency response vehicles. For the purposes of asphalt pavement design, the pracedures described by the "Asphalt Institute's Thickness Design Manual" (latest edition) will be accepted by the Department, Alternate design procedures or materials may be used if approved by the Department through the pracess listed in RMC 4- 9-250.E. SECTION XLVII. Subsection 4-6-060.G.2, Exemptions, of Chapter 6, Street and Utiiity Standards, of Title IV (Development Regulations} of the Renton Municipal Code, is amended as follows: 69 �RDINANCE NQ. 5828 2. Exemptions: Pedestrian and bicycle facilities are not required to be established when it is cor�cluded by the Administrator that application of complete streets principies is unnecessary ar inapprapriate: a. Where their establishment wauld be contrary#o public safety; ar b. When the cost wouid be excessively disproportionate to the need or probable use; or c. Where there is no identified long-term need; or d. Where the establishment would violate Camprehensive Plan policies, or e. Where trails in common areas are pravided in lieu of sidewalks, ar when ve�etated best mana�ement practices such as biaretention is pra�osed, or soil conservation or critical area protection is necessary; or �f. Where the Administrator grants a documented exemption which may only be authorized in specific situations where conditions warrant. Such site-speci#ic exemptians shall not constitute genera4 changes to the minimum street star�dards established in this Section. SECTION XlVlll. Subsection 4-6-Q60.H.4, Cul-de-Sac Design, af Chapter 6, Street and Utility Standards, of Ti#le IV {Devebpment Regulatians) of the Renton Municipal Code, is amended as foilows: 4. Cul•de-Sac Design: Cul-de-sacs shall have a minimum paved and landscaped radius af forty-_five feet (45') with a right-of-way radius of fifty_five feet (55') for the turnaround. A landscaped center island with a radius of twenty 70 ORDINANCE NO. 5828 feet (20') delineated by curbing shall be provided in the cul-de-sac. Low impact development best mana�ement practices shall be arovided in the center island where feasible and consistent with Citv standard details and the Surface Water Desi�n Manual. The landscaping shall be maintained by the homeowners' association or adjacent property owners. The cul-de-sac turnaround shall have a design approved by the Administrator and the Fire Department. SECTION XLIX. Subsection 4-6-060.Q Variations from Standards, of Chapter 6, Street and Utility Standards, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: Q. VARIATIONS FROM STANDARDS: 1. Alternates, Modifications, Waivers, Variances: See RMC 4-9-250. 2. Half Street Improvements: a. When Permitted: Half street improvements may be allowed for a residential access street by the Administrator when it is determined that the adjacent parcel of property has the potential for future development and dedication of the right-of-way necessary for the completion of the street right- of-way. b. Minimum Design Standards: The right-of-way for the half street improvement must be a minimum of thirty_five feet (35') with twenty feet (20') paved. A curb, planting strip area, and sidewalk shall be installed on the development side of the street according to the minimum design standards for public streets. If the street is permitted a cul-de-sac, then the right-of-way for 71 �RDINANCE NO. 5828 the half of the cui-de-sac shali be dedicated, with installation of a temporary hamrnerhead turnaround. The property shail aiso dedicate easements ta the City for street lighting and fire hydrants. Additional easements shall be provided for the franchise utilities outside af the dedicated right-af-way. c. Standards for Completian af the Half Street: When the adjacent parcel is platted ar develaped, the additional right-of-way width needed to complete the type of street classification shall be dedicated from the developing property. The pavement shall then be widened to the width needed to complete the type of street classificatian, curb, planting strip, and sidewalk shall be installed on the develaping side af the street. If the street is a dead end street requiring a cui-de-sac, then the develaping parcel shali dedicate the remainder of the right-af-way for the cul-de-sac and construct the finai compiete cul-de- sac, including curb, sidewalk, and other required improvements. 3, Cul-de-Sac Modifications for Low lm�act Develapment: The Admir�istrator mav madifv cul-de-sac desi�n standards to allow low im�act develapment facilitv installatian and reduce imperviaus sur#aces: provided that turnaraund desi�n modificatians shall allow safe access and emer�encv respanse. SECTION L. Subsection 4-7-150.E.7, Cul-de-Sac Streets, of Chapter 7, Subdivisian Regulations, of Title IV (Development Regulations) of the Renton Municipai Code, is amended as follaws: 72 C}R�INANCE NO. 5828 7. Cul•de-Sac Streets: Cul-de-sac streets may only be permitted where no future connection to a larger street pattern is physicaily possible due ta demonstrable physical canstraints. See RMC 4-6-060.H. Dead End Streets, for standards and aptions. SECTION LI. Subsection 4-7-200,B, Storm Drainage, af Chapter 7, Subdivisian Regulations, of Title IV (Development Regulationsj of the Renton Municipal Code, is amended as follaws: B,STORM DRAlNAGE: An adequate drainage system shall be pravided far the praper drainage of afl surface water. Low impact devefopment best mana�ement prac#ices are required cansistent with the Surface Water Desi,�n Manual. Cross drains shall be provided to accommodate all natural water flow and shali be of sufficient length to permit full-width roadway and required slopes. The drainage system shall be designed per the requirements of RMC 4-6-030, Drainage (Surface Water) Standards. The drainage system shall include detention capacity for the new street areas. Residential plats sha11 also include detention capacity far future development of the lats. Water quaHty features shall afso be designed to pravide capacity for the new street paving for the plat. Drainage vaults, pands, etc., shall be located within dedicated tracts. SECTION LII. Subsection 47-220.C.2, Grading, of Chapter 7, Subdivision Regulations, of Title IV (Development Regulations� of the Renton Municipal Code, is amended as follows: 73 ORDINANCE N0. 5828 2. Grading: Detailed plans for any proposed cut and fill operations shall be submitted. These plans shall include the angle of slope, contours, compaction, and retaining walls. Plans shall be consistent with requirements in RMC 4-4-060„ Gradin�, Excavation and Minin� Re�ulations. SECTION LIII. Subsection 4-9-150.D.2.d, Use of Sustainable Development Techniques, of Chapter 9, Permits — Specific, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: d. Use of Sustainable Development Techniques: Design which results in a sustainable development; such as LEED certification, energy efficiency, use of alternative energy resources, low impact development techniques bevond that reauired bv the Surface Water Desi�n Maniaal, etc.; or SECTION LIV. Subsection 4-9-200.E.3.b.iii, Loading and Storage Areas, of Chapter 9, Permits — Specific, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: iii. Utilities, Loading and Storage Areas: Locating, designing and screening storage areas, utilities, rooftop equipment, loading areas, and refuse and recyclables to minimize views from surrounding properties. Locate utilities under�round consistent with RMC 4-6-090; SECTION LV. Subsection 4-9-200.E.3.c, On-Site Impacts, of Chapter 9, Permits — Specific, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: c. On-Site Impacts: Mitigation of impacts to the site, including: 74 ORDINANCE N0. 5828 i. Structure Placement: Provisions for privacy and noise reduction by building placement, spacing and orientation; ii. Structure Scale: Consideration of the scale of proposed structures in relation to natural characteristics, views and vistas, site amenities, sunlight, prevailing winds, and pedestrian and vehicle needs; iii. Natural Features: Protection of the natural landscape by retaining existing vegetation and soils, using topography to reduce undue cutting and filling, and limiting and disconnectin� impervious surfaces;a� iv. Reducin� Parkin� Impervious Areas: Desi�n parkin� areas to minimize impervious surfaces, includin� but not limited to: 11 breakin� up parkin� areas and directin� stormwater flows to multiple low impact development features such as bioretention areas; 21 locatin� parkin� near trees to provide storm water uptake: 31 retainin� or addin� ve�etation to parkin� areas: 4) ptacin� existin� parkin� that exceeds maximum parkin� ratios in permeable pavement desi�ned consistent with the Surface Water Desi�n Manual in RMC 4-6-030: and 5) usin� other low impact development techniques consistent with RMC 4-6-030; and +v. Landscaping: Use of landscaping to soften the appearance of parking areas, to provide shade and privacy where needed, to define and enhance open spaces, and generally to enhance the appearance of the project. Landscaping also includes the design and protection of planting areas so that 75 ORDINANCE N0. 5828 they are less susceptible to damage from vehicles or pedestrian movements. Landscapin�shall be consistent with RMC 4-4-070. SECTION LVI. Subsection 4-9-200.E.3, Criteria, of Chapter 9, Permits — Specific, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: 3. Criteria: The Administrator must find a proposed project to be in compliance with the following: a. Compliance and Consistency: Conformance with plans, policies, regulations and approvals, including: i. Comprehensive Plan: The Comprehensive Plan, its elements, goals, objectives, and policies, especially those of the applicable land use designation, and any applicable adopted Community Plan; ii. Applicable land use regulations; iii. Relevant Planned Action Ordinance and Development Agreements; and iv. Design Regulations: Intent and guidelines of the design regulations located in RMC 4-3-100. b. Off-Site Impacts: Mitigation of impacts to surrounding properties and uses, including: i. Structures: Restricting overscale structures and overconcentration of development on a particular portion of the site; ii. Circulation: Providing desirable transitions and linkages between uses, streets, walkways and adjacent properties; 76 ORDINANCE N�. 5828 iii. Laading and Storage Areas: Locating, designing and screening storage areas, utilities, rooftap equipment, loading areas, and refuse and recyciables to minimize views from surrounding properties; iv. Views: Recognizing the public benefit and desirability of maintaining visua( accessibility to attractive natural features; v. Landscaping: Using landscaping to provide transitions between development and surrounding properties to reduce noise and glare, maintain privacy, and generally enhance the appearance of the project; and vi. Lighting: Designing and(or pfacing exterior lighting and glazing in arder ta avaid excessive brightness ar gfare ta adjacent properties and streets. c. t3n-Site Impacts: Mitigation of impacts to the site, including: i. Structure Placernent: Provisions for privacy and noise reduction by building placement, spacing and orientation; ii. Structure Scale: Consideration of the scale of proposed structures in relation to natural characteristics, views and vistas, site arnenities, suniight, prevailing winds, and pedestrian and vehide needs, iii. Natural Features: Protection of the natural landscape by retaining existing vegetatian and soils, using topagraphy to reduce undue cutting and filling, and limiting impervious surfaces; and iv. Landscaping: Use of landscaping to soften the appearance of parking areas, to provide shade and privacy where needed, to define and enhance open spaces, and generally to enhance the appearance of the project. 77 ORDINANCE N0. 5828 Landscaping also includes the design and protection of planting areas so that they are less susceptible to damage from vehicles or pedestrian movements. d. Access and Circulation: Safe and efficient access and circulation for all users, including: i. Location and Consolidation: Providing access points on side streets or frontage streets rather than directly onto arterial streets and consolidation of ingress and egress points on the site and, when feasible, with adjacent properties; ii. Internal Circulation: Promoting safety and efficiency of the internal circulation system, including the location, design and dimensions of vehicular and pedestrian access points, drives, parking, turnarounds, walkways, bikeways, and emergency access ways; iii. Loading and Delivery: Separating loading and delivery areas from parking and pedestrian areas; iv. Transit and Bicycles: Providing transit, carpools and bicycle facilities and access; and v. Pedestrians: Providing safe and attractive pedestrian connections between parking areas, buildings, public sidewalks and adjacent properties. e. Open Space: Incorporating open spaces to serve as distinctive project focal points and to provide adequate areas for passive and active recreation by the occupants/users of the site. 78 C}RDlNANCE N0. 5828 #, Views and Public Access: When possible, providing view corridors to shorelines and Mt. Rainier, and incorporating pubfic access to shorelines. g. Natura) Systems: Arranging project eEements to pratect existing natural systems where applicable. h. Services and infrastructure: Making available public services and facilities ta accommodate the proposed use. i. Phasing: Including a detailed sequencing plan with development phases and estimated time frames, for phased projects. a. Starmwater: Pravidin� optimal lacatians of stormwater infiltratin� !ow impact develo�ment facilities. Avoidin� placement of buildin�s or imperviaus areas on soils with infiltratian ca�abilitv to the maximum extent practicable. SECTION LVII. 5ection 4-11-060, Definitions F, of Chapter 11, Definitions, of Title IV (Development Regulations) af the Renton Municipal Code, is amended to add a definition of "Feasible," in alphabetical order,ta read as follows: FEASIB�E (with re�ard to application af the Surface Water Desi�n Manuat in RMC 4-6-0301: An LID best mana�emerrt practice that is cansidered capable af implementatian €ollowing consideratian of the Surface Water Desi�n Manua! infeasibilitv criteria. Provided. an LID best mana�ement practice is not feasible if it would canflict with requirements of federal or state iaw, zanin� district desi�n criteria, public health and safetv, transportation re�ulatians, re�ulatians protectin� tree species. a local code or rule adopted as �art of a Wellhead 79 ORDINANCE N0. 5828 Protection Pro�ram established under the Federal Safe Drinkin� Water Act, or a local code or rule adopted to protect a Critical Aauifer Rechar�e Area established under the State Growth Mana�ement Act. SECTION LVIII.The definition of "Impervious Surface" in section 4-11-090, Definitions I, of Chapter 11, Definitions, of Title IV (Development Regulations) of the Renton Municipal Code, is amended as follows: IMPERVIOUS SURFACE: A non-ve�etated surface area that either prevents or retards the entry of water into the soil mantle as under natural conditions before development; or that causes water to run off the surface in �reater quantities or at an increased rate of flow compared to the flow present under natural conditions prior to development. Common impervious surfaces include, but are not limited to, roofs, walkwavs, patios, drivewavs, parkin� lots, stora�e areas, areas that are paved, �raveled or made of packed or oiled earthen materia�s, or other surfaces that similarlv impede the natural infiltration of surface water or stormwater. �e�+��.. ..M:�ase�:,;tu ..., .. "u .. ,. o;+�„ .�.,��+� ;a��+e�eez.:-� :"�� ,-.,+-„-,a�+ti„ ,,..+.,,„�..,.,+,,,- ;.,+„+�„ �,.,i ..... �+ie ., r,�,,.- �-,+�„--,t � ..,a;+�.,.,.- ti„�,,,-„ Elavalraramant• �r th�t �-►�� +�,-+„ .- ��+ti � + +•+• r � 4+ -. .,.J .--.+., .,��I.,...f..,.�. +4.., FI + .a... .,M+..... �6 �eVe�Ap111�F1�. E6CT}171vii i�i�pcivni'c►6�ia"ttc'ree�-i*t"t*'titr:� �;�� ..,"� •�•+ I�.v.�+.,.�1 +.. i Eensre�e;as�hal�ree#s; L�•,-•••`� EFl:S�-lE��$C�E�v�FAS��rrc�:� :'.w+::."�� 9i�Af .,+4..,. ., ..i.,�� ., � /.,...,., �.�L..,.. �.,+ '+1, I �I1. .J +6, + ' + � $� QRDINANCE N�. 5828 ; � M� Mu.L:�� $'.:�'�� ,.. c��}C�1"�rc'a'r'�ccc� ±�::+ .. �� . � .."�"�is Eer�sr�e, .,� ..�,�„ .,-,.,�.� �n� , , ,..�.. ..wr.. , 0 , , � 1 �.�� .. .. � � . � I �-. � � _,,,,._._,._ _ r � � f �lmgerw�ou5 5urfac�ng�Lot Coverage SECTION LIX. Section 4-11-090, Definitions I, of Chapter 11, Definitions, of Title IV (Development Regulations) of the Renton Municipal Code, is amended to add a new definition of"lnfiltratian Facility," in alphabetical order,to read as fallows: 81 4R�INANCE NO. 5828 INFILTRATION FACIlITY: Infiltration facilitv means a draina�e facilitv desi�ned to use the hvdrolo�ic process of water soakin� into the �round (commonlv referred to as percolatian)ta dispose of surface and storm water runoff. SECTION LX. Sectian 4-11-220, Definitions L, of Chapter 11, Definitions, af Title iV (Development Regufations) of the Renton Municipa) Code, is amended to add new definitions af "LID Best Management Practices" and "Low Impact Development (LID)," in alphabetical order,to read as follows: L!D BEST MANAGEMENT PRACTICES: LID Best Manaaement Practices are referred to as BMPs. LID BMPs are distrib�ated starmwater mana!�ement practices, inte�rated into a proiect desi�n. that emphasize pre-disturbance hvdrolo�ic processes of infiltratian, filtration, stora�e, evaparatian and transpiration, LID structural BMPs are referred to as flaw cantrol BMPs and include, but are not limited to, bioretention, permeable qavements. roof dawnspout controls. dispersion. soil qualitv and depth. and minimal excavation faundations (pursuant to the Surface Water Desian Manuall. LOW IIVIPACT DEVElt?PMENT (I.ID): A stormwater and (and use mana�ement strate�v that strives to mimic are-disturbance hvdrolo�ic pracesses af infiitratian, filtration, stora�e, evaporatian and transpiration ba emphasizing conservation, use of on-site natural features, site plannin�, and distributed stormwater mana�ement aractices that are inte�rated into a qroiect desi�n faursuant to the Surface Water Desi�n Manual). 82 C}RDiNANCE N0. 5828 SECTION LXI, Section 4-11-160, Definitions P, of Chapter 11, Definitions, of Title !V (Development Regulations} of the Renton Munieipal Code, is amended to add a new definition of"Pervious Surface," in alphabeticai order,to read as faifaws: PERVIUUS SURFACE: Anv surface materia) that allaws stormwater to infiltrate into the �raund. Examp{es include lawn, landscape, pasture, and native vegetation areas. Note for purpases of Surface Water Desi,�n Manual (RMC 4-6- 030) threshold determination and runoff volume modelin� far detention and treatment, ve�etated roofs and permeable pavements are to be considered impervious surfaces alon� with lawns, landscapin�, sports fields, �alf courses, ar�d ather areas that have modified runaff characteristics resultir�� fram the addition of underdrains. SECTION LXII. Section 411-190, Definitions S, of Chapter 11, Definitions, af Title 1V (Develapment Regulatians} of the Renton Municipaf Code, is amended to add a new definitian af "Stormwater Facility," in alphabetical order, to read as shown below, and amend the definition of"Surface Water Design Manual, as follaws; STQRMWATER FACI�.ITY: A constructed component of a stormwater draina�e svstem. desi�ned or constructed to perfarm a particular function, or muftipfe functions. Stormwater facilities include, but are not limited #o, pipes. swales, ditches, culverts, street �utters, detention qonds. retention ponds, constructed wetlands, infiltration devices, catch basins, ail/water separators, and biofiitration swales. Thev mav also include low impact develo�ment BMPs/facilities. Also referred to as Draina�e Facilitv. 83 ORDINANCE N0. 5828 SURFACE WATER DESIGN MANUAL: Shall be the 2016 King County, Washin�ton Surface Water Design Manual, _�� ,... w��s�ria�e}�re�are� ��—m;��o�� .,.J D�.L� ��,- - -- -- - � �, as ��e amended, ��MY!�menTe� an� rx�o�i#+ed by the City of Renton Amendments to the King County Surface Water Design Manual, dated December 12, 2016+�,.,+ �Mnr�f[�� i,,,..,i ,-„�,,,,.,,.,.,,,.,+� -,�,� N ,� ,� ,. •h• � � ,� rt�.Km.uuztar rlrssL•,•, �•,� � -,1.,.-c� r .,+� a ,.1 •.� a � �M�� • SECTION LXIII.Subsection 9-10-2.F of Chapter 10, Street Excavations, of Title IX (Public Ways and Property) of the Renton Municipal Code, is amended as follows: F. All existing r+^��^ �^.•,^� draina�e facilities and outer utilities that are moved� e� disconnected, or disturbed during such work shall be replaced or repaired immediately as directed. A concrete saw shall be used to cut all pavement so as to produce a reasonable square and true edge without spalling or cracking into adjacent pavement. SECTION LXIV. Subsection 9-10-11.F.4, Pavement Removal in Lieu of Grinding, of Chapter 10, Street Excavations, of Title tX (Public Ways and Property) of the Renton Municipal Code, is amended as follows: 4. Pavement Removal in Lieu of Grinding: The contractor in all cases can remove the pavement in the replacement area instead of grinding out the specified two inches (2") of asphalt. Full pavement replacement to meet or exceed the existing pavement depth will be required for the area of pavement 84 4RDINANCE NO. 5$28 removal. Patchin� of pervious concrete shall foliow American Concrete Institute, (ACI} 522.1-13. Porous asphalt shail be specified on a pro�ect-bv-proiect basis by the engineer. Permeabfe materials should be replaced in-kind where feasible. Patchin� uaraus asphalt with conventianal asphalt is acce�tabie if it is no mare than ten percent (10%1 of the tatal faciiitv area ar does not impact the overali facilitv functian. Appropriate precautions shall be taken durin� pavement repair and replacement efforts to prevent clo��in�of adiacent surfaces. SECTIQN �XV. Subsectians 9-10-11.F.5.c and 9-1Q-11.F.5.h, of Chapter 10, Street Excavations, of Title IX (Public Ways and Praperiy} of the Renton Municipal Cade, are amended as follows: c. All trenching within the top four feet (4'} shall be backfilled with crushed surfacing materials conforming to Section 4-04 of the Standard Specifications. Any trenching over four feet (4') in depth may use materials approved by the Engineer ar Materials Lab for backfilling below the faur-foot (4'� depth. If the existing material {ar other material) is determined by the Engineer to be suitable far backfill, the contractor may use the native materiaf except that the top six inches (6") shall be crushed surfacing tap course material. The trench shall be compacted to a minimum ninety-five percent (95%) density, as described in Section 2-03 af the Standard Specifications. In the top six feet (6'� af any trench, backfill compaction shall be performed in ei ht 8� to twelve-inch {12"�-i� lifts. Any trench deeper than six feet (6'� may be 85 ORDINANCE N0. 5828 compacted in twentv-four inch {24",�+�e#lifts, up to the tap six-foat (6') zone. All trench backfill shall be firm and unvieldin� but in no case shali be compacted ta more than ninetv-twa percent (92°/al of maximum densitv in permeable pavement areas. All compactian shali be performed by mechanical methods. The compaction tests may be performed in faur-foot (4') vertical increments maximum. The test results shall be given to the Engineer for review and approval prior ta paving. The number and lacation of tests required shall be determined by the Engineer. n. AI! joints, except those associated with aermeable t�avement, shall be sealed using paving asphalt AR-4000tN. SECTiON LXVI. Section 9-15-1, Weeds, of Chapter 15, Weeds and Noxious Matter, of Title IX (Public Ways and Property) of the Renton Municipal Code, is amended as follows: 9-15-1 WEEDS OR VEGETATfVE ENCROACHMENTS: No awner or occupant of any lot, place or area within the City, ar the agent of any such owner or accupant, shall permit an such lot, place or area: A. Any naxious weeds ar deleteriaus, unheafthful growth or other noxious matter that may be growing, lying ar located therean. B, Any encroachment of any hedge, fence, vegetation, trees, bushes or other obstructians an any public alley, street or other public right-of-way, inciuding sidewalks or walkways. 86 �R�iNANCE NO. 5828 1. Planter strips and ve�etated planter islands in ri�hts-of-way are permitted subiect ta Citv landscapin�, maintenance, and street standards in RMC 4-4-070 and 4-6-060. and shaii be mana�ed to maintain pedestrian and vehicular passa�e and si�ht iines. 2. Ve�etated faw impact develapment facilities shali further be desianed consistent with RMC 4-6-030 and the adopted Surface Water Desi�n Manual. SECTION LXVII. The City Clerk andfar other entity responsible for codification of this ordinance are authorized to make necessary corrections to this ardinance including correctian of scriuener'sJclerical errors, references, ordinance numbering, sectionJsubsection numbering and any references thereto. SECTION LXVIII. This ordinance shall be in full force and effect on December 31, 2016. PASSED BY THE CiTY COUNCIL this 12th day of December , 2016. > � � f Jason A Seth, Cit lerk APPROVED BY TNE MAYOR this 22th day af December , 2Q16. Denis Law, Mayor 87 C}RDINANCE Nt3. 5828 Approved as to form: �t��r i��tr ��•``G�'(Y OF `'��y �r�.� �.�.. `��,..� t``�is�+��rt��t�i��fi�� awrence J. Warren, City Attorney i '� �''`� �'`,�'.��% = � * �� " , = z' � ''A = Z� Date af Publication: December 26, 2026 fsummarv) = o ; �;�� ± = "A '"r Y i �` T♦ ORD:1937:11j30J16:scr '�r°��9p�,��Srir++ytt�ti`�;�0�`��+.�`~ f�jt�N�i�P��6��'`� 88 CITY OF RENTON SURFACE WATER DESIGN MANUAL CITY OF RENTON PUBLIC WORKS DEPARTMENT SURFACE WATER UTILITY December 12, 2016 Note: Some pages in this document have been purposely skipped or blank pages inserted so that this document will copy correctly when duplexed. INTRODUCTION TABLE OF CONTENTS AND OVERVIEW Item° {f' /i CITY OF RENTON SURFACE WATER DESIGN MANUAL 2017 City of Renton Surface Water Design Manual 12/12/2016 TABLE OF CONTENTS AND OVERVIEW (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL TABLE OF CONTENTS CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS Section Page 1.4 Adjustment Process 1-99 1.1 Drainage Review 1-11 1.4.1 Adjustment Authority 1-99 1.1.1 Projects Requiring Drainage Review 1-12 1.4.2 Criteria for Granting Adjustments 1-99 1.1.2 Drainage Review Types and 1.4.3 Adjustment Application Process 1-100 Requirements 1-12 1.4.4 Adjustment Review Process 1-101 1.1.3 Drainage Review Required By Other 1.4.5 Appeals 1-101 Agencies 1-22 1.1.4 Drainage Design Beyond Minimum Compliance 1-22 1.2 Core Requirements 1-23 1.2.1 Core Requirement#1: Discharge at the Natural Location 1-23 1.2.2 Core Requirement#2: Offsite Analysis 1-24 1.2.3 Core Requirement#3: Flow Control Facilities 1-35 1.2.4 Core Requirement#4: Conveyance System 1-50 1.2.5 Core Requirement#5: Construction Stormwater Pollution Prevention 1-54 1.2.6 Core Requirement#6: Maintenance and Operations 1-59 1.2.7 Core Requirement#7: Financial Guarantees and Liability 1-61 1.2.8 Core Requirement#8: Water Quality Facilities 1-63 1.2.9 Core Requirement#9: On-Site BMPs 1-73 1.3 Special Requirements 1-89 1.3.1 Special Requirement#1: Other Adopted Area-Specific Requirements 1-89 1.3.2 Special Requirement#2: Flood Hazard Area Delineation 1-90 1.3.3 Special Requirement#3: Flood Protection Facilities 1-91 1.3.4 Special Requirement#4: Source Controls 1-92 1.3.5 Special Requirement#5: Oil Control 1-94 1.3.6 Special Requirement#6: Aquifer Protection Area 1-97 2017 City of Renton Surface Water Design Manual 12/12/2016 TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS CHAPTER 2 CHAPTER 3 DRAINAGE PLAN SUBMITTAL HYDROLOGIC ANALYSIS & DESIGN Section Page Section Page 2.1 Plans for Permits and Drainage 3.1 Hydrologic Design Standards and Review 2-3 Principles 3-3 2.1.1 Plans Required for Pre-Application 3.1.1 Hydrologic Impacts and Mitigation 3-3 Submittal 2-3 3.1.2 Flow Control Standards 3-5 2.1.2 Site Plans Required for Drainage 3.1.3 Hydrologic Analysis Using Continuous Review 2-3 Models 3-5 2.2 Plans Required with Construction 3.2 Runoff Computation and Analysis Permit Application 2-5 Methods 3-9 2.2.1 Subdivision, PUD, and Binding Site 3.2.1 Rational Method 3-11 Plans 2-6 2.2.2 Short Subdivisions 2-7 3.2.2 Continuous Models and the Runoff Files Method 3-19 2.2.3 Commercial Site Development 2-7 3.2.3 The Approved Model 3-30 2.2.4 Single-Family Residential 2-7 3.2.4 The HSPF Model 3-30 2.2.5 Other Permits 2-7 3.3 Hydrologic Design Procedures and 2.3 Drainage Review Plan Specifications 2-9 Considerations 3-33 2.3.1 Engineering Plan Specifications 2-10 3.3.1 General Hydrologic Design Process 3-33 2.3.2 Projects in Targeted Drainage Review 3.3.2 Flow Control Design Using the Runoff (TDR) 2-34 Files Method 3-34 2.4 Plans Required After Drainage 3.3.3 Conveyance System Design with the Review 2-35 Runoff Files Method 3-37 2.4.1 Plan Changes After Permit Issuance 2-35 3.3.4 Safety Factors in Hydrologic Design 3-38 2.4.2 Final Corrected Plan Submittal 2-35 3.3.5 Design Options for Addressing 2.4.3 Final Plat, Short Plat, and Binding Site Downstream Drainage Problems 3-38 Plan Submittals 2-36 3.3.6 Point of Compliance Analysis 3-38 3.3.7 Onsite Closed Depressions and Ponding Areas 3-41 12/12/2016 2017 City of Renton Surface Water Design Manual TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & CHAPTER 5 DESIGN FLOW CONTROL DESIGN Section Page Section Page 4.1 Route Design and Easement 5.1 Detention Facilities 5-3 Requirements 4-3 5.1.1 Detention Ponds 5-3 4.1.1 Route Design 4-3 5.1.2 Detention Tanks 5-17 4.1.2 Easement and Setback Requirements 4-3 5.1.3 Detention Vaults 5-21 4.2 Pipes, Outfalls,and Pumps 4-7 5.1.4 Control Structures 5-25 4.2.1 Pipe Systems 4-7 5.1.5 Parking Lot Detention 5-35 4.2.2 Outfall Systems 4-30 5.1.6 Roof Detention 5-35 4.2.3 Pump Systems 4-36 5.1.7 Simple Detention Pond for Cleared 4.3 Culverts and Bridges 4-37 Areas 5-35 4.3.1 Culverts 4-37 5.2 Infiltration Facilities 5-43 4.3.2 Culverts Providing for Fish 5.2.1 General Requirements for Infiltration Passage/Migration 4-50 Facilities 5-43 4.3.3 Bridges 4-52 5.2.2 Infiltration Ponds 5-54 4.4 Open Channels, Floodplains, and 5.2.3 Infiltration Tanks 5-57 Floodways 4-55 5.2.4 Infiltration Vaults 5-60 4.4.1 Open Channels 4-55 5.2.5 Infiltration Trenches 5-62 4.4.2 Floodplain/Floodway Analysis 4-71 5.2.6 Alternative Infiltration Systems 5-63 5.2.7 Small Infiltration Basins 5-64 2017 City of Renton Surface Water Design Manual 12/12/2016 TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS CHAPTER 6 WATER QUALITY DESIGN Section Page Section Page 6.1 Water Quality Menus 6-3 6.6 Oil Control Facility Designs 6-127 6.1.1 Basic Water Quality Menu 6-5 6.6.1 Catch Basin Inserts 6-127 6.1.2 Enhanced Basic Water Quality Menu 6-8 6.6.2 Oil/Water Separators 6-127 6.1.3 Sensitive Lake Protection Menu 6-10 6.7 Proprietary Facility Designs 6-141 6.1.4 Sphagnum Bog Protection Menu 6-14 6.7.1 Ecology Requirements 6-141 6.1.5 High-Use Menu 6-15 6.7.2 City of Renton Requirements 6-141 6.1.6 Pretreatment Facilities 6-17 6.8 Bioretention Facility Designs 6-145 6.2 General Requirements for WQ 6.8.1 Bioretention 6-145 Facilities 6-19 6.9 WSDOT WQ Facility Designs 6-159 6.2.1 Water Quality Design Flows and 6.9.1 Media Filter Drain 6-159 Treatment Volumes 6-19 6.2.2 Sequence of Facilities 6-22 6.9.2 Compost-Amended Filter Strips 6-169 6.2.3 Setbacks, Slopes, and Embankments 6-24 6.9.3 Compost-Amended Biofiltration Swales 6-170 6.2.4 Facility Liners 6-28 6.2.5 Flow Splitter Designs 6-32 6.2.6 Flow Spreading Options 6-36 6.3 Vegetated Flowpath Facility Designs 6-41 6.3.1 Basic Bioswales 6-41 6.3.2 Wet Bioswales 6-57 6.3.3 Lateral Inflow Bioswales 6-59 6.3.4 Standard Filter Strips 6-60 6.3.5 Narrow Area Filter Strips 6-68 6.4 Wetpool Facility Designs 6-69 6.4.1 Wetponds—Basic and Large 6-69 6.4.2 Wetvaults 6-84 6.4.3 Stormwater Wetlands 6-90 6.4.4 Combined Detention and Wetpool Facilities 6-96 6.5 Filtration Facility Designs 6-101 6.5.1 General Requirements For Filtration Facilities 6-101 6.5.2 Sand Filters— Basic and Large 6-102 6.5.3 Sand Filter Vaults 6-118 6.5.4 Linear Sand Filters 6-123 12/12/2016 2017 City of Renton Surface Water Design Manual TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS DEFINITIONS REFERENCE (continued) 7. Engineering Plan Support APPENDICES A King County Standard Map Symbols B Standard Plan Notes and Example Construction APPENDIX A Sequence Maintenance Requirements for Stormwater C Stormfilter Facility Access and Cartridge Facilities and On-site BMPs Configuration 8. Forms and Worksheets APPENDIX B A Technical Information Report(TIR)Worksheet Master Drainage Plan Objective,Criteria, B Offsite Analysis Drainage System Table Components and Review Process C Water Quality Facility Sizing Worksheets APPENDIX C D Flow Control and Water Quality Facility Summary Sheet and Sketch Simplified Drainage Requirements E CSWPP Worksheet Forms F Adjustment Application Form and Process APPENDIX D Guidelines Construction Stormwater Pollution Prevention G Dedication and Indemnification Clause— Standards Final Recording H Bond Quantities Worksheet REFERENCE I Maintenance and Defect Agreement 1. Surface Water Runoff Policy J Drainage Facility Covenant 2. Adopted Critical Drainage Areas K Drainage Release Covenant 3. Other Adopted Area Specific Drainage L Drainage Easement Requirements M On-Site BMP Covenant and Maintenance 4. Other Drainage Related Regulations and Instructions (recordable format) Guidelines N Impervious Surface Limit Covenant A Grading Code Soil Amendment Standard O Clearing Limit Covenant B Clearing&Grading Seasonal Limitations P River Protection Easement C Landscape Management Plan Guidelines Q Leachable Metals Covenant D Shared Facility Maintenance Responsibility R Agreement to Construct Improvements Guidance 5. Wetland Hydrology Protection Guidelines 9. Interim Changes to Requirements A Blanket Adjustments 6. Hydrologic/Hydraulic Design Methods B Administrative Changes A Infiltration Rate Test Methods B Pond Geometry Equations 10. King County-Identified Water Quality Problems C Introduction to Level Pool Routing 11. Materials D Supplemental Modeling Guidelines A (VACANT) B (VACANT) C Bioretention Soil Media Standard Specifications D (VACANT) E Roofing Erodible or Leachable Materials 2017 City of Renton Surface Water Design Manual 12/12/2016 TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS 12. (VACANT) 15. Reference Maps 13. (VACANT) A Flow Control Application Map 14. Supplemental Approved Facilities B Groundwater Protection Areas A City of Renton Approved Proprietary Facilities C Soil Survey for Use on Private Development Projects B City of Renton Approved Proprietary Facilities for Use in Public Projects 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL INTRODUCTION OVERVIEW The intent of this document is to provide requirements and guidance for the design,construction, and maintenance of on-site best management practices(BMPs), flow control facilities and water quality treatment facilities that are approved through the development permit process. This document is based on the 2016 King County Surface Water Design Manual with some modifications. Revisions have been made to the 2016 King County Surface Water Manual to reflect City of Renton-specific requirements. PURPOSE OF AND NEED FOR THIS DOCUMENT The City's adoption this manual is required to comply with federal stormwater regulations. Specifically, the City's Phase II National Pollutant Discharge Elimination System(NPDES)municipal stormwater permit establishes regulations for jurisdictions that: 1. Own and operate a storm drain system; 2. Discharge to surface waters; 3. Are located in urbanized areas; and 4. Have a population greater than 1,000. Washington State's Department of Ecology(Ecology),who oversees stormwater requirements in the state, has developed the 2012 Stormwater Management Manual for Western Washington, as amended in 2014, which complies with the NPDES requirements. In addition,Ecology has approved the 2016 King County Surface Water Design Manual as equivalent to the 2012 Stormwater Management Manual for Western Washington, as amended in 2014.The new surface water standards outlined in these manuals generally increase the use of low impact development(LID)principles and practices. The new standards add requirements to manage stormwater on-site to minimize impacts to water quality and aquatic habitat in the City. ORGANIZATION The chapters of this manual are organized as follows: Chapter 1 — DRAINAGE REVIEW AND REQUIREMENTS Describes the basic drainage requirements that implement the City of Renton's adopted surface water runoff policies and explains how these requirements are applied to proposed projects through the drainage review process. Chapter 2 — DRAINAGE PLAN SUBMITTAL Describes the requirements and specifications for submittal of design plans for drainage review,including report and plan formats, and scopes. 2017 City of Renton Surface Water Design Manual 12/12/2016 1 INTRODUCTION Chapter 3 — HYDROLOGIC ANALYSIS AND DESIGN Presents the acceptable methods of hydrologic analysis used to estimate runoff and design flow control,conveyance, and water quality facilities. Chapter 4 — CONVEYANCE SYSTEM ANALYSIS AND DESIGN Presents the acceptable methods, details, and criteria for analysis and design of conveyance systems. Chapter 5 — FLOW CONTROL DESIGN Presents the acceptable methods, details, and criteria for analysis and design of flow control facilities. Chapter 6 — WATER QUALITY DESIGN Presents the acceptable methods, details, and criteria for analysis and design of water quality facilities. DEFINITIONS — A comprehensive list of the words,terms, and abbreviations accompanied by their meaning as applied in this manual. APPENDICES: • APPENDIX A — MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND ON-SITE BMPs Contains the thresholds and standards for maintenance of all flow control facilities, on-site BMPs,conveyance systems, and water quality facilities required in this manual. • APPENDIX B — MASTER DRAINAGE PLAN OBJECTIVES, CRITERIA AND COMPONENTS, AND REVIEW PROCESS Describes in a general outline,the objectives,criteria,components and review process for Master Drainage Plans prepared for Urban Planned Developments and very large projects. • APPENDIX C — SIMPLIFIED DRAINAGE REQUIREMENTS Describes,the simplified drainage requirements for smaller projects that qualify for Simplified Drainage Review. • APPENDIX D — CONSTRUCTION STORMWATER POLLUTION PREVENTION STANDARDS Describes,the required measures to be implemented during construction to prevent discharges of sediment-laden runoff from the project site. It also describes effective management practices for spill control and chemical pollutants used during construction that may be needed to supplement the required erosion and sedimentation control measures. REFERENCE — Includes materials that are strictly for reference only and have not been adopted by the public rule adopting this manual. The applicant is responsible to ensure that the most current materials are used in preparing a permit application. 12/12/2016 2017 City of Renton Surface Water Design Manual 2 KEY REVISIONS KEY REVISIONS This section identifies the key revisions that the City has made to the 2016 King County Surface Water Design Manual.These revisions were necessary to address specific City of Renton requirements and to address deficiencies within the 2016 King County Surface Water Design Manual. 1. Aquifer Protection Area—The City of Renton Surface Water Design Manual includes an additional special requirement(Special Requirement 46)related to the Aquifer Protection Area. Certain facilities are restricted in Zone 1 of the Aquifer Protection Area. 2. On-site BMPs—Core Requirement 49 was renamed to On-site BMPs instead of Flow Control BMPs to avoid confusion with Flow Control Facilities (Core Requirement 43). 3. On-site BMP Sizing Credits— a) Explicit modeling of BMP infiltration for facility sizing is also allowed instead of applying the sizing credits in Table 1.2.9.A in Chapter 1 b) Credits were added to Table 1.2.9.A for vegetated roofs 4. Additional On-site BMP Options for Core Requirement 49— a) Rain Gardens b) Soil Amendment(included in King County Code,but details were not included in the King County Surface Water Design Manual) c) Tree Retention Credit d) Vegetated Roofs(optional) 5. LID Performance Standard—The LID performance standard is optional,but is not required for any projects located within the City of Renton. 6. Flow Control Standards—The City developed specific terminology for flow control standards in Core Requirement 43 in the Amendments to the 2009 King County Surface Water Design Manual that were retained in this manual: a) Flow Control Duration Standard—Matching Forested b) Flow Control Duration Standard—Matching Existing c) Peak Rate Flow Control Standard d) Flood Problem Flow Control Standard 7. Additional Water Quality Facility Options for Core Requirement 48— a) The following facilities are available as options on the Basic WQ Menu: i. Bioretention ii. WSDOT WQ Facilities—Media Filter Drain, Compost-amended Vegetated Filter Strips, and Compost-amended Biofiltration Swales b) The following facilities are available as options on the Enhanced Basic WQ Menu: i. Bioretention it. Proprietary Facilities iii. WSDOT WQ Facilities—Media Filter Drain, Compost-amended Vegetated Filter Strips, and Compost-amended Biofiltration Swales c) The following facilities are available as options on the Sensitive Lake Protection Menu: i. Proprietary Facilities 2017 City of Renton Surface Water Design Manual 12/12/2016 3 INTRODUCTION 8. Proprietary Facilities—Added specific proprietary facilities to Chapter 6,Appendix A, and Reference Section 14-A. 9. Element#11 of Core Requirement#2—This Construction Stormwater Pollution Prevention element was modified to be more inclusive of other infiltration facilities that are not considered to be On-site BMPs. 10. Bioretention Design and Construction—The City incorporated the following modifications to the bioretention design site suitability factors and design criteria: a) Specific information added regarding perpendicular utility crossings b) Minimum bottom with is 18 inches c) Minimum ponding depth is 2 inches d) Maximum side slopes are 2.5 to 1 e) Minimum berm top width is 6 inches f) Minimum shoulder between road edge and bioretention side slope is 6 inches g) Water tolerant plant list has been revised h) Underdrains are allowed for bioretention facilities designed to meet Core Requirement 48 i) Added a section on construction sequencing 11. Permeable Pavement Design and Construction—The City incorporated the following modifications to the permeable pavement design criteria: a) Run-on is not allowed from pervious surfaces b) Underdrains are allowed in specific settings c) Added design criteria regarding the overflow d) Added figures depicting permeable pavement cross sections and permeable pavement with check dams e) Added a reference to ASTM C1701 and ASTM C1781 for infiltration rate verification f) Added a section on construction sequencing 12. Core Requirement#6 and#7—Language specific to the City of Renton has been incorporated into Core Requirement 46(Maintenance and Operations)and Core Requirement 47 (Financial Guarantees and Liabilities). 13. Basic WQ Thresholds—The thresholds for basic WQ treatment in Core Requirement 48 have been adjusted for consistency with the Amendments to the 2009 King County Surface Water Design Manual and the 2012 Stormwater Management Manual for Western Washington, as amended in 2014. 14. Target impervious surface—The City of Renton does not require including existing impervious surfaces added on or after January 8,2001 in the definition of target impervious surfaces. 15. Continuous modeling timestep—Precipitation with a 15-minute precipitation is available for the entire City of Renton, so references to the 1-hour timestep were removed. 16. Continuous modeling precipitation series—The City of Renton allows either rain gage data(Sea- Tac Airport) or the 158-year extended precipitation timeseries(Puget East)to be used for modeling. 17. Allowable pipe materials—The City of Renton added a table of allowable pipe materials and minimum cover to Chapter 4 for consistency with the Amendments to the 2009 King County Surface Water Design Manual. 12/12/2016 2017 City of Renton Surface Water Design Manual 4 KEY REVISIONS 18. Conveyance requirements—The City of Renton added requirements to Chapter 4 for changes in pipe size, structures,pipe cover,pipe clearances,pipe system connections, and pump systems. 19. Fencing requirements—The City of Renton added specific fencing requirements to Chapter 5 and Chapter 6 for detention ponds and wet ponds related to a City policy decision. 20. Seeding—The City of Renton revised seeding rates,timing, and mixes in Chapter 5, Chapter 6, and Appendix C, and Appendix D. 21. Removing terminology that does not apply—Terminology related to the Urban Growth Area,Urban Planned Development(UPD),Agricultural Projects, Critical Drainage Areas,Large Rural Lots, Stormwater Compliance Plans (SWCPs),Experimental Design Adjustments,recreational tracts, and Landscape Management Plan was removed. 22. Removed BMPs that do not apply—Catch Basin Inserts(Chapter 6),Narrow Area Filter Strips (Chapter 6), and Farmland Dispersion(Appendix Q. 23. References a) Provided links to the City's website for covenants, easements, agreements, and worksheets b) Added Reference Section 15-A(Reference Maps) for consistency with the Amendments to the 2009 King County Surface Water Design Manual OTHER APPLICABLE REFERENCES The City also adopts,by reference,the 2016 King County Stormwater Pollution Prevention Manual for determining source control requirements. 2017 City of Renton Surface Water Design Manual 12/12/2016 5 INTRODUCTION (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 6 CHAPTER I Section Page 1.1 Drainage Review 1-11 1.1.1 Projects Requiring Drainage Review 1-12 DRAINAGE REVIEW 1.1.2 Drainage Review Types and Requirements 1-12 AND REQUIREMENTS 1.1.3 Drainage Review Required By Other Agencies 1-22 1.1.4 Drainage Design Beyond Minimum Compliance 1-22 rr, 1.2 Core Requirements 1-23 1.2.1 Core Requirement#1: Discharge at the Natural Location 1-23 1.2.2 Core Requirement#2: Offsite Analysis 1-24 1.2.3 Core Requirement#3: Flow Control i Facilities 1-35 1.2.4 Core Requirement#4: Conveyance System 1-50 1.2.5 Core Requirement#5: Construction Stormwater Pollution Prevention 1-54 1.2.6 Core Requirement#6: Maintenance and Operations 1-59 CITY OF RENTON 1.2.7 Core Requirement#7: Financial Guarantees and Liability 1-61 SURFACE WATER 1.2.8 Core Requirement#8: Water Quality Facilities 1-63 1.2.9 Core Requirement#9: On-Site BMPs 1-73 DESIGN MANUAL 1.3 Special Requirements 1-89 1.3.1 Special Requirement#1: Other Adopted Area-Specific Requirements 1-89 1.3.2 Special Requirement#2: Flood Hazard Area Delineation 1-90 1.3.3 Special Requirement#3: Flood Protection Facilities 1-91 1.3.4 Special Requirement#4: Source Controls 1-92 1.3.5 Special Requirement#5: Oil Control 1-94 1.3.6 Special Requirement#6: Aquifer Protection Area 1-97 1.4 Adjustment Process 1-99 1.4.1 Adjustment Authority 1-99 1.4.2 Criteria for Granting Adjustments 1-99 1.4.3 Adjustment Application Process 1-100 1.4.4 Adjustment Review Process 1-101 1.4.5 Appeals 1-101 2017 City of Renton Surface Water Design Manual 12/12/2016 CITY OF RENTON SURFACE WATER DESIGN MANUAL (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS This chapter describes the drainage review procedures and types,the drainage requirements, and the adjustment procedures necessary to implement surface water runoff policies codified in Chapter 4-6-030 of the City of Renton Municipal Code(RMC). It also provides direction for implementing the more detailed procedures and design criteria found in subsequent chapters of this manual. Chapter Organization The information presented in Chapter 1 is organized into four main sections as follows: • Section 1.1,"Drainage Review" • Section 1.2,"Core Requirements" • Section 1.3,"Special Requirements" • Section 1.4,"Adjustment Process" Each of these sections begins on an odd page so the user can insert tabs if desired for quicker reference. Formatting of Chapter Text The text of Chapter 1 and subsequent chapters has been formatted using the following conventions to aid the user in finding,understanding, and properly applying the thresholds,requirements, and procedures contained in this manual: • Italic is used to highlight the following: (a)terms when they are first introduced and defined within the same paragraph; (b) special notes that supplement or clarify thresholds,requirements, and procedures; (c)sentences considered important for purposes of understanding thresholds, requirements, and procedures; and(d)titles of publications. • Bold italic is used to highlight terms considered key to understanding and applying drainage review thresholds,requirements, and procedures. These are called"key terms"and are defined below. This convention applies after the key term is defined and does not necessarily apply to tables and figures. • Bold is used to highlight words and phrases that are not key terms but are considered important to emphasize for purposes of finding and properly applying thresholds,requirements, and procedures. Key Terms and Definitions (a complete list of definitions follows Chapter 6) Proper application of the drainage review and requirements in this chapter requires an understanding of the following key terms and their definitions. Other key terms may be defined in subsequent chapters.All such key terms are highlighted in bold italic throughout the manual. Other important terms that are not key terms are defined in the text when they are first introduced. These are highlighted in italic when they are first introduced but are not highlighted throughout the manual.All terms defined in this chapter are also found in the "Definitions"section of this manual as are other important terms defined throughout the Manual. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-1 CHAPTER DRAINAGE REVIEW AND REQUIREMENTS Aquifer Protection Area(APA)means the portion of anaquifer within the zone ofcapture and recharge area for uwell orwell field owned oroperated hvthe City of Renton delineated inthe Aquifer Protection map found inthe City of Renton Map Gallery (<htti)://rentonwa.gov/government/default.aspx?id=29885>) or viewed via COR Maps ( ). Arterial—&ligh traffic-volume road orstreet primarily for through traffic. The term generally includes roads or streets considered collectors. It does not include local access roads which are generally limited 10providing access 10abutting property. Refer oothe City ofRenton Map Gallery for umap of arterial streets( )orviep/viuC0|lB4upu ( ). Biouetom6om—&non-site and water quality treatment best management practice consisting ofushallow landscaped depression designed 10temporarily store and promote infiltration ofu1ormo'u1errunoff. Standards for biou:1endondesign,including soil mix,plants, storage volume and G:omibili criteria, are Umanual. meet 43, 48 and/or 9. CED means the Community and Economic Development Department. Certified Erosion and Sediment Control Lead(CESCL)means an individual who has current certification through an approved erosion and sediment control training program that meets the minimum training standards established hvthe Wuu6in81onState Department of Ecology(Ecolo8v).ACESCLiu knowledgeable *in the principles undprudiceuof erosion and sediment control. The CESCLmust have the skills 10assess site conditions and construction activities that could impact the quality of u1oonp/cuerand,the effectiveness of erosion and sediment control measures used 10control the quality of storrnwater discharges. Certification is obtained through an Ecology approved erosion and sediment control course. Civil engineerzneunuuperuonliccmuedhvt6euta1eofVVuu6in81onomupnoG:uuionulcm8ineerincivil engineering. Construct oxn/oJifymeans ooinstall unew drainage pipe ordhc6orznuke improvements 1ounexisting drainage pipe or ditch, for purposes other than maintenance,that either serves to concentrate pu:vzouu}v unconcentrated surface-p��ror�oonp��rrunoff oru�rv�u1oincrease, d decrease or rediu:dthe conveyance ofuurf�epateroru1oroou1errunoff. Conveyance system nuisance problem means a flooding or erosion problem that does not constitute a severeflooding problem or severe erosion problem and that results from the overflow of a constructed conveyance system for runoff events less than or equal to a 10-year event. Examples include inundation of a shoulder or lane of a roadway, overflows collecting in yards or pastures, shallow flows across driveways,minor flooding of crawl spaces or unheated garages/outbuildings, and minor erosion. Doro/*pn/om/ The division ofuparcel ofland into two (2)ormore parcels;the construction, reconstruction,conversion, structural alteration,relocation or enlargement of any structure; any mining, excavation,landfill or land disturbance and any use or extension of the use ofland. Development review engineer—T6eCi1yof|len1onemployeeu:uponsiblefort6econddionin8,reviep/, inspection, and approval of ri8lu'o(-p/oyuse permits, and road and drainage improvements constructed uupart ofdevelopment peondxudzniniuu:u:dhvCED. Effective impervious surface—Those impervious surfaces that are connected via sheet flow ordiscrete conveyance to a drainage system. Impervious surfaces are considered ineffective if. l)the runoff iu fully dispersed as described inAppendix C ofthis manual;2)residential roof runoff iuinfiltrated in accordance with the full infiltration BMP described in Appendix C of this manual; or 3)approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated. 12/12/2016 20l7City^[Renton Surface Water Design Manual 1-2 CHAPTER 1—KEY TERMS AND DEFINITIONS Erodible or leachable materials, wastes, or chemicals are those materials or substances that,when exposed to rainfall,measurably alter the physical or chemical characteristics of the rainfall runoff (Examples include but are not limited to erodible soil,uncovered process wastes,manure, fertilizers, oily substances, ashes,kiln dust,garbage dumpster leakage,commercial-scale vehicle and animal wash waste,galvanized structural, architectural,cabinet, and utility steel, architectural copper,bronze, brass, and lead,treated lumber, etc.). Erosion hazard area is the critical area designation, defined and regulated in RMC 4-3-050,that is applied to areas underlain by soils that are subject to severe erosion when disturbed. Refer to the City of Renton Map Gallery for a map of erosion hazard areas (<http://rentonwa.gov/government/default.aspx?id=29885>) or view via COR Maps (<htt :�//rp.rentonwa.gov/SilverlightPublieNiewer.html9Viewer=COR-Maps>). Existing site conditions means those that existed prior to May 1979 as determined from aerial photographs and,if necessary,knowledge of individuals familiar with the area,unless a drainage plan for land cover changes has been approved by the City of Renton since May 1979 as part of a City permit or approval(or County-approved permit if in an area that has been annexed by the City). If so, existing site conditions are those created by the site improvements and drainage facilities constructed per the approved drainage plan. Exposed means subject to direct or blown-in precipitation and/or direct or blown in runoff.Not fully covered. Exposed area or exposed material means not covered sufficiently to shield from rainfall and stormwater runoff.At a minimum, full coverage to not be considered exposed requires a roof with enough overhang in conjunction with walls of sufficient height to prevent rainfall blow-in; and the walls must extend into the ground or to a berm or footing to prevent runoff from being blown in or from running onto the covered area. Flood hazard area is the critical area designation, defined and regulated in RMC 4-3-050,that is applied to areas subject to flooding. Refer to the City of Renton Map Gallery for a map of flood hazard areas (<http://rentonwa.gov/government/default.aspx?id=29885>) or view via COR Maps (<bttp://rp.rentonwa.gov/SilverlightPublie/Viewer.html?Viewer=COR-Maps>). Flow control facility means a drainage facility designed in accordance with the drainage requirements in this manual to mitigate the impacts of increased stormwater runoff generated by site development.A "flow control facility"is designed either to hold water for a considerable length of time and then release it by evaporation,plant transpiration or infiltration into the ground or to hold runoff for a short period of time and then release it to the conveyance system. Fully covered means covered sufficiently to shield from rainfall and stormwater runoff.At a minimum, full coverage requires a roof with enough overhang in conjunction with walls of sufficient height to prevent rainfall blow-in; and the walls must extend into the ground or to a berm or footing to prevent runoff from being blown in or from running onto the covered area.Not exposed. Fully dispersed means the runoff from an impervious surface or nonnative pervious surface has dispersed per the criteria for fully dispersed surface in Section 1.2.3.2 of this manual. Groundwater protection areas include the Cedar Valley Sole Source Aquifer Project Review Area designated by the federal Environmental Protection Agency,Wellfield Capture Zones as mapped by the Washington State Department of Health, and the Aquifer Protection Areas as mapped by the City. The combined area described by these criteria is represented in Reference Section 15-13, Groundwater Protection Areas in the City of Renton. High-use site means a commercial or industrial site that(1)has an expected average daily traffic(ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area; (2)is subject to petroleum storage or transfer in excess of 1,500 gallons per year,not including delivered heating oil; or(3)is subject to use, storage,or maintenance of a fleet of 25 or more vehicles that are over 2017 City of Renton Surface Water Design Manual 12/12/2016 1-3 CHAPTER I DRAINAGE REVIEW AND REQUIREMENTS 10 tons net weight(trucks,buses,trains,heavy equipment, etc.).Also included is any road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use improvements.For the purposes of this definition,commercial and industrial site means that portion of a site's developed area associated with an individual commercial or industrial business(e.g.,the area occupied by the business's buildings and required parking). Historical site conditions means those that existed on the site prior to any development in the Puget Sound region.For lands not currently submerged(i.e., outside the ordinary high water mark of a lake, wetland, or stream),historical site conditions shall be assumed to be forest cover unless reasonable, historical,site-specific information is provided to demonstrate a different vegetation cover. The historical site conditions exception in the King County Surface Water Design Manual does not apply to the City. Impaired waterbody or impaired receiving water means where the receiving waterbody is either(1)listed as impaired for metals or organic pollutants according to Ecology's Water Quality Assessment categories 2,4,or 5—water or sediment and/or(2)where subject to any other local, state, or federal cleanup plan. Impervious surface means a non-vegetated surface area that either prevents or retards the entry of water into the soil mantle as under natural conditions before development; or that causes water to run off the surface in greater quantities or at an increased rate of flow compared to the flow present under natural conditions prior to development(see also new impervious surface). Common impervious surfaces include,but are not limited to,roof,walkways,patios, driveways,parking lots, or storage areas, areas that are paved,graveled or made of packed or oiled earthen materials or other surfaces that similarly impede the natural infiltration of surface water or stormwater.For the purposes of applying the impervious surface thresholds and exemptions contained in this manual,permeable pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater runoff are considered impervious surface while an open uncovered flow control or water quality facility is not. However, for the purposes of computing runoff,uncovered flow control or water quality facilities shall be modeled as impervious surfaces as specified in Chapter 3. Land disturbing activity means any activity that results in a change in the existing soil cover,both vegetative and non-vegetative, or the existing soil topography. Land disturbing activities include,but are not limited to demolition,construction,clearing,grading, filling, excavation, and compaction. Land disturbing activity does not include tilling conducted as part of agricultural practices,landscape maintenance, or gardening. Landslide hazard is the critical area designation,defined and regulated in RMC 4-3-050,that is applied to areas subject to severe risk of landslide due to topography, soil conditions, and geology. Refer to the City of Renton Map Gallery for a map of landslide hazards (<http://rentonwa.gov/government/default.aspx?id=29885>) or view via COR Maps (<htt :�//rp.rentonwa.gov/SilverlightPublieNiewer.html9Viewer=COR-Maps>). Landslide hazard drainage area means an area that has overland flows from a project may pose a significant threat to health and safety because of its close proximity to a landslide hazard. Local drainage system means any natural or constructed drainage feature that collects and concentrates runoff from the site and discharges it downstream. Low Impact Development(LID)—A stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation,use of onsite natural features, site planning, and distributed stormwater management practices that are integrated into a project design. LID Best Management Practices—Distributed stormwater management practices,integrated into a project design,that emphasize pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration. LID 13MPs are referred to as on-site 13MPs in this manual and include, 12/12/2016 2017 City of Renton Surface Water Design Manual 1-4 CHAPTER 1—KEY TERMS AND DEFINITIONS but are not limited to,bioretention,permeable pavements, limited infiltration systems,roof downspout controls, dispersion, soil quality and depth, and minimum disturbance foundations. LID Principles—Land use management strategies that emphasize conservation,use of onsite natural features, and site planning to minimize impervious surfaces,native vegetation loss, and stormwater runoff. Maintenance means those usual activities taken to prevent a decline,lapse, or cessation in the use of currently serviceable structures, facilities,BMPs, equipment, or systems if there is no expansion of any of these, and there are no significant hydrologic impacts. Maintenance includes the repair or replacement of non-functional facilities and BMPs,and the replacement of existing structures(e.g., catch basins,manholes,culverts)with different types of structures,if the repair or replacement is required to meet current engineering standards or is required by one or more environmental permits and the functioning characteristics of the original facility or structure are not changed.For the purposes of applying this definition to the thresholds and requirements of this manual, CED will determine whether the functioning characteristics of the original facility, structure, or BMP will remain sufficiently unchanged to consider replacement as maintenance. Note: The following pavement maintenance practices are exempt from drainage review: • Pothole and square cut patching • Overlaying existing non-permeable asphalt or non-permeable concrete pavement with asphalt or concrete without expanding the area of coverage • Shoulder grading • Reshaping/regrading drainage systems • Crack sealing • Resurfacing with in-kind material without expanding the road prism,pavement preservation activities that do not expand the paved prism • Vegetation maintenance. The following pavement maintenance practices are not categorically exempt from drainage review: • Removing and replacing a paved surface to base course or lower, or repairing the pavement base (i.e., "replaced impervious surfaces"). • Extending the edge of pavement without increasing the size of the paved area • Resurfacing that meets the definition of new impervious surface in this manual. Major receiving water means a large receiving water that has been determined by City of Renton to be safe for the direct discharge of increased runoff from a proposed project without a flow control facility, subject to the restrictions on such discharges set forth in Core Requirement 43, Section 1.2.3. A list of major receiving waters is provided in Section 1.2.3.1. Major receiving waters are also considered safe for application of Basic WQ treatment in place of otherwise required Enhanced Basic WQ treatment(see Section 1.2.8.1), except where the receiving water is either(1)listed as impaired for metals or organic pollutants according to Ecology's Water Quality Assessment categories 2,4,or 5, and/or(2)where subject to any other local, state, or federal cleanup plan. In any of these cases,the major receiving water exception for Enhanced Basic treatment is superseded by 1.2.2.3 Water Quality Problem Impact Mitigation for Metals(Type 4). Native vegetated surface means a surface in which the soil conditions,ground cover, and species of vegetation are like those of the original native condition for the site. More specifically,this means (1)the soil is either undisturbed or has been treated according to the "native vegetated landscape" specifications in Appendix C, Section C.2.1.8; (2)the ground is either naturally covered with vegetation litter or has been top-dressed between plants with 4 inches of mulch consistent with the native vegetated landscape specifications in Appendix C; and(3)the vegetation is either(a)comprised predominantly of plant species, other than noxious weeds,that are indigenous to the coastal region of the Pacific Northwest and that reasonably could have been expected to occur naturally on the site or 2017 City of Renton Surface Water Design Manual 12/12/2016 1-5 CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS (b)comprised of plant species specified for a native vegetated landscape in Appendix C. Examples of these plant species include trees such as Douglas fir,western hemlock,western red cedar, alder,big- leaf maple and vine maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous plants such as sword fern, foam flower, and fireweed. Natural discharge area means an onsite area tributary to a single natural discharge location. Natural discharge location means the location where surface and storm water runoff leaves(or would leave if not infiltrated or retained)the site or project site under existing site conditions. New impervious surface means the addition of a man-made,modified, or compacted surface like roofs, pavement,gravel, or dirt; or the addition of a more compacted surface, such as resurfacing by upgrading from dirt to gravel, asphalt, or concrete;upgrading from gravel to asphalt, or concrete; or upgrading from a bituminous surface treatment("chip seal")to asphalt or concrete.Permeable pavement and vegetated roofs are considered new impervious surface for purposes of determining whether the thresholds for application of minimum requirements are exceeded, as are lawns, landscaping, sports fields,golf courses, and other areas that have modified runoff characteristics resulting from the addition of underdrains designed to collect stormwater runoff. Open,uncovered retention/detention facilities shall not be considered impervious surfaces for purposes of determining whether the thresholds for application of minimum requirements are exceeded. Open,uncovered retention/detention facilities shall be considered impervious surfaces for purposes of runoff modeling. New pervious surface means the conversion of a native vegetated surface or other native surface to a nonnative pervious surface(e.g.,conversion of forest or meadow to pasture land,grass land,cultivated land,lawn,landscaping,bare soil, etc.), or any alteration of existing nonnative pervious surface that significantly increases surface and storm water runoff(e.g.,conversion of pasture land,grass land, or cultivated land to lawn,landscaping, or bare soil; or alteration of soil characteristics). New PGIS means new impervious surface that is pollution-generating impervious surface or any alteration of existing pollution-generating impervious surface that changes the type of pollutants or results in increased pollution loads and/or concentrations. New PGPS means new pervious surface that is pollution generating pervious surface or any alteration of existing pollution-generating pervious surface that changes the type of pollutants or results in increased pollution loads and/or concentrations. Offsite means any area lying upstream of the site that drains onto the site and any area lying downstream of the site to which the site drains including frontage improvements. Onsite means the entire site that includes the proposed development. On-site BMP means a small scale drainage facility or feature that is part of a development site strategy to use processes such as infiltration, dispersion, storage, evaporation,transpiration, forest retention, and reduced impervious surface footprint to mimic pre-developed hydrology and minimize stormwater runoff. Permeable pavement means pervious concrete,porous asphalt,permeable pavers or other forms of pervious or porous paving material intended to allow passage of water through the pavement section. It often includes an aggregate base that provides structural support and acts as a stormwater reservoir. Pervious Surface—Any surface material that allows stormwater to infiltrate into the ground.Examples include lawn,landscape,pasture, and native vegetation areas.Note for purposes of threshold determination and runoff volume modeling for detention and treatment,vegetated roofs and permeable pavements are to be considered impervious surfaces along with lawns,landscaping, sports fields,golf courses, and other areas that have modified runoff characteristics resulting from the addition of underdrains. Pollution-generating impervious surface(PGIS)means an impervious surface considered to be a significant source of pollutants in stormwater runoff. Such surfaces include those that are subject to vehicular use,industrial activities, or storage of erodible or leachable materials, wastes, or chemicals 12/12/2016 2017 City of Renton Surface Water Design Manual 1-6 CHAPTER I KEY TERMS AND DEFINITIONS and that receive direct rainfall or the run-on or blow-in of rainfall.A covered parking area would be considered PGIS if runoff from uphill could regularly run through it or if rainfall could regularly blow in and wet the pavement surface. Metal roofs are also considered PGIS unless they are coated with an inert,non-leachable material(see Reference Section I I-E); or roofs that are exposed to the venting of significant amounts of dusts,mists, or fumes from manufacturing,commercial, or other indoor activities.PGIS includes vegetated roofs exposed to pesticides, fertilizers, or loss of soil. Other roofing types that may pose risk but are not currently regulated are listed Reference Section I I-E. Lawns,landscaping, sports fields,golf courses, and other areas that have modified runoff characteristics resulting from the addition of underdrains that have the pollution generating characteristics described under the "pollution-generating pervious surface"definition are also considered PGIS. Pollution generating pervious surface(PGPS)means a non-impervious surface considered to be a significant source of pollutants in surface and storm water runoff. Such surfaces include those that are subject to vehicular use,industrial activities, storage of erodible or leachable materials, wastes, or chemicals, and that receive direct rainfall or the run-on or blow-in of rainfall; or subject to use of pesticides and fertilizers, or loss of soil. Such surfaces include,but are not limited to,the lawn and landscaped areas of residential,commercial, and industrial sites or land uses,golf courses,parks, sports fields(natural and artificial turf),cemeteries, and grassed modular grid pavement. Project means any proposed action to alter or develop a site. The proposed action of a permit application or an approval,which requires drainage review. Project site means that portion of a site and any offsite areas subject to proposed project activities, alterations, and improvements including those required by this manual. Rain Garden means a shallow,landscaped depression with compost-amended native soils and adapted plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent areas, and to allow stormwater to pass through the amended soil profile. Rain gardens can only be used to meet Core Requirement 49. Receiving waters means bodies of water, surface water systems, or groundwater receiving water from upstream man-made or natural systems. Redevelopment project means a project that proposes to add,replace,or modify impervious surfaces(e.g., building,parking lot)for purposes other than a residential subdivision or maintenance on a site that is already substantially developed in a manner consistent with its current zoning or with a legal non- conforming use, or has an existing impervious surface coverage of 35%or more. The following examples illustrate how this definition may apply to residential and commercial sites. Redevelopment Project that Redevelopment Project that Redevelopment Project that Adds and Replaces Impervious Adds New Impervious Surface Replaces Impervious Surface Surface Residential Site Commercial Site Commercial Site Existing Bldg. Existing — Existing Impervious ——— Impervious i Area(35%) lExisting i i Area(35%) :Bldg. Bldg. Existing Existing Pervious Existing i New Parking Area Parking Parking i (65%) i Replaced impervious surface means any existing impervious surface on the project site that is proposed to be removed and re-established as impervious surface, excluding impervious surface removed for the 2017 City of Renton Surface Water Design Manual 12/12/2016 1-7 CHAPTER I DRAINAGE REVIEW AND REQUIREMENTS sole purpose of installing utilities or performing maintenance on underground infrastructure. For structures,removed means the removal of buildings down to the foundation. For other impervious surfaces,removed means the removal down to base course or bare soil. For purposes of this definition, base course is the layer of crushed rock that typically underlies an asphalt or concrete pavement. It does not include the removal of pavement material through grinding or other surface modification unless the entire layer of PCC or AC is removed. Replaced impervious surface also includes impervious surface that is moved from one location to another on the project site where the following two conditions are met: (A)runoff characteristics and volumes remain the same or are improved in the area where the existing impervious surface is removed, and(B)impervious surface at the new location is either designated as non-pollution generating or the pollution generating characteristics remain unchanged compared to that of the original location. Replaced PGIS means replaced impervious surface that is pollution generating impervious surface. Sensitive lake means a designation applied by the City to lakes that are particularly prone to eutrophication from development-induced increases in phosphorus loading. Severe building flooding problem means there is flooding of the finished floor area' of a habitable building,2 or the electrical/heating system of a habitable building for runoff events less than or equal to a 100-year event. Examples include flooding of finished floors of homes and commercial or industrial buildings, or flooding of electrical/heating system components in the crawl space or garage of a home. Severe erosion problem means there is an open drainage feature with evidence of or potential for erosion/incision sufficient to pose a sedimentation hazard to downstream conveyance systems or pose a landslide hazard by undercutting adjacent slopes. Severe erosion problems do not include roadway shoulder rilling or minor ditch erosion. Severe flooding problem means a severe building flooding problem or a severe roadway flooding problem. Severe roadway flooding problem means there is flooding over all lanes of a roadway,3 or a sole access driveway4 is severely impacted, for runoff events less than or equal to the 100-year event.A severely impacted sole access driveway is one in which flooding overtops a culverted section of the driveway, posing a threat of washout or unsafe access conditions due to indiscernible driveway edges, or flooding is deeper than 6 inches on the driveway,posing a severe impediment to emergency access. Single family residential project means any project that(a)constructs or modifies a single family dwelling unit, (b)makes improvements(e.g., driveways,roads, outbuildings,play courts, etc.)or clears native vegetation on a lot that contains or will contain a single family dwelling unit, or(c)is a plat, short plat, or boundary line adjustment that creates or adjusts lots that will contain single family dwelling units. Site means a single parcel; or,two or more contiguous parcels that are under common ownership or documented legal control; or a portion of a single parcel under documented legal control separate from the remaining parcel,used as a single parcel for a proposed project for purposes of applying for authority from the City to carry out a proposed project.For projects located primarily within dedicated rights-of-way,the length of the project site and the right-of-way boundaries define the site. 1 Finished floor area,for the purposes of defining severe building flooding problem,means any enclosed area of a building that is designed to be served by the building's permanent heating or cooling system. 2 Habitable building means any residential,commercial,or industrial building that is equipped with a permanent heating or cooling system and an electrical system. 3 Roadway,for the purposes of this definition, means the traveled portion of any public or private road or street classified as such in the City of Renton Standard Details and City of Renton Transportation Department guidelines. 4 Sole access driveway means there is no other unobstructed,flood-free route for emergency access to a habitable building. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-8 CHAPTER 1—KEY TERMS AND DEFINITIONS Steep slope hazard area is the critical area designation, defined and regulated in RMC 4-3-050,that is applied to areas where extra protection of sensitive slopes is required.Refer to the City of Renton Map Gallery for a map of steep slope hazard areas (<httg://rentonwa.gov/government/default.aM2i9id=29885>). Structure means a catch basin or manhole in reference to a storm drainage system. Subject to vehicular use means the surface is regularly used by motor vehicles including but not limited to motorcycles,cars,trucks,busses, aircraft,tractors, and heavy equipment. The following surfaces are considered regularly used by motor vehicles: roads,un-vegetated road shoulders,bike lanes within the traveled lane of a roadway, driveways,parking lots,unrestricted access fire lanes,vehicular equipment storage yards, and airport taxiways and runways. The following surfaces are not considered regularly used by motor vehicles:paved bicycle pathways separated from and not subject to drainage from roads for motor vehicles, fenced or restricted access fire lanes, and maintenance access roads with a recurring use of no more than one routine vehicle access per week. Target impervious surface means that portion of a site's new and/or replaced impervious surface from which runoff impacts are required to be mitigated by a particular set of drainage requirements (flow control facility,water quality facility, and/or on-site BMP). Type of Development Target Impervious Surface New development New plus replaced impervious surface Redevelopment with <5,000 sf impervious or New impervious surface improvements < 50% of the assessed value of the existing site improvements Redevelopment with >_ 5,000 sf impervious New plus replaced impervious surface and improvements >_ 50% of the assessed value of the existing site improvements Target pervious surface means all areas subject to clearing and grading that have not been covered by an impervious surface,incorporated into a drainage facility, or engineered as structural fill or slope. Threshold discharge area means an onsite area draining to a single natural discharge location, or multiple natural discharge locations that combine within one-quarter-mile downstream(as determined by the shortest flowpath). The examples below illustrate this definition. This term is used to clarify how the thresholds, exemptions, and exceptions of this manual are applied to sites with multiple discharge locations. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-9 CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS Example of a Project Site Example of a Project Site Example of a Project Site with a Single Natural with Multiple Natural with Multiple Natural Discharge and a Single Discharges and a Single Discharges and Multiple Threshold Discharge Area Threshold Discharge Area Threshold Discharge Areas \ Natural __Natural Natural Natural Natural Discharge Discharge ------ Dischjr g------- Dischar --_____ _Dischai$� UUU g Area Area 1 Area 2 Area 1 Area 2 THRESHOLD -------TTIRES OLD IRFSUQLD HRESHOLD DISCS ARGEDISC GE ;=-`-� SCH fAR6jE ISCHAROr-- -- --` AREA ARhA A 1 AICA 2 V (Sh__Ted) (Sh = [Shaed)e ' NaturalNatural Natural ' Discharge Discharge Discharge r` Location �'; Locations Locations r r '/4 Mile Downstream (shortest flow path) -- -------------------------` Transportation redevelopment project means a stand-alone transportation improvement project that proposes to add,replace, or modify impervious surface, for purposes other than maintenance,within a length of dedicated public or private road right-of-way that has an existing impervious surface coverage of thirty-five percent or more. Road right-of-way improvements required as part of a subdivision, commercial,industrial or multifamily project may not be defined as a separate transportation redevelopment project. Zone I of the Aquifer Protection Area means the land area situated between a well or well field owned by the City of Renton and the one-year groundwater travel time contour and not otherwise designated as Zone 1 Modified as depicted in the Aquifer Protection map found in the City of Renton Map Gallery (<http://rentonwa.gov/government/default.aspx?id=29885>).Zone 1 of the APA is shown on Reference Section 15-13, Groundwater Protection Areas in the City of Renton. Zone 2 of the Aquifer Protection Area means the land area situated between the one-year groundwater travel time contour and the boundary of the zone of potential capture for a well or well field owned or operated by the City. If the aquifer supplying water to such a well,well field, or spring is naturally protected by confining overlying and underlying geologic layers,the City may choose not to subdivide an Aquifer Protection Area into two (2)zones. In such a case,the entire Aquifer Protection Area will be designated as Zone 2. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-10 CITY OF RENTON SURFACE WATER DESIGN MANUAL 1.1 DRAINAGE REVIEW Drainage review is the evaluation by City of Renton staff of a proposed project's compliance with the drainage requirements of this manual. The City of Renton department responsible for drainage review is the Community and Economic Development(CED)Department unless otherwise specified in RMC 4-6-060.Drainage review by CED is an integral part of its permit review process for development projects. This section describes when and what type of drainage review is required for a proposed project and how to determine which drainage requirements apply. The section covers the following topics related to drainage review: • "Projects Requiring Drainage Review,"Section 1.1.1 • "Drainage Review Types and Requirements,"Section 1.1.2 • "Drainage Review Required By Other Agencies,"Section 1.1.3 • "Drainage Design Beyond Minimum Compliance," Section 1.1.4 Guide to Using Section 1.1 The following steps are recommended for efficient use of Section 1.1: 1. Determine whether your proposed project is subject to the requirements of this manual by seeing if it meets any of the thresholds for drainage review specified in Section 1......1.1.Making this determination requires an understanding of the key terms defined at the beginning of this chapter. 2. If drainage review is required per Section 1.1.1,use the flow chart in Figure 1.1.2.A to determine what type of drainage review will be conducted by the City. The type of drainage review defines the scope of drainage requirements that will apply to your project as summarized in Table 1.1.2.A. 3. Check i the more detailed threshold information in Section 1.1.2 to verify;that you have determined the correct type of drainage review. 4. After verifying the type of drainage review,use the information in Section 1.1.2 to determine which core requirements(found in Section 1.2)and which special requirements(found in Section 1.3) must be evaluated for compliance by your project. To determine how to icomply with each applicable core and special requirement, see the more detailed information on these requirements contained in Sections 1.2 and 1.3 of this chapter. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-11 SECTION 1.1 DRAINAGE REVIEW 1.1.1 PROJECTS REQUIRING DRAINAGE REVIEW Drainage review is required for any proposed project(except those proposing only maintenance) that is subject to a City of Renton development permit or approval,including but not limited to those listed at right,AND that meets any one of the following conditions: 1. The project adds or will result in 2,000 square feet5 or more of new impervious surface, replaced impervious City of Renton Permits surface, or new plus replaced impervious surface, OR and 2. The project proposes 7,000 square feet' or more of land Approvals disturbing activity, OR Building Permits/Combination 3. The project proposes to construct or modify a drainage Building Permits pipe/ditch that is 12 inches or more in size/depth, or Construction Permits receives storm water runoff or surface water from a Demolition Permits drainage pipe/ditch that is 12 inches or more in Flood Control Zone Permits size/depth, OR Grading/Filling Permit Land Use Permit 4. The project contains or is adjacent to a flood, erosion, or Mining,Excavation or Grading Permit steep slope hazard area as defined in RMC 4-3-050, or or License projects located within a Landslide Hazard Drainage Planned Urban Development Area, OR Rezones 5. Condition 45 does not apply to the City,6 OR Right-of-Way Permits Right-of-Way Use Application 6. The project is a redevelopment project proposing Site Plan Approvals $100,0007 or more of improvements to an existing high- Shoreline Permits use site. Short Subdivision Developments If drain (Short Plat) age review is required for the proposed project,the Special Permits type of drainage review must be determined based on project Subdivision Developments(Plats) and site characteristics as described in Section 1.1.2.The type Temporary Permits when involving of drainage review defines the scope of drainage requirements land disturbance that must be evaluated for compliance with this manual. Other City of Renton permits as required 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS For most projects resulting in 2,000 square feet or more of new plus replaced impervious surface,the full range of core and special requirements contained in Sections 1.2 and 1.3 must be evaluated for compliance through the drainage review process. However, for some types of projects,the scope of requirements applied is narrowed to allow more efficient,customized review. Each of the following five drainage review types tailors the review process and application of drainage requirements to a project's size, location,type of development, and anticipated impacts to the local and regional surface water system: • "Simplified Drainage Review,"Section 1.1.2.1 • "Targeted Drainage Review,"Section 1.1.2.2 • "Directed Drainage Review,"Section 1.1.2.3 • "Full Drainage Review," Section 1.1.2.4 • "Large Project Drainage Review,"Section 1.1.2.5. 5 The thresholds for new impervious surface,replaced impervious surface,and land disturbing activity shall be applied by project site and in accordance with the definitions of these surfaces and activities. 6 Footnote 6 is not used. 7 This is the"project valuation"as declared on the permit application submitted to CED.The dollar amount of this threshold may be adjusted on an annual basis using the local consumer price index(CPI). 12/12/2016 2017 City of Renton Surface Water Design Manual 1-12 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS Each project requires only one of the above drainage review types,with the single exception that a project that qualifies for Simplified Drainage Review may also require Targeted Drainage Review.Figure 1.1.2.A can be used to determine which drainage review type is required. However,this may entail consulting the more detailed thresholds for each review type specified in the above-referenced sections. Table 1.1.2.A can be used to quickly identify which requirements are applied in each type of drainage review. The applicant must evaluate the requirements "checked"for a particular drainage review type to determine what is necessary for compliance. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-13 SECTION IA DRAINAGE REVIEW [FIGUIWI-1.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED Is the project a single family residential project that results in: 0 >2,000 sf of new plus replaced impervious surface or>7,000 sf of land disturbing activity,AND 0 <5,000 sf of new plus replaced pollution generating impervious surface,AND 0 <1/.acre of pollution generating pervious surfaces AND does the project meet one of the following criteria: • The project results in<5001 sf of new plus replaced impervious surface,AND<%acres of new pervious surface. • For projects on predominately till soils:'The project results in<7,947 sf of target impervious surfaces(as defined in Section 1.1.2 1) AND proposed pervious area is<14,941-1-88 x(total target impervious surfaces) • For projects an predominately o�utwash soils:The project results in<6,872 sf of target impervious surfaces(as defined in Section 1,11.2.1) AND proposed pervious area is.<.210,,343-2.96 x(total target impervious surfaces) ...............................................................................................................................................................................................................................I....................................................... No —Yes SIMPLtFIED DRAINAGE ......................................................................................................................................... REVIEW Is the project a single family residential project that results2','1 iin 2,000 sf of Section 1 A new plus replaced impervious surface or>7,000 sf of land disturbing activity Note:The project may also be subject to Targeted Drainage ...................................................................................................................................................................................................................................................................................... Review as determined below I I ............................1_11...... checking to DIRECTED DRAINAGE see it Targeted No —Ye_ REVIEW Drainage Review Section 1,1.2.3 applies Does the project have the characteristics of one or more of the following categories If projects(see more detailed threshold language in Table 1.1.2.A)'? Does the new or I- Projects containing or adjacent to a flood,erosion,or steep slope hazard redevelopment project result area or documented drainage problem;or projects within a Landslide Hazard in>2,000 sf of new plus replaced impervious surface No Drainage Area or Aquifer Protection Area. or 2,7,000 sf of land 2. Projects proposing to construct or modify a drainage pipelditch that is 12"or disturbing activity? larger or receives runoff from a 12"or larger drainage pipelditch- ..................................................... 3. Redevelopment projects proposing>$100,000 in improvements to an existing high-use site- No_ Yes Yes Reassess whether drainage TARGETED,DRAINAGE review is required per Section REVIEW 1.1.1,or use Simplified Section 1.1.2.2 Drainage Review(if applicable). ------------------------------------------------------------------------------ Does the project result in>50 acres of new impervious surface within a No FULL DRAINAGE REVIEW subbasin or multiple subbasins that Section 1.1.2.4 are hydraulically connected? ----------------------------------------------------------------------------- LARGE PROJECT DRAINAGE Yes 10 REVIEW Section 1.1,2.5 12/12/2016 2017 City of Renton Surface Water Design Manual 1-14 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE Single family residential projects that result in>_2,000 sf of new plus replaced impervious Simplified surface or>_7,000 sf of land disturbing activity but do not exceed the new plus replaced PGIS, new PGPS, and new pervious surface thresholds specified in Sec. 1.1.2.1. Note: The project may also be subject to Targeted Drainage Review. Single family residential projects that result in>_2,000 sf of new plus replaced impervious Directed surface or>_7,000 sf of land disturbing activity that are not subject to Simplified Drainage Review or Large Project Drainage Review. The project may also be subject to Targeted Drainage Review. New and redevelopment projects that are not subject to Directed, Full or Large Project Drainage Review,AND have characteristics of one or more of the following categories of projects: 1. Projects containing or adjacent to a flood, erosion, or steep slope hazard area; or projects Targeted within a Landslide Hazard Drainage Area or Aquifer Protection Area. 2. Projects that construct or modify a drainage pipe/ditch that is 12"or larger or receive runoff from a 12"or larger drainage pipe/ditch. 3. Redevelopment projects with Z$100,000 in improvements to a high-use site.(') All projects that result in)_2,000 sf of new plus replaced impervious surface or>_7,000 sf of land Full disturbing activity but are not subject to Simplified Drainage Review, Directed Drainage Review, OR Large Project Drainage Review. Projects that result in>_50 acres of new impervious surface within a subbasin or multiple subbasins Large Project that are hydraulically connected. DRAINAGE REVIEW TYPE Targeted Large Simplified Directed Categ 1 Categ 2 Categ 3 Full Project SIMPLIFIED DRAINAGE REQUIREMENTS SEE NOTE 4 CORE REQUIREMENT#1 ✓i4> -,/(2,3) *(2) ✓ ✓ Discharge at Natural Location CORE REQUIREMENT#2 ✓i4> -,/(2,3) *(2) _43> _43> _43> Offsite Analysis CORE REQUIREMENT#3 ✓i4> -,/(2,3) *(2) _43> _43> Flow Control Facilities CORE REQUIREMENT#4 _144> -,/(2,3) *(2) ✓ ✓ ✓ Conveyance System CORE REQUIREMENT#5 _144> -,/(2,3) ✓ ✓ ✓ ✓ ✓ Erosion&Sediment Control CORE REQUIREMENT#6 _144> -,/(2,3) *(2) ✓ ✓ ✓ ✓ Maintenance&Operations CORE REQUIREMENT#7 _144> -,/(2,3) *(2) _043> _143> _043> _143> Financial Guarantees&Liability CORE REQUIREMENT#8 _144> -,/(2,3) *(2) _043> _143> Water Quality Facilities CORE REQUIREMENT#9 _144) ✓ ✓ ✓ On-site BMPs 2017 City of Renton Surface Water Design Manual 12/12/2016 1-15 SECTION 1.1 DRAINAGE REVIEW TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE DRAINAGE REVIEW TYPE Targeted Large Simplified Directed Categ 1 Categ 2 Categ 3 Full Project SPECIAL REQUIREMENT#1 ✓ia> -,/(2,3) ✓(3> _043> _143> Other Adopted Area-Specific Requirements SPECIAL REQUIREMENT#2 ✓ia> -,/(2,3) ✓(3> _043> _143> Flood Hazard Area Delineation SPECIAL REQUIREMENT#3 ✓ia> -,/(2,3) ✓(3> _043> _143> Flood Protection Facilities SPECIAL REQUIREMENT#4 ✓ia> -,/(2,3) ✓(3> _043> _143> _043> _143> Source Control SPECIAL REQUIREMENT#5 _44> -,/(2,3) ✓(3> _143> _43> Oil Control SPECIAL REQUIREMENT#6 ✓(2,3) _43> _143> _43> _143> _43> Aquifer Protection Areas (1) Category 3 projects installing oil controls that construct or modify a 12-inch pipe/ditch are also Category 2 projects. (2) May be applied by CED based on project or site-specific conditions. Documentation of compliance required. (3) These requirements have exemptions or thresholds that may preclude or limit their application to a specific project. (4) A proposed project subject to Simplified Drainage Review that complies with the Simplified drainage requirements detailed in Appendix C is presumed to comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that would apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2. 1.1.2.1 SIMPLIFIED DRAINAGE REVIEW Simplified Drainage Review is for small residential building projects or clearing projects that meet the threshold requirements below. The core and special requirements applied under Full Drainage Review are replaced with simplified drainage requirements that can be applied by a non-engineer. These requirements include simple stormwater dispersion,infiltration, and site design techniques called flow control Best Management Practices(BMPs),which provide the necessary mitigation of flow and water quality impacts for small projects.Also included are simple measures for erosion and sediment control(ESC).This simplified form of drainage review acknowledges that drainage impacts for many small project proposals can be effectively mitigated without construction of costly flow control and water quality facilities. The Simplified Drainage Review process minimizes the time and effort required to design, submit,review, and approve drainage facilities for these proposals. In most cases,the requirements can be met with submittals prepared by contractors, architects, or homeowners without the involvement of a civil engineer. Note:some projects subject to Simplified Drainage Review may also require Targeted Drainage Review if they meet any of the threshold criteria in Section 1.1.2.2. Threshold Simplified Drainage Review is required for any single family residential project that will result in 2,000 square feet$or more of new impervious surface,replaced impervious surface, or new plus replaced 8 The thresholds of 2,000 and 7,000 square feet shall be applied by project site.All other thresholds specified in terms of square feet of impervious or pervious surface shall be applied by threshold discharge area and in accordance with the definitions of these surfaces in Section 1.1. Note:the calculation of total impervious surface may exclude any such added impervious surface that is confirmed by CED staff to be already mitigated by a City approved and inspected flow control facility or on-site BMP. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-16 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS impervious surface,or 7,000 square feet$ or more of land disturbing activity,AND that meets the following criteria: The project will result in less than 5,000 square feet of new plus replaced pollution generating impervious surface,will result in less than 3/4 acre of new pollution generating pervious surfaces,AND meets one of the following three additional criteria: 1. The project meets the Basic Exemption from flow control in Core Requirement#3: a)the project results in less than 5,000 square feet of new plus replaced impervious surface,AND b)less than 3/4 acres of new pervious surface will be added.Note the Basic Exemption thresholds are applied by project site. 2. For nroiects on predominately till soils: The project results in no more than 7,947 square feet of target impervious surfaces as defined below AND proposed pervious area is equal to or less than 14,941 —1.88 x(total target impervious surfaces). 3. For nroiects on predominately outwash soils: The project results in no more than 6,872 square feet of target impervious surfaces as defined below AND proposed pervious area is equal to or less than 20,343 —2.96 x(total target impervious surfaces). Determination of Target Impervious Surface • If the project is a New Development project, then target impervious surfaces include new plus proposed replaced impervious surface. • If the project is a Redevelopment project where o New impervious surface is less than 5,000 square feet or o Valuation of improvements is less than 50%of the assessed value of the existing site improvements, then target impervious surfaces include new impervious surface. • If the project is a Redevelopment project where o New impervious surface is greater than or equal to 5,000 square feet and o Valuation of improvements is greater than or equal to 50%of the assessed value of the existing site improvements, then target impervious surfaces include new plus proposed replaced impervious surface. Note:for the purposes applying this threshold to a proposed single family residential subdivision (i.e.,plat or short plat project), the impervious surface coverage assumed on each created lot shall be 4,000 square feet or the maximum allowed per Section 3.2.1, whichever is less.A lower impervious surface coverage may be assumed for any lot in which the lower impervious surface coverage is set as the maximum through a declaration of covenant recorded for the lot.Also, the new pervious surface assumed on each created lot shall be the entire lot area, except the assumed impervious portion and anyportion in which native conditions are preserved by a clearing limit per RMC IV, a covenant or easement recorded for the lot, or a tract dedicated by the proposed subdivision. Scope of Requirements IF Simplified Drainage Review is required,THEN the proposed project must comply with the simplified project submittal and drainage design requirements detailed in Simplified Drainage Requirements adopted as Appendix C to this manual. These requirements include simplified BMPs/measures for flow control and erosion and sediment control. Presumption of Compliance with Core and Special Requirements The simplified drainage requirements applied under Simplified Drainage Review are considered sufficient to meet the overall intent of the core and special requirements in Sections 1.2 and 1.3, except under certain conditions when a proposed project has characteristics that trigger Targeted Drainage Review(see the threshold for Targeted Drainage Review in Section 1.1.2.2)and may require the involvement of a civil 2017 City of Renton Surface Water Design Manual 12/12/2016 1-17 SECTION 1.1 DRAINAGE REVIEW engineer. Therefore,any proposed project that is subject to Simplified Drainage Review as determined above and complies with the Simplified drainage requirements detailed in Appendix C is presumed to comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that would apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2. 1.1.2.2 TARGETED DRAINAGE REVIEW Targeted Drainage Review(TDR)is an abbreviated evaluation by CED permit review staff of a proposed project's compliance with selected core and special requirements. Projects subject to this type of drainage review are typically Simplified Drainage Review proposals or other small projects that have site-specific or project-specific drainage concerns that must be addressed by a civil engineer or CED engineering review staff.Under Targeted Drainage Review, engineering costs associated with drainage design and review are kept to a minimum because the review includes only those requirements that would apply to the particular project. Threshold Targeted Drainage Review is required for any proposed project that is subject to drainage review as determined in Section 1.1.1,but is not subject to Directed,Full or Large Project Drainage Review as determined in Sections 1.1.2.3, 1.1.2.4 and 1.1.2.5,AND that has the characteristics of one or more of the following project categories: • TDR Project Category#1:Projects that contain or are adjacent to a flood hazard,erosion hazard area, or steep slope hazard area as defined in RMC 4-3-050; OR projects located within a Landslide Hazard Drainage Area or Aquifer Protection Area.Note:at the discretion of CED, this category may also include any project in Simplified Drainage Review that has a design or site-specific issue that must be addressed by a civil engineer.A project is considered adjacent to a flood,erosion, or steep slope hazard area if any portion of the project site is within 50 feet. • TDR Project Category#2:Projects that propose to construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth or receives surface and storm water runoff from a drainage pipe/ditch that is 12 inches or more in size/depth. • TDR Project Category#3:Redevelopment projects that propose$100,000 or more of improvements to an existing high-use site. Scope of Requirements IF Targeted Drainage Review is required,THEN the applicant must demonstrate that the proposed project complies with the selected core and special requirements corresponding to the project category or categories that best match the proposed project. The project categories and applicable requirements for each are described below and summarized in Table 1.1.2.A. Note:If theproposedproject has the characteristics of more than oneproject category, the requirements of each applicable category shall apply. Compliance with these requirements requires the submittal of engineering plans and calculations stamped by a civil engineer,unless deemed unnecessary by CED and the City of Renton.The engineer need only demonstrate compliance with those core and special requirements that have been predetermined to be applicable based on specific project characteristics as detailed below. The procedures and requirements for submitting engineering plans and calculations can be found in Section 2.3. TDR Project Category#1 This category includes projects that are too small to trigger application of most core requirements,but may be subject to site-specific floodplain or drainage requirements related to certain critical areas, or other area-specific drainage requirements adopted by the City. Such projects primarily include single family residential projects in Simplified Drainage Review. IF the proposed project meets the characteristics of TDR Project Category 41,THEN the applicant must demonstrate that the project complies with the following requirements: 12/12/2016 2017 City of Renton Surface Water Design Manual 1-18 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS • "Core Requirement 45: Erosion and Sediment Control," Section 1.2.5 • "Special Requirement 41: Other Adopted Area-Specific Requirements,"Section 1.3.1 • "Special Requirement 42:Floodplain/Floodway Analysis,"Section 1.3.2 • "Special Requirement 43:Flood Protection Facilities,"Section 1.3.3 • "Special Requirement 44: Source Control,"Section 1.3.4 • "Special Requirement 46:Aquifer Protection Area,"Section 1.3.6. In addition, CED may require the applicant to demonstrate compliance with any one or more of the remaining seven core requirements in Section 1.2 based on project or site-specific conditions.For example,if the proposed project discharges to an erosion or steep slope hazard area as defined in RMC 4-3-050, CED may require compliance with"Core Requirement 41: Discharge at the Natural Location"(Section 1.2.1). This may in turn require compliance with"Core Requirement 42: Offsite Analysis"(Section 1.2.2)if a tightline is required by Core Requirement 41. If a tightline is found to be infeasible, CED may instead require a flow control facility per"Core Requirement 43: Flow Control" (Section 1.2.3). If a tightline is feasible, "Core Requirement 44: Conveyance System"(Section 1.2.4) would be required to ensure proper size and design.Any required flow control facility or tightline system may also trigger compliance with"Core Requirement 46: Maintenance and Operations" (Section 1.2.6), "Core Requirement 47:Financial Guarantees and Liability"(Section 1.2.7), and possibly"Core Requirement 48,Water Quality"(Section 1.2.8)if runoff from pollution-generating impervious surfaces is collected. The applicant may also need to address compliance with any applicable critical areas requirements in RMC 4-3-050 as determined by CED. TDR Project Category#2 This category is intended to apply selected core and special requirements to those projects that propose to construct or modify a drainage system of specified size,but are not adding sufficient impervious surface to trigger Full Drainage Review or Large Project Drainage Review. IF the proposed project meets the characteristics of TDR Project Category 42,THEN the applicant must demonstrate that the proposed project complies with the following requirements: • "Core Requirement 41: Discharge at the Natural Location," Section 1.2.1 • "Core Requirement 42: Offsite Analysis,"Section 1.2.2 • "Core Requirement 44: Conveyance System,"Section 1.2.4 • "Core Requirement 45: Erosion and Sediment Control," Section 1.2.5 • "Core Requirement 46: Maintenance and Operations,"Section 1.2.6 • "Core Requirement 47:Financial Guarantees and Liability,"Section 1.2.7 • "Special Requirement 44: Source Control," Section 1.3.4. TDR Project Category#3 This category is intended to improve water quality by applying source control and oil control requirements to redevelopment projects located on the most intensively used sites developed prior to current water quality requirements. These are referred to as high-use sites. IF the proposed project meets the characteristics of TDR Project Category 43,THEN the applicant must demonstrate that the proposed project complies with the following requirements: • "Core Requirement 45: Erosion and Sediment Control," Section 1.2.5 • "Core Requirement 46: Maintenance and Operations,"Section 1.2.6 • "Core Requirement 47:Financial Guarantees and Liability,"Section 1.2.7 • "Special Requirement 44: Source Control,"Section 1.3.4 • "Special Requirement 45: Oil Control," Section 1.3.5. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-19 SECTION 1.1 DRAINAGE REVIEW Note:In some cases, CED may determine that application of these requirements does not require submittal of engineering plans and calculations stamped by a civil engineer. For example, if catch basin inserts are proposed to meet oil control requirements, engineered plans and calculations may not be necessary.A plot plan showing catch basin locations may suffice. 1.1.2.3 DIRECTED DRAINAGE REVIEW Directed Drainage Review(DDR)is an evaluation of a proposed single family residential project by CED permit review staff to determine a specialized list of submittal(plans,technical reports, etc.) and engineering requirements that ensures compliance with all core and special requirements in this chapter. Projects subject to this type of drainage review are single family residential projects that do not qualify for Simplified Drainage Review. CED staff will review proposals and determine the following:whether the project is exempt from a given core or special requirement based on exemptions and exceptions listed in this Manual;whether a pre- engineered solution is available and feasible for meeting a given core or special requirement;whether a licensed civil engineer is required to comply with a given core or special requirement; and the type of technical report and plan submittal required to document compliance with the core and special requirements. Depending upon a project's site specific conditions,DDR may result in requirements for engineering or documentation that range from following the requirements of Appendix C to those required for full drainage review. CED will provide and/or require documentation of the DDR process and decision making to be included in the project file that demonstrates how compliance with all core and special requirements in this Manual are achieved. Under Directed Drainage Review, engineering costs associated with drainage design and review are minimized because the review is tailored to the particular project. Threshold Directed Drainage Review is required for any single family residential project that results in 2,000 square feet or more of new plus replaced impervious surface or 7,000 square feet or more of land disturbing activity(refer to Section 1.1.1)but is not subject to Simplified Drainage Review or Large Project Drainage Review as determined in Sections 1.1.2.1 and Section 1.1.2.5. Scope of Requirements IF Directed Review is required,THEN the proposed project must comply with the following requirements: All nine core requirements in Section 1.2 All six special requirements in Section 1.3 Engineering plans and calculations stamped by a civil engineer may be required be submitted to demonstrate compliance with these requirements. The procedures and requirements for submittal of engineering plans and calculations are as directed by CED in the DDR process. 1.1.2.4 FULL DRAINAGE REVIEW Full Drainage Review is the evaluation by City staff(CED unless otherwise specified in RMC 4-6-060) of a proposed project's compliance with the full range of core and special requirements in this chapter. This review addresses the impacts associated with changing land cover on typical sites. Threshold Full Drainage Review is required for any proposed project,including a redevelopment project,that is subject to drainage review as determined in Section 1.1.1, OR that meets one or more of the following criteria: 12/12/2016 2017 City of Renton Surface Water Design Manual 1-20 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS • The project will result in 2,000 square feet9 or more of new impervious surface,replaced impervious surface, or new plus replaced impervious surface but is not subject to Simplified Drainage Review or Directed Drainage Review as determined in Sections 1.1.2.1 and 1.1.2.3, OR • The project will result in 7,000 square feet'or more of land disturbing activity but is not subject to Simplified Drainage Review or Directed Drainage Review as determined in Sections 1.1.2.1 and 1.1.2.3. Scope of Requirements IF Full Drainage Review is required,THEN the applicant must demonstrate that the proposed project complies with the following requirements: • All nine core requirements in Section 1.2 • All six special requirements in Section 1.3 Engineering plans and calculations stamped by a civil engineer must be submitted to demonstrate compliance with these requirements. The procedures and requirements for submittal of engineering plans and calculations are found in Section 2.3. 1.1.2.5 LARGE PROJECT DRAINAGE REVIEW Large Project Drainage Review is applied to development proposals that are large and/or involve resources or problems of special sensitivity or complexity. Because of the large size and complexities involved,there is usually a greater risk of significant impact or irreparable damage to sensitive resources. Such proposals often require a more definitive approach to drainage requirements than that prescribed by the core and special requirements in Sections 1.2 and 1.3;it may be appropriate to collect additional information about site resources,use more sophisticated models, and prepare special studies not specified in this manual. Large Project Drainage Review entails preparation of a master drainage plan (MDP) or limited scope MDP that is reviewed and approved by CED. Threshold Large Project Drainage Review is required for any proposed project that is subject to drainage review as determined in Section 1.1.1,AND that would, at full buildout,result in 50 acres or more of new impervious surface within a single subbasin or multiple subbasins that are hydraulically connected'o across subbasin boundaries.Hydraulically connected means connected through surface flow or water features such as wetlands or lakes. Scope of Requirements IF Large Project Drainage Review is required,THEN the applicant must do the following: 1. Prepare a MDP,limited scope MDP, or special study in accordance with the process and requirements described in the MDP guidelines,Master Drainage Planning for Large or Complex Site Developments, available from King County Department of Natural Resources and Parks(DNRP)or CED. The MDP or special study shall be completed, or a schedule for completion identified and agreed to by CED,prior to permit approval.Note: Generally, it is most efficient for the MDP process to parallel the State Environmental Policy Act(SEPA)process. 2. Demonstrate that the proposed project complies with all the core and special requirements in Sections 1.2 and 1.3,with some potential modifications as follows: • Core Requirement 42,Offsite Analysis,is typically modified during MDP scoping. • Core Requirement 43,Flow Control,may be modified to require more sophisticated hydrologic modeling. • Core Requirement 45,ESC,may be modified to require enhanced construction monitoring. 9 The thresholds of 2,000, 5,000,and 7,000 square feet shall be applied by project site. 10 Hydraulically connected means connected through surface flow or water features such as wetlands or lakes. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-21 SECTION 1.1 DRAINAGE REVIEW • Core Requirement 47,Financial Guarantees and Liability,may be modified to implement a monitoring fund. • Special pre-and post-development monitoring may also be required if deemed necessary by CED to adequately characterize sensitive site and downstream resources, and to ensure that onsite drainage controls and mitigation measures are effective in protecting sensitive or critical resources. Detailed guidelines for monitoring are appended to the MDP guidelines referenced above. 1.1.3 DRAINAGE REVIEW REQUIRED BY OTHER AGENCIES Drainage review for a proposed project's impact on surface and storm waters may be addressed by processes or requirements apart from the City's.Agencies such as those listed below may require some form of drainage review and impose drainage requirements that are separate from and in addition to the City's drainage requirements. The applicant is responsible for coordinating with these agencies and resolving any conflicts in drainage requirements. Agency PermiVApproval Seattle/King County Department of Public Onsite Sewage Disposal and Well permits Health Washington State Department of Transportation Developer/Local Agency Agreement Department of Fish and Wildlife Hydraulic Project Approval Department of Ecology Short Term Water Quality Modification Approval Dam Safety permit NPDES Stormwater permit Department of Natural Resources Forest Practices Class IV permit United States Army Corps of Engineers Sections 10, 401, and 404 permits 1.1.4 DRAINAGE DESIGN BEYOND MINIMUM COMPLIANCE This manual presents the City of Renton's minimum standards for engineering and design of drainage facilities. While the City believes these standards are appropriate for a wide range of development proposals,compliance solely with these requirements does not relieve the professional engineer submitting designs of his or her responsibility to ensure drainage facilities are engineered to provide adequate protection for natural resources and public and private property. Compliance with the standards in this manual does not necessarily mitigate all probable and significant environmental impacts to aquatic biota. Fishery resources and other living components of aquatic systems are affected by a complex set of factors. While employing a specific flow control standard may prevent stream channel erosion or instability, other factors affecting fish and other biotic resources(e.g.,increases in stream flow velocities)are not directly addressed by this manual. Likewise, some wetlands,including bogs, are adapted to a very constant hydrologic regime. Even the most stringent flow control standard employed by this manual does not prevent increases in runoff volume,which can adversely affect wetland plant communities by increasing the duration and magnitude of water level fluctuations. Thus,compliance with this manual should not be construed as mitigating all probable and significant stormwater impacts to aquatic biota in streams and wetlands; additional mitigation may be required. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-22 CITY OF RENTON SURFACE WATER DESIGN MANUAL 1.2 CORE REQUIREMENTS This section details the following nine core requirements: • "Core Requirement 41: Discharge at the Natural Location," Section 1.2.1 • "Core Requirement 42: Offsite Analysis,"Section 1.2.2 • "Core Requirement 43:Flow Control," Section 1.2.3 • "Core Requirement 44: Conveyance System," Section 1.2.4 • "Core Requirement 45: Erosion and Sediment Control," Section 1.2.5 • "Core Requirement 46: Maintenance and Operations,"Section 1.2.6 • "Core Requirement 47:Financial Guarantees and Liability,"Section 1.2.7 • "Core Requirement 48: Water Quality,"Section 1.2.8 • "Core Requirement 49: On-site BMPs,"Section 1.2.9 1.2.1 CORE REQUIREMENT #1: DISCHARGE AT THE NATURAL LOCATION All storm water runoff and surface water from a project must be discharged at the natural location so as not to be diverted onto or away from downstream properties. The manner in which stormwater runoff and surface water are discharged from the project site must not create a significant adverse impact to downhill properties or drainage facilities (see"Discharge Requirements"below). Drainage facilities as described above means a constructed or engineered feature that collects,conveys, stores,treats, or otherwise manages surface water or stormwater runoff. "Drainage facility"includes,but is not limited to, a constructed or engineered stream,lake,wetland, or closed depression, or a pipe,channel, ditch,gutter, flow control facility,on-site BMP,water quality facility, erosion and sediment control facility, and any other structure and appurtenance that provides for drainage. Note:Projects that do not discharge all project site runoff at the natural location will require an approved adjustment of this requirement(see Section 1.4). CED may waive this adjustment, however,forprojects in which only a smallportion of the project site does not discharge runoff at the natural location and the runofffrom that portion is unconcentrated and poses no significant adverse impact to downstream properties. Intent: To prevent adverse impacts to downstream properties caused by diversion of flow from one flowpath to another, and to discharge in a manner that does not significantly impact downhill properties or drainage systems. Diversions can cause greater impacts(from greater runoff volumes)than would otherwise occur from new development discharging runoff at the natural location. Diversions can also impact properties that rely on runoff water to replenish wells and ornamental or fish ponds. ❑ DISCHARGE REQUIREMENTS Proposed projects must comply with the following discharge requirements (1,2, and 3)as applicable: 1. Where no conveyance system exists at the abutting downstream property line and the natural (existing)discharge is unconcentrated, any runoff concentrated by the proposed project must be discharged as follows: a) IF the 100-year peak discharge" is less than or equal to 0.2 cfs under existing conditions and will remain less than or equal to 0.2 cfs under developed conditions,THEN the concentrated runoff may be discharged onto a rock pad or to any other system that serves to disperse flows. Peak discharges for applying this requirement are determined using the approved runoff model with 15-minute time steps as detailed in Chapter 3. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-23 SECTION 1.2 CORE REQUIREMENTS b) IF the 100-year peak discharge is less than or equal to 0.5 cfs under existing conditions and will remain less than or equal to 0.5 cfs under developed conditions,THEN the concentrated runoff may be discharged through a dispersal trench or other dispersal system provided the applicant can demonstrate that there will be no significant adverse impact to downhill properties or drainage systems. c) IF the 100-year peak discharge is greater than 0.5 cfs for either existing or developed conditions, or if a significant adverse impact to downhill properties or drainage systems is likely,THEN a conveyance system must be provided to convey the concentrated runoff across the downstream properties to an acceptable discharge point.12 Drainage easements for this conveyance system must be secured from downstream property owners and recorded prior to engineering plan approval. 2. IF a proposed project, or any natural discharge area within a project,is located within a Landslide Hazard Drainage Area and drains over the erodible soils of a landslide hazard with slopes steeper than 15%,THEN a tightline system must be provided through the landslide hazard to an acceptable discharge point unless one of the following exceptions applies. The tightline system must comply with the design requirements in Core Requirement 44 and in Section 4.2.2 unless otherwise approved by CED. Drainage easements for this system must be secured from downstream property owners and recorded prior to engineering plan approval. Exceptions:A tightline is not required for any natural discharge location where CED approves an alternative system based on a geotechnical evaluation/recommendation from a licensed geotechnical engineer that considers cumulative impacts on the hazard area under built out conditions AND one of the following conditions can be met: a) Less than 2,000 square feet of new impervious surface will be added within the natural discharge area, OR b) The developed conditions runoff from the natural discharge area is less than 0.1 cfs for the 100-year runoff event and will be infiltrated for runoff events up to and including the 100-year event, OR c) The developed conditions runoff volume13 from the natural discharge area is less than 50% of the existing conditions runoff volume from other areas draining to the location where runoff from the natural discharge area enters the landslide hazard onto slopes steeper than 15%,AND the provisions of Discharge Requirement 1 are met, OR d) CED determines that a tightline system is not physically feasible or will create a significant adverse impact based on a soils report by a geotechnical engineer. 3. For projects adjacent to or containing a landslide hazard,steep slope hazard area, or erosion hazard area as defined in RMC 4-3-050,the applicant must demonstrate that onsite drainage facilities and/or on-site BMPs will not create a significant adverse impact to downhill properties or drainage systems. 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS All proposed projects must submit an offsite analysis report that assesses potential offsite drainage and water quality impacts associated with development of the project site, and that proposes appropriate 12 Acceptable discharge point means an enclosed drainage system(i.e., pipe system,culvert,or tightline)or open drainage feature(e.g.,ditch,channel,swale,stream, river, pond, lake,or wetland)where concentrated runoff can be discharged without creating a significant adverse impact. 13 For the purposes of applying this exception,the developed conditions runoff volume is the average annual runoff volume as computed per Chapter 3.The analysis is performed using the entire period of record.The total volume is divided by the number of full water years being analyzed to determine the annual average runoff volume.Any areas assumed not to be cleared when computing the developed conditions runoff volume must be set aside in an open space tract or covenant in order for the proposed project to qualify for this exception. Preservation of existing forested areas in Landslide Hazard Drainage Areas is encouraged. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-24 1.2.2 CORE REQUIREMENT#2:OFFSITE ANALYSIS mitigation of those impacts. The initial permit submittal shall include, at minimum, a Level I downstream analysis as described in Section 1.2.2.1 below. If impacts are identified,the proposed projects shall meet any applicable problem-specific requirements specified in Section 1.2.2.2 for mitigation of impacts to drainage problems and Section 1.2.2.3 for mitigation of impacts to water quality problems. Intent: To identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project, and to ensure appropriate measures are provided for preventing creation or aggravation of those problems. In addition,this requirement is intended to ensure appropriate provisions are made, as needed,to mitigate other identified impacts associated with the quantity and quality of surface and storm water runoff from the project site(e.g.,impacts to the hydrology of a wetland as may be identified by a"critical area report"per RMC 4-3-050). The primary component of an offsite analysis report is the downstream analysis,which examines the drainage system within one-quarter mile downstream of the project site or farther as described in Section 1.2.2.1 below. It is intended to identify existing or potential/predictable downstream flooding, erosion, and water quality problems so that appropriate mitigation, as specified in Sections 1.2.2.2 and 1.2.2.3,can be provided to prevent aggravation of these problems.A secondary component of the offsite analysis report is an evaluation of the upstream drainage system to verify and document that significant flooding and erosion impacts will not occur as a result of the proposed project. The evaluation must extend upstream to a point where any backwater effects created by the project cease. ❑ EXEMPTION FROM CORE REQUIREMENT#2 With the exception of: • Projects that trigger Core Requirement 43 (Flow Control Facilities)which must at minimum perform offsite analysis sufficient to identify and address "Downstream Drainage Problems Requiring Special Attention(Section 1.2.2.1.1),Problem Type 4(Potential Impacts to Wetland Hydrology problem)," and • Projects that trigger Core Requirement 4 8 (Water Quality Facilities)which must at minimum perform offsite analysis sufficient to identify and address"Downstream Water Quality Problems Requiring Special Attention(Section 1.2.2.1.2)," a proposed project is exempt from Core Requirement 42 if any one of the following is true: 1. The City of Renton determines there is sufficient information for them to conclude that the project will not have a significant adverse impact on the downstream and/or upstream drainage system, OR 2. The project adds less than 2,000 square feet of new impervious surface,AND less than 3/4 acre of new pervious surface,AND does not construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth or that receives runoff from a drainage pipe/ditch that is 12 inches or more in size/depth, AND does not contain or lie adjacent to a landslide hazard,steep slope hazard area, or erosion hazard area as defined in RMC 4-3-050, OR 3. The project does not change the rate,volume, duration, or location of discharges to and from the project site(e.g.,where existing impervious surface is replaced with other impervious surface having similar runoff-generating characteristics, or where pipe/ditch modifications do not change existing discharge characteristics). 1.2.2.1 DOWNSTREAM ANALYSIS The level of downstream analysis required depends on specific site and downstream conditions. Each project submittal must include at least a Level 1 downstream analysis.Upon review of the Level 1 analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional,more detailed analysis may be required. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-25 SECTION 1.2 CORE REQUIREMENTS The Level l downstream analysis is a qualitative survey of each downstream system and is the first step in identifying flooding problems, erosion problems, or potential impacts to wetland hydrology problems as described below under"Downstream Drainage Problems Requiring Special Attention."The Level 1 analysis also identifies water quality problems as described below under"Downstream Water Quality Problems Requiring Special Attention."Each Level 1 analysis is composed of four tasks at a minimum: • Task 1: Define and map the study area • Task 2: Review all available information on the study area • Task 3:Field inspect the study area • Task 4: Describe the drainage system, and its existing and predicted drainage and water quality problems. Upon review of the Level 1 analysis, CED may require a Level 2 or 3 downstream analysis, depending on the presence of existing or predicted flooding, erosion, or nuisance problems identified in the Level 1 analysis. Levels 2 and 3 downstream analysis quantify downstream flooding, erosion, or nuisance problems by providing information on the severity and frequency of an existing problem or the likelihood of creating a new problem.A Level 2 analysis is a rough quantitative analysis(non-survey field data,uniform flow analysis). Level 3 is a more precise analysis(e.g., survey field data,backwater analysis)of significant problems. If conditions warrant, additional,more detailed analysis may be required beyond Level 3. For Levels 2 and 3 downstream analyses, an additional Task 5, addressing mitigation of existing and potential flooding, erosion, or nuisance problems,will be required. Extent of Downstream Analysis The downstream analysis must consider the existing conveyance system(s) for a minimum flowpath distance downstream of one-quarter mile and beyond that, as needed,to reach a point where the project site area constitutes less than 15%of the tributary area. This minimum distance may be increased as follows: • Task 2 of a Level 1 downstream analysis(described in detail in Section 2.3.1.1)is a review of all available information on the downstream area and is intended to identify existing drainage and water quality problems.In all cases, this information review shall extend one mile downstream of the project site. The existence of flooding or erosion problems further downstream may extend the one-quarter- mile/15%minimum distance for other tasks to allow evaluation of impacts from the proposed development upon the identified flooding or erosion problems. The existence of documented water quality problems beyond the one-quarter-mile/15% distance may in some cases require additional mitigation of impacts as determined necessary by CED based on the type and severity of problem. • If a project's impacts to flooding or erosion problems are mitigated by improvements to the downstream conveyance system,the downstream analysis will extend a minimum of one-quarter mile beyond the improvement. This is necessary because many such improvements result in a reduction of stormwater storage or an increase in peak flows from the problem location. • At their discretion, CED may extend the downstream analysis beyond the minimum distance specified above on the reasonable expectation of drainage or water quality impacts. A detailed description of the scope of offsite analysis and submittal requirements is provided in Section 2.3.1.1. Hydrologic analysis methods and requirements for Levels 2 and 3 downstream analyses are contained in Chapter 3;hydraulic analysis methods are contained in Chapter 4. 1.2.2.1.1 DOWNSTREAM DRAINAGE PROBLEMS REQUIRING SPECIAL ATTENTION While the area-specific flow control facility requirement in Core Requirement 43 (Section 1.2.3.1) serves to minimize the creation and aggravation of many types of downstream drainage problems,there are some types that are more sensitive to creation/aggravation than others depending on the nature or severity of the problem and which flow control facility standard is being applied. In particular,there are four types of downstream drainage problems for which the City has determined that the nature and/or severity of the 12/12/2016 2017 City of Renton Surface Water Design Manual 1-26 1.2.2 CORE REQUIREMENT#2:OFFSITE ANALYSIS problem warrants additional attention through the downstream analysis and possibly additional mitigation to ensure no creation/aggravation: 1. Conveyance system nuisance problem. 2. Severe erosion problem. 3. Severe flooding problem. 4. Potential Impacts to Wetland Hydrology problem. These four types of downstream drainage problem are further described below and precisely defined at the beginning of Chapter 1. Conveyance System Nuisance Problem (Type 1) Conveyance system nuisance problems are minor but chronic flooding or erosion problems that result from the overflow of a constructed conveyance system that is substandard or has become too small as a result of upstream development. Such problems warrant additional attention because of their chronic nature and because they result from the failure of a conveyance system to provide a minimum acceptable level of protection. If a conveyance system nuisance problem is identified or predicted downstream,the need for additional mitigation must be evaluated as specified in Section 1.2.2.2 under"Drainage Problem-Specific Mitigation Requirements". This may entail additional onsite flow control or other measures as needed to prevent creation or significant aggravation of the problem. For any other nuisance problem that may be identified downstream,this manual does not require mitigation beyond the area-specific flow control facility requirement applied in Core Requirement 43 (Section 1.2.3.1)because preventing aggravation of such problems(e.g.,those caused by the elevated water surfaces of ponds,lakes,wetlands, and closed depressions or those involving downstream erosion) can require two to three times as much onsite detention volume,which is considered unwarranted for nuisance problems. However,if under some unusual circumstance,the aggravation of such a nuisance problem is determined by CED to be a significant adverse impact, additional mitigation may be required. Severe Erosion Problem (Type 2) Severe erosion problems can be caused by conveyance system overflows or the concentration of runoff into erosion-sensitive open drainage features.Severe erosion problems warrant additional attention because they pose a significant threat either to health and safety or to public or private property. If a severe erosion problem is identified or predicted downstream, additional mitigation must be considered as specified in Section 1.2.2.2 under"Drainage Problem-Specific Mitigation Requirements". This may entail additional onsite flow control or other measures as needed to prevent creation or aggravation of the problem. Severe Flooding Problem (Type 3) Severe flooding problems(i.e., a severe building flooding problem or severe roadway flooding problem) can be caused by conveyance system overflows or the elevated water surfaces of ponds,lakes,wetlands, or closed depressions.Severe flooding problems warrant additional attention because they pose a significant threat either to health and safety or to public or private property. If a severe flooding problem is identified or predicted downstream,the need for additional mitigation must be evaluated as specified in Section 1.2.2.2 under"Drainage Problem-Specific Mitigation Requirements". This may entail consideration of additional onsite flow control or other measures as needed to prevent creation or significant aggravation of the problem. Potential Impacts to Wetlands Hydrology Problem (Type 4) Potential impacts to wetlands hydrology can be caused by changes in the rate, duration, and quantity of stormwater discharged from the project site to a wetland. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-27 SECTION 1.2 CORE REQUIREMENTS Where wetlands are identified on the site,the applicant shall submit a critical area report at a level determined by CED to adequately evaluate the proposal and probable impacts. Where wetlands are identified off the site AND the project is not exempt from Core Requirement 43,the applicant shall submit a critical area report at a level determined by CED to adequately evaluate the proposal and probable impacts. Based upon the critical area report, CED will determine if the quantity of surface and storm water runoff from a proposed project or threshold discharge area within a proposed project could significantly alter the hydrology of a wetland--in which case, CED will require (as described in Section 1.2.2.2 under"Drainage Problem-Specific Mitigation Requirements"),implementation of additional flow control or other measures to mitigate the adverse impacts of this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5. 1.2.2.1.2 DOWNSTREAM WATER QUALITY PROBLEMS REQUIRING SPECIAL ATTENTION A water quality problem, for the purposes of impact mitigation in this manual,is a situation in which a waterbody of the State is documented by the Federal Government, State, or City to be exceeding or at concern of exceeding the State's numeric water quality standards, or is subject to a federal, state, or City cleanup program or action. Water quality problems and associated water quality standards encompass surface water,groundwater, and sediment quality. The goal of this manual is to prevent creation or significant aggravation of such problems to the maximum extent practicable. While the area-specific water quality facility requirement in Section 1.2.8.1,the source controls required in Section 1.3.4, and the oil controls required in Section 1.3.5 all serve to minimize the creation and aggravation of many types of downstream water quality problems,there are some types that are either not addressed by these requirements(e.g.,temperature problems) or warrant additional measures/considerations to minimize the proposed project's impacts to the maximum extent practicable. In particular,there are currently 7 types of downstream water quality problems for which the City has determined that additional attention needs to be given to preventing or minimizing increases in the pollutant or pollutants of concern discharging from the site. These are as follows: 1. Bacteria Problem 2. Dissolved Oxygen Problem 3. Temperature Problem 4. Metals Problem 5. Phosphorus Problem 6. Turbidity Problem 7. High pH Problem These problems are defined below and the mitigation of impacts to them is addressed in Section 1.2.2.3. Bacteria Problem (Type 1) A bacteria problem is defined as a stream reach,lake, or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standard for fecal coliform as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer14 of these waterbodies,or(2)is currently designated by the City as a bacteria problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standard for fecal coliform.15 14 The link to the Query Tool is<https://fortress.wa.gov/ecy/wets/approvedsearch.aspx>;select all appropriate mediums. The Map Tool is at<https-.//fortress.wa.qov/ecy/wgamapyiewer/default.aspx?res=1280x1024>. 15 Footnote 15 is not used. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-28 1.2.2 CORE REQUIREMENT#2:OFFSITE ANALYSIS Dissolved Oxygen (DO) Problem (Type 2) A dissolved oxygen problem is defined as a stream reach,lake, or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standard for dissolved oxygen as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer14 of these waterbodies, or(2)is currently designated by the City as a DO problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standard for DO. Temperature Problem (Type 3) A temperature problem is defined as a stream reach,lake,or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standard for temperature as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer14 of these waterbodies, or(2)is currently designated by the City as a temperature problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standard for temperature. Metals Problem (Type 4) A metals problem is defined as a stream reach,lake, or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standards for metals(e.g.,copper,zinc,lead, mercury,etc.)as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer16 of these waterbodies, or(2)is currently designated by the City as a metals problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standards for metals(e.g.,copper,zinc,lead, mercury,etc.). Phosphorus Problem (Type 5) A phosphorus problem is defined as a stream reach,lake,or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric action standard for total phosphorus as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer16,14 of these waterbodies, or(2)is currently designated by the City as a nutrient problem based on credible data indicating exceedance or concern for exceedance of the state's numeric action standard for total phosphorus.17,15 Turbidity Problem (Type 6) A turbidity problem is defined as a stream reach,lake, or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standard for turbidity as documented in the state's Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer14,16 of these waterbodies, or(2)is currently designated by the City as a turbidity problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standard for turbidity.17 High pH Problem (Type 7) A High pHproblem is defined as a stream reach,lake, or other waterbody of the state that is either (1)currently designated by the state as a Category 5,4,or 2 Waterbody due to exceedance or concern for exceedance of the state's numeric water quality standard for high pH as documented in the state's 16 The link to the Query Tool is<https://fortress.wa.gov/ecy/wets/approvedsearch.aspx>;select all appropriate mediums. The Map Tool is at<https-.//fortressma.qov/ecy/wgamapyiewer/default.aspx?res=1280x1024>. 17 Footnote 17 is not used. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-29 SECTION 1.2 CORE REQUIREMENTS Water Quality Assessment 303(d)/305(b)Integrated Report and as displayed in WA Ecology's electronic database and map viewer16 of these waterbodies, or(2)is currently designated by the City as a pH problem based on credible data indicating exceedance or concern for exceedance of the state's numeric water quality standard for pH .17 1.2.2.2 DRAINAGE PROBLEM IMPACT MITIGATION A proposed project must not significantly aggravate existing downstream drainage problems or create new problems as a result of developing the site. This manual does not require development proposals to fix or otherwise reduce the severity of existing downstream drainage problems, although doing so may be an acceptable mitigation. Principles of Impact Mitigation for Drainage Problems Aggravation of an existing downstream drainage problem means increasing the frequency of occurrence and/or severity of the problem. Increasing peak flows at the location of a problem caused by conveyance system overflows can increase the frequency of the problem's occurrence. Increasing durations of flows at or above the overflow return frequency can increase the severity of the problem by increasing the depth and duration of flooding. Controlling peaks and durations through onsite detention can prevent aggravation of such problems by releasing the increased volumes from development at return frequencies below the conveyance overflow return frequency,which limits their effect to just causing the conveyance system to flow full for a longer period of time. When a problem is caused by high water-surface elevations of a volume-sensitive water body, such as a lake,wetland, or closed depression, aggravation is the same as for problems caused by conveyance overflows. Increasing the volume of flows to a volume-sensitive water body can increase the frequency of the problem's occurrence. Increasing the duration of flows for a range of return frequencies both above and below the problem return frequency can increase the severity of the problem;mitigating these impacts requires control of flow durations for a range of return frequencies both above and below the problem return frequency. The net effect of this duration control is to release the increased volumes from development only at water surface elevations below that causing the problem,which in turn can cause an increase in these lower,but more frequently occurring,water surface elevations. This underscores an unavoidable impact of development upstream of volume-sensitive water bodies:the increased volumes generated by the development will cause some range of increase in water surface elevations,no matter what detention standard is applied. Creating a new drainage problem means increasing peakflows and/or volumes so that after development, the frequency of conveyance overflows or water surface elevations exceeds the thresholds for the various problem types discussed in Section 1.2.2.1.For example, application of the Peak Rate Flow Control Standard requires matching the existing site conditions 2-and 10-year peak flows. The 100-year peak flow is only partially attenuated, and the flow increase may be enough to cause a severe flooding problem as described in Section 1.2.2.1.1. The potential for causing a new problem is often identified during the Level 1 downstream analysis,where the observation of a reduction in downstream pipe sizes, for example, may be enough to predict creation of a new problem.A Level 2 or 3 analysis will typically be required to verify the capacity of the system and determine whether 100-year flows can be safely conveyed. Significance of Impacts to Existing Drainage Problems The determination of whether additional onsite mitigation or other measures are needed to address an existing downstream drainage problem depends on the significance of the proposed project's predicted impact on that problem.For some identified problems, CED will make the determination as to whether the project's impact is significant enough to require additional mitigation. For Type 1,2, and 3 downstream drainage problems described in Section 1.2.2.1.1,this threshold of significant impact or aggravation is defined below.For a Type 4, "Potential Impacts to Wetland Hydrology problem,"CED will make this determination based on required critical area report findings,the wetland hydrology protection guidelines found in Reference Section 5,the project's relative contribution to the identified wetland's hydrology, and the mitigation proposed in meeting other requirements(e.g., flow control facilities and on-site BMPs). 12/12/2016 2017 City of Renton Surface Water Design Manual 1-30 1.2.2 CORE REQUIREMENT#2:OFFSITE ANALYSIS For conveyance system nuisance problems,the problem is considered significantly aggravated if there is any increase in the project's contribution to the frequency of occurrence and/or severity of the problem for runoff events less than or equal to the 10-year event.Note:Increases in the project's contribution to this type ofproblem are considered to be prevented if suff cient onsite flow control and/or offsite improvements are provided as specified in Table 1.2.3.A. For severe erosion problems,the problem is considered significantly aggravated if there is any increase in the project's existing contribution to the flow duration18 of peak flows ranging from 50%of the 2-year peak flow up to the full 50-year peak flow at the eroded area.Note:Increases in the project's contribution to this type ofproblem are considered to be prevented if Flow Control Duration Standard or offsite improvements are provided as specified in Table 1.2.3.A. For severe building flooding problems,the problem is considered significantly aggravated if there is any increase in the project's existing contribution19 to the frequency, depth, or duration of the problem for runoff events less than or equal to the 100-year event. For severe roadway flooding problems,the problem is considered significantly aggravated if any of the following thresholds are exceeded and there is any increase in the project's existing contribution19 to the frequency, depth, or duration of the problem for runoff events less than or equal to the 100-year event: • The existing flooding20 over all lanes of a roadway or overtopping the culverted section of a sole access driveway is predicted to increase in depth more than a quarter-inch or 10%(whichever is greater)for the 100-year runoff event. • The existing flooding over all lanes of a roadway or severely impacting a sole access driveway is more than 6 inches deep or faster than 5 feet per second for runoff events less than or equal to the 100-year event.A severely impacted sole access driveway is one in which flooding overtops a culverted section of the driveway,posing a threat of washout or unsafe access conditions due to indiscernible driveway edges, or flooding is deeper than 6 inches on the driveway,posing a severe impediment to emergency access. • The existing flooding over all lanes of a sole access roadway2l is more than 3 inches deep or faster than 5 feet per second for runoff events less than or equal to the 100-year event, or is at any depth for runoff events less than or equal to the 10-year event. ❑ DRAINAGE PROBLEM-SPECIFIC MITIGATION REQUIREMENTS L IF a proposed project or threshold discharge area within a project drains to one or more of Type 1, Type 2, or Type 3 downstream drainage problems described in Section 1.2.2.1 as identified through a downstream analysis,THEN the applicant must do one of the following: a) Submit a Level 2 or Level 3 downstream analysis per Section 2.3.1 demonstrating that the proposed project will not create or significantly aggravate the identified downstream drainage problem(s), OR b) Show that the natural discharge area or threshold discharge area draining to the identified problem(s) qualifies for an exemption from Core Requirement 43: Flow Control(Section 1.2.3)or 18 Flow duration means the aggregate time that peak flows are at or above a particular flow rate(e.g.,the amount of time over the last 50 years that peak flows were at or above the 2-year flow rate). Note:flow duration is not considered to be increased if it is within the tolerances specified in Chapter 3. 19 Increases in the project's contribution are considered to be prevented if sufficient onsite flow control and/or offsite improvements are provided as specified for severe flooding problems in Table 1.2.3.A. For severe flooding problems located within the mapped 100-year floodplain of a major receiving water(see Table 1.2.3.B)or the mapped 100-year floodplain of a major stream for which there is an adopted basin plan, increases in the project's contribution are considered negligible(zero)regardless of the flow control standard being applied, unless CED determines there is a potential for increased flooding separate from that associated with the existing 100-year floodplain. 20 Existing flooding,for the purposes of this definition,means flooding over all lanes of the roadway or driveway has occurred in the past and can be verified by County records, County personnel, photographs,or other physical evidence. 21 Sole access roadway means there is no other flood-free route for emergency access to one or more dwelling units. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-31 SECTION 1.2 CORE REQUIREMENTS an exception from the applicable area-specific flow control facility requirement per Section 1.2.3.1, OR c) Document that the applicable area-specific flow control facility requirement specified in Core Requirement 43 is adequate to prevent creation or significant aggravation of the identified downstream drainage problem(s)as indicated in Table 1.2.3.A with the phrase, "No additional flow control needed,"OR d) Provide additional onsite flow control necessary to prevent creation or significant aggravation of the downstream drainage problem(s) as specified in Table 1.2.3.A and further detailed in Section 3.3.5, OR e) Provide offsite improvements necessary to prevent creation or significant aggravation of the identified downstream drainage problem(s) as detailed in Chapter 3 unless identified as not necessary in Table 1.2.3.A, OR f) Provide a combination of additional onsite flow control and offsite improvements sufficient to prevent creation or significant aggravation of the downstream drainage problem(s) as demonstrated by a Level 2 or Level 3 downstream analysis. 2. IF it is identified that the manner of discharge from a proposed project may create a significant adverse impact as described in Core Requirement 41,THEN CED may require the applicant to implement additional measures or demonstrate that the impact will not occur. 3. IF it is identified through a critical area review as described under"Potential Impacts to Wetlands Hydrology Problem(Type 4),"that the quantity of surface and storm water runoff from a proposed project or threshold discharge area within a proposed project could significantly alter the hydrology of a wetland(Type 4 problem),THEN CED shall require the applicant to implement additional flow control or other measures to mitigate the adverse impacts of this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5. Intent: To ensure provisions are made (if necessary)to prevent creation or significant aggravation of the four types of downstream drainage problems requiring special attention by this manual, and to ensure compliance with the discharge requirements of Core Requirement 41. In addressing downstream drainage problems per Problem-Specific Mitigation Requirement 1 above, additional onsite flow control will often be the easiest provision to implement. This involves designing the required onsite flow control facility to meet an additional set of performance criteria targeted to prevent significant aggravation of specific downstream drainage problems. To save time and analysis, a set of predetermined flow control performance criteria corresponding to each of the three types of downstream drainage problems is provided in Table 1.2.3.A and described in more detail in Chapter 3. Note that in some cases the area-specific flow control facility requirement applicable to the proposed project per Section 1.2.3.1 is already sufficient to prevent significant aggravation of many of the defined downstream drainage problem types. Such situations are noted in Table 1.2.3.A as not needing additional onsite flow control or offsite improvements.For example, if the project is located within a Flow Control Duration Standard Area subject to the Flow Control Duration Standard per Section 1.2.3.1.13, and a conveyance system nuisance problem is identified through offsite analysis per Core Requirement 42,no additional onsite flow control is needed, and no offsite improvements are necessary. 1.2.2.3 WATER QUALITY PROBLEM IMPACT MITIGATION As stated in Section 1.2.2.1,the goal of this manual is to prevent creation and/or significant aggravation of water quality problems to the maximum extent practicable. This is accomplished through a number of Mitigation requirements,including(1)the area-specific water quality facility requirement in Section 1.2.8.1, (2)any mitigation required by other adopted area-specific requirements per Special Requirement 41, Section 1.2.9, (3)the source controls required in Special Requirement 44, Section 1.3.4, (4)the oil control required in Special Requirement 45, Section 1.3.5, and(5)the water quality problem- 12/12/2016 2017 City of Renton Surface Water Design Manual 1-32 1.2.2 CORE REQUIREMENT#2:OFFSITE ANALYSIS specific mitigation requirements presented in this section.Note that this manual does not require development proposals to fix or otherwise reduce the severity of existing downstream water quality problems, although doing so may be an acceptable mitigation. ❑ WATER QUALITY PROBLEM-SPECIFIC MITIGATION REQUIREMENTS IF a proposed project drains to one or more of the 7 types of downstream water quality problems defined in Section 1.2.2.1 as identified through a downstream analysis,THEN the applicant must comply with the following problem-specific mitigation requirements that apply.Note that CED may require additional measures if the opportunity exists to further mitigate the pollutants of concern associated with these types of problems. Bacteria Problem (Type 1) IF the proposed project drains to a bacteria problem located within the quarter mile/15%distance downstream(or beyond as deemed necessary by CED),THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement 48,THEN a sand filter or stormwater wetland shall be used to meet the area-specific water quality facility requirement. Sand filters are the preferred option. Other treatment options for meeting the area-specific facility requirement may be used in lieu of a sand filter or stormwater wetland only if combined with an emerging technology treatment method that provides equivalent removal of fecal coliform as demonstrated through an experimental design adjustment per Section 1.4. 2. IF the proposed project is a residential subdivision,THEN signage shall be provided in the subdivision's public areas(i.e.,recreation/open space areas and right-of-way)requesting that pet waste be picked up in order to protect downstream water quality. The extent and location of this signage shall be reviewed and approved by CED. 3. IF the proposed project is a multifamily development with a recreation/open area or is a park improvement,THEN signage shall be provided requesting that pet waste be picked up in order to protect downstream water quality. The extent and location of this signage shall be reviewed and approved by CED. Dissolved Oxygen (DO) Problem (Type 2) IF the proposed project drains to a DO problem located within the quarter mile/15%distance downstream (or beyond as deemed necessary by CED),THEN the following requirements must be met as applicable: L IF the proposed project includes a wetpond or wetvault,THEN the wetpool depth shall not exceed 6 feet,AND the outflow system shall include a measure designed to promote aeration of the facility's discharges for 2-year runoff events and smaller. One way to do this is to create a drop in flow elevation within a manhole by placing the outlet invert of the incoming pipe a minimum of 12 inches above the 2-year headwater elevation of the outgoing pipe.Alternatively,if the outflow system discharges to an open channel,the same drop in flow elevation could be achieved by placing the outlet invert a minimum of 12 inches above the 2-year tailwater elevation created by the channel. Other equivalent approaches may be used as approved by CED. 2. IF the proposed project includes a wetvault,THEN the required ventilation area specified in Chapter 6 shall be doubled. 3. IF the DO problem is documented to be caused by excessive phosphorus and a water quality facility is required per Core Requirement 48,THEN a water quality facility option from the Sensitive Lake Protection menu shall be a component of the required treatment system. Temperature Problem (Type 3) IF the proposed project drains to a temperature problem located within the quarter mile/15%distance downstream(or beyond as deemed necessary by CED),THEN the following requirements must be met as applicable: 2017 City of Renton Surface Water Design Manual 12/12/2016 1-33 SECTION 1.2 CORE REQUIREMENTS L IF a water quality facility is required per Core Requirement 48,THEN use of a wetpond is prohibited unless it will be at least 50%shaded at midday in the summer or its discharges will flow through 200 feet or more of open channel that is at least 50% shaded at midday in the summer. CED shall review and approve the extent and location of this shading. 2. IF the proposed project includes open drainage features,THEN vegetation or other means shall be used where practicable to maximize shading of the drainage features, except bioswales and filter strips. The extent and location of this shading shall be reviewed and approved by CED. Metals Problem (Type 4) IF the proposed project drains to a metals problem located within the quarter mile/15%distance downstream(or beyond as deemed necessary by CED),THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement 48,THEN a water quality facility option from the Enhanced Basic WQ menu shall be a component of the project's required treatment system. 2. IF the proposed project is a residential subdivision,THEN a covenant shall be recorded for each lot and common area tract prohibiting use of leachable heavy metals(e.g.,galvanized metals)that will be exposed to the weather(use the covenant in Reference Section 8-Q). 3. IF the proposed project includes road right-of-way improvements,THEN use of leachable heavy metals(e.g.,galvanized metals)that will be exposed to the weather(e.g.,guard rails,street lights, etc.)shall be avoided. Phosphorus Problem (Type 5) IF the proposed project drains to a phosphorus problem located within the quarter mile/15%distance downstream(or beyond as deemed necessary by CED),THEN the following requirements must be met as applicable: L IF a water quality facility is required per Core Requirement 48,THEN the project shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area for the purposes of applying the area-specific water quality treatment requirement in Section 1.2.8.1. 2. For the purposes of applying the Erosion and Sediment Control Standards in Appendix D,the project shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area. Turbidity Problem (Type 6) IF the proposed project drains to a turbidity problem located within the quarter mile/15% distance downstream(or beyond as deemed necessary by CED)AND the downstream flow path from the project site to the turbidity problem is through a landslide hazard,steep slope hazard area,erosion hazard area or any actively eroding area,THEN the project shall provide a tightline system through the area in accordance with the same criteria and exceptions specified in Core Requirement 41,Discharge Requirement 2 for projects located within a designated Landslide Hazard Drainage Area. Other means for safely conveying project site discharges through the area of concern for erosion may be proposed subject to approval by CED. High pH Problem (Type 7) IF the proposed project drains to a pH problem located within the quarter mile/15%distance downstream (or beyond as deemed necessary by CED)AND the proposed project includes a concrete vault structure for stormwater control purposes,THEN the vault's submerged surfaces shall be coated or otherwise treated to prevent alteration of pH. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-34 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES All proposed projects,including redevelopment projects,must provide onsite flow control facilities to mitigate the impacts of storm and surface water runoff generated by new impervious surface,new pervious surface, and replaced impervious surface targeted for flow mitigation as specified in the following sections. Flow control facilities must be provided and designed to perform as specified by the area-specific flow control facility requirement in Section 1.2.3.1 and in accordance with the applicable flow control facility implementation requirements in Section 1.2.3.2. Intent: To ensure the minimum level of control needed to protect downstream properties and resources from increases in peak, duration, and volume of runoff generated by new development. The level of control varies depending on location and downstream conditions identified under Core Requirement 42. ❑ EXEMPTION FROM CORE REQUIREMENT#3 There is a single exemption from the flow control provisions of Core Requirement 43: Basic Exemption A proposed project is exempt if it meets the following criteria: 1. Less than 5,000 square feet of new plus replaced impervious surface will be created,AND 2. Less than 3/4 acres of new pervious surface will be added. 1.2.3.1 AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Projects subject to Core Requirement 43 must provide flow control facilities as specified by the area- specific facility requirements and exceptions for the designated flow control area in which the proposed project or threshold discharge area of the proposed project is located as described in Subsections A,B, and C below. Guide to Applying the Area-Specific Flow Control Facility Requirement The flow control facility requirement varies across the City according to the flow control area within which the project or a threshold discharge area of the project is located. There are currently four such flow control areas,three of which are depicted on the City of Renton Flow Control Applications Map adopted in Reference Section 15-A of this manual or viewed via COR Maps ( htt :// rentonwa. ov/Silverli htPublic/�Iiewer,html?�Jiewer=COR.-Ma sem). These are referred to as follows.' 1. Flow Control Duration Standard-Matching Forested site conditions for areas draining to streams and subject to flow-related water quality problems such as erosion or sedimentation. 2. Flow Control Duration Standard Matching Existing site conditions in designated highly urbanized areas draining to streams that are currently stable or showing no impacts caused by high flows. 3. 'Peak Rate Flow Control Standard—Matching Existing site conditions 2, 10 and 100-year peak- rate runoff for areas draining to constructed(man-made)or highly modified drainage systems so as not to create a downstream flooding problem. 4. Flood Problem Flow Control Standard-Although no Flood Problem Flow Control Standard areas are currently shown on the map,the City may apply this standard where projects discharge to a severe flooding or erosion problems.The standard includes flowcontrol for duration matching forested (historical)conditions for the 2-year through the 100-year return frequencies. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-35 SECTION 1.2 CORE REQUIREMENTS Guide to Applying the Area-Specific Flow Control Facility Requirement (cont.) Note that the minimum required performance of the facility as specified by this requirement may need to be increased to ensure that downstream drainage problems are not created or significantly aggravated as set forth in Section 1.2.2.2,"Drainage Problem-Specific Mitigation Requirements."Table 1.2.3.A provides a quick guide for selecting the flow control performance criteria necessary to meet both the area-specific flow control facility requirement and the problem-specific mitigation;requirement. This is further explained in Step 4 below. For efficient application of the flow control facility requirement,the following steps are recommended: 1. Check the Direct Discharge Exemption in Section 1.2.3.1 to determine if and/or which portions of your project are exempt from the flow control facility requirement. If exempt from the flow control facility requirement,proceed to Step 6. 2. 'Use the City of Renton Flow Control Applications Map(Reference Section 15-A)to determine the flow control area in which your project is located. 3. Consult the detailed requirement and exception language for the identified flow control area to determine if and how the flow control facility requirement applies to your project. This requirement and exception language is detailed on subsequent pages for each of the flow control areas depicted on the City of Renton Flow Control Applications Map.If a flow control facility is not applicable per the area-specific exceptions,proceed to Step 6. 4. If downstream drainage problems were identified throughoffsite analysis per Core Requirement 42 and are proposed to be addressed through onsite flow control,use Table 1.2.3.A to determine if and what additional flow control,performance is necessary to mitigate impacts(i.e.,to prevent creation or aggravation of the identified problems). 5. Use Section 1.2.3.2 to identify the applicable requirements for implementing the flow control facility' requirement. These requirements cover facility siting,analysis and design,unusual situations,and other site-specific considerations. 6. Use Core Requirement 49 to identify the on-site BMPs that must be applied to yourproject site regardless of whether a flow control facility is required. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-36 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES TABLE 1.2.3.A SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT MITIGATION"' AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Flow Control Flow Control Duration IDENTIFIED Peak Rate Flow Duration Standard Standard Matching Flood Problem Flow PROBLEM Control Standard Matching Existing Forested Condition Control Standard DOWNSTREAM Areas Condition Areas Areas Areas No Problem Apply the Peak Rate Apply the Flow Apply the Flow Control Apply the existing or Identified Flow Control Control Duration Duration Standard historical site Apply the minimum Standard,which Standard,which which matches the flow conditions Flow area-specific flow matches the 2-, 10-, matches the flow duration of pre- Control Duration control performance and 100-year peaks duration of pre- developed rates for Standard (whichever criteria. developed rates for forested (historical) is appropriate based existing site site conditions over on downstream flow conditions over the the range of flows control areas)AND range of flows extending from 50%of match existing site extending from 50% 2-year up to the full 50- conditions 100-year of 2-year up to the year flow AND matches peaks full 50-year flow AND peaks for the 2-and matches peaks for 10-year return periods the 2-and 10-year return periods. Type 1 Drainage Additional Flow Additional Flow No additional flow No additional flow Problem Control Control control or other control or other Conveyance Hold 10-year peak to The City may require mitigation is needed mitigation is needed System Nuisance overflow Tr peak(2)(3) design adjustments Problem to meet the Flow Control Duration Standard matching forested (historical) conditions. Type 2 Drainage Additional Flow Additional Flow No additional flow No additional flow Problem Control Control control is needed, but control is needed, but Severe Erosion Apply the Flow Apply the Flow other mitigation may be other mitigation may Problem Control Duration Control Duration required(4) be required(4) Standard matching Standard matching forested (historical) forested (historical) conditions(3)(4) conditions.(3)(4) Type 3 Drainage Additional Flow Additional Flow Additional Flow Additional Flow Problem Control Control Control Control Severe Flooding Apply the Flow Apply the Flow If flooding is from a If flooding is from a Problem Control Duration Control Duration closed depression, closed depression, Standard matching Standard matching make design make design forested(historical) forested(historical) adjustments as needed adjustments as conditions. If conditions. If to meet the"special needed to meet the flooding is from a flooding is from a provision for closed "special provision for closed depression, closed depression, depressions"(3)(5) closed depressions" make design make design (3)(5) adjustments as adjustments as needed to meet the needed to meet the "special provision for "special provision for closed closed depressions"(3)(5) depressions"(3)(5) 2017 City of Renton Surface Water Design Manual 12/12/2016 1-37 SECTION 1.2 CORE REQUIREMENTS TABLE 1.2.3.A SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT MITIGATION"' AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Flow Control Flow Control Duration IDENTIFIED Peak Rate Flow Duration Standard Standard Matching Flood Problem Flow PROBLEM Control Standard Matching Existing Forested Condition Control Standard DOWNSTREAM Areas Condition Areas Areas Areas Type 4 Potential Additional Flow Additional Flow Additional Flow Additional Flow Impact to Wetland Control Control Control Control Hydrology as The City may require The City may require The City may require The City may require Determined through design adjustments design adjustments design adjustments per design adjustments a Critical Area per the wetland per the wetland the wetland hydrology per the wetland Review per RMC hydrology protection hydrology protection protection guidelines in hydrology protection Title IV guidelines in guidelines in Reference Section 5. guide-lines in Reference Section 5. Reference Section 5. Reference Section 5. Notes: (1) More than one set of problem-specific performance criteria may apply if two or more downstream drainage problems are identified through offsite analysis per Core Requirement#2. If this happens,the performance goals of each applicable problem-specific criterion must be met.This can require extensive,time-consuming analysis to implement multiple sets of outflow performance criteria if additional onsite flow control is the only viable option for mitigating impacts to these problems. In these cases,it may be easier and more prudent to implement the Flow Control Duration Standard matching forested conditions standard in place of the otherwise required area- specific standard. Use of the Flow Control Duration Standard matching forested conditions standard satisfies the specified performance criteria for all the area-specific and problem-specific requirements except if adjustments are required per the special provision for closed depressions described below in Note 5. (2) Overflow T,is the return period of conveyance system overflow.To determine T,requires a minimum Level 2 downstream analysis as detailed in Section 2.3.1.1.To avoid this analysis,a T,.of 2 years may be assumed. (3) Offsite improvements may be implemented in lieu of or in combination with additional flow control as allowed in Section 1.2.2.2 and detailed in Section 3.3.5. (4) A tightline system may be required regardless of the flow control standard being applied if needed to meet the discharge requirements of Core Requirement#1 or the outfall requirements of Core Requirement#4,or if deemed necessary by the City of Renton where the risk of severe damage is high. (5) Special Provision for Closed Depressions with a Severe Flooding Problem: IF the proposed project discharges by overland flow or conveyance system to a closed depression experiencing a severe flooding problem AND the amount of new impervious surface area proposed by the project is greater than or equal to 10%of the 100-year water surface area of the closed depression,THEN use the"point of compliance analysis technique"described in Section 3.3.6 to verify that water surface levels are not increasing for the return frequencies at which flooding occurs, up to and including the 100-year frequency. If necessary, iteratively adjust onsite flow control performance to prevent increases. Note:The point of compliance analysis relies on certain field measurements taken directly at the closed depression(e.g.,soils tests,topography, etc.).If permission to enter private property for such measurements is denied, the City of Renton may waive this provision and apply the Flow Control Duration Standard matching forested conditions standard with a mandatory 20%safety factor on the storage volume. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-38 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES ❑ DIRECT DISCHARGE EXEMPTION Any onsite natural drainage area is exempt from the flow TABLE 1.2.3.B control facility requirement if the area drains to one of the MAJOR RECEIVING WATERS22 major receiving waters listed in Table 1.2.3.13,AND meets the following criteria for direct discharge23 to that receiving . Cedar River downstream of Taylor water: Creek confluence 1. The flowpath from the project site discharge point to the • Johns Creek downstream of edge of the 100-year floodplain of the major receiving Interstate-405 (1-405)east right-of- water will be no longer than a half mile, except for way discharges to Lake Washington,AND • Lake Washington 2. The conveyance system between the project site and the Note: The major receiving waters listed major receiving water will extend to the ordinary high above do not include side adjacent or water mark, and will be comprised of manmade associated channels, spring-or conveyance elements (pipes, ditches, etc.)and will be groundwater-fed streams, or wetlands. within public right-of-way or a public or private drainage easement,AND 3. The conveyance system will have adequate capacity24 to convey the 25-year peak flow(per Core Requirement 44,Conveyance System), for the entire contributing drainage area, assuming build-out conditions to current zoning for the equivalent area portion(the area that is contained within an arc formed by the shortest, straight line distance from the conveyance system discharge point to the furthermost point of the proposed project)and existing conditions for the remaining area,AND 4. The conveyance system will be adequately stabilized to prevent erosion,assuming the same basin conditions as assumed in Criteria(c)above,AND 5. The direct discharge proposal will not divert flows from or increase flows to an existing wetland or stream sufficient to cause a significant adverse impact. A. PEAK RATE FLOW CONTROL STANDARD AREAS The Peak Rate Flow Control Standard is a peak-rate matching standard intended to prevent increases of peak flows for specific events rather than match flow-durations over a range of flows.The standard is appropriate for use in areas where the concern is flooding rather than stream bed erosion. Within the City of Renton,this standard is allowed for those areas that are highly urbanized prior to 1985 and that drain to pipes or non-fish bearing constructed conveyance systems leading to the lower Cedar River, Lake Washington or the portion of the Green River Valley floor located in Renton. Minimum Required Performance Facilities in Peak Rate Flow Control Standard Areas must comply with the following flow control performance standards and assumptions unless modified by offsite analysis per Core Requirement 42(see Table 1.2.3.A): Peak Rate Flow Control Standard: Match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10-, and 100-year return periods. Intent The Peak Rate Flow Control Standard is intended to protect flow-carrying capacity and limit increased erosion within the downstream conveyance system for runoff events less than or equal to the 10-year event. Matching the 2-, 10-, and 100-year peak flows is intended to prevent increases in return-frequency peak flows less than or equal to the 100-year peak flow down to the 2-year peak 22 Footnote 22 is not used. 23 Direct discharge means undetained discharge from a proposed project to a major receiving water. 24 Note: The City does not charge a special use fee. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-39 SECTION 1.2 CORE REQUIREMENTS flow. This level of control is also intended to prevent creation of new conveyance system nuisance problems as described in Section 1.2.2.1. Effectiveness in Addressing Downstream Drainage Problems While the Peak Rate Flow Control Standard provides reasonable protection from many development-induced conveyance problems(up to the 100-year event),it does not prevent increases in runoff volumes or flow durations that tend to aggravate the three types of downstream drainage problems described in Section 1.2.2.1. Consequently,if one or more of these problems are identified through offsite analysis per Core Requirement 42, additional onsite flow control and/or offsite improvements will likely be required(see "Drainage Problem-Specific Mitigation Requirements"in Section 1.2.2.2). Target Surfaces Facilities in Peak Rate Flow Control Standard Areas must mitigate (either directly or in effect)the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New impervious surface that is not fully dispersed per the criteria in Section 1.2.3.2.0 as specified in Appendix C. For individual lots within residential subdivision projects,the extent of new impervious surface shall be assumed as specified in Chapter 3.Note, any new impervious surface such as a bridge or boardwalk that spans the ordinary high water of a stream,pond, or lake may be excluded as a target surface if the runofffrom such span is conveyed to the ordinary high water area in accordance with Criteria(b), (c), (d), and(e)of the `Direct Discharge Exemption" (p 1-39). 2. New pervious surface that is not fully dispersed as specified in Appendix C.For individual lots within residential subdivision projects,the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion and any portion in which native conditions are preserved by covenant,tract, or easement. In addition,the new pervious surface on individual lots shall be assumed to be 100%grass. Exceptions The following exceptions apply only in Peak Rate Flow Control Standard Areas: 1. The facility requirement in Peak Rate Flow Control Standard Areas is waived for any threshold discharge area in which the target surfaces subject to this requirement will generate no more than a 0.15-cfs increase(when modeled using 15 minute time steps)in the existing site conditions 100-year peak flow(modeled using same time step unit(e.g., 15-minute)used to calculate the developed flow). Note:for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A. 2. The facility requirement in Peak Rate Flow Control Standard Areas may be waived for any threshold discharge area of a redevelopment project in which all of the following criteria are met: a) The target surfaces subject to the Peak Rate Flow Control Standard Areas facility requirement will generate no more than a 0.15-cfs increase(when modeled using 15-minute time steps)in the existing site conditions 100-year peak flow(modeled using same time step unit(e.g., 15-minute) used to calculate the developed flow) at any natural discharge location from the project site (note:for the purposes of this calculation, target surfaces served by on-site BMPs perAppendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A,AND b) The increased runoff from target surfaces will not significantly impact a critical area,severe flooding problem, or severe erosion problem. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-40 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES B. FLOW CONTROL DURATION STANDARD AREAS The flow control duration standard requires runoff from urban developments to be detained and released at a rate that matches the flow duration of predeveloped rates over the range of flows extending from'/z of the 2-year up to the 50-year flow.Also match developed peak discharge rates to predeveloped peak discharge rates for the 2-and 10-year return periods. Flow duration specifies the cumulative amount of time that various flows are equaled or exceeded during a long-term simulation using historical rainfall. The target flow duration may be the"historical"(i.e., fully forested condition) or in specific situations it may be the existing site or"pre-project"condition as described below. The Flow Control Applications Map shows the areas where the "forested"and"existing"conditions are allowed. Forested land cover—Runoff from the developed site will be controlled and released at a rate that matches the flow duration for a forested("historical")land cover. The"historical"land cover is the default standard required by the technical requirements of the NPDES permit. The standard is applicable to those areas draining to streams that have erodible channels where runoff from urban areas has the potential to destabilize the channel. Existing land cover—Runoff from the developed site will be controlled and released at a rate that matches the flow duration for the site conditions existing before the development. These are areas that have been developed for years and drain to stream channels that have become stabilized to a new hydrologic regime. Ecology has proposed that the existing land cover can be used in basins that have had at least 40%total impervious surface area for the 20 years preceding Ecology's adoption of the 2005 Stormwater Management Manual for Western Washington(called the 40/20 rule) and the stream channels receiving the runoff are considered stable from the standpoint of excessive erosion or sedimentation. In developing the"40/20 rule"for highly urbanized basins,Ecology conducted a preliminary analysis and produced maps that identify those areas that may meet the criteria.Portions of Renton were included in the initial maps prepared by Ecology. These maps have been adjusted to better represent the areas that were 40%impervious in 1985 as well as drainage basin divides within the City. Flow control facilities designed to the"40/20 rule"will only have to mitigate for the added impervious surface.As a result,these flow control facilities will be smaller than those required to be designed to match runoff from a fully forested site. Minimum Required Performance Facilities in Flow Control Duration Standard Areas must comply with the following flow control performance standard and assumptions unless modified by offsite analysis per Core Requirement 42(see Table 1.2.3.A): Flow Control Duration Standard Matching Forested Site Conditions: Developed discharge durations shall not exceed predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Developed peak discharge rates shall not exceed predeveloped peak discharge rates for the 2- and 10-year return periods.Assume forested (historical)site conditions as the predeveloped condition. Flow Control Duration Standard Matching Existing Site Conditions: Developed discharge durations shall not exceed predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow.Developed peak discharge rates shall not exceed predeveloped peak discharge rates for the 2-and 10-year return periods. Assume existing site conditions as the predeveloped condition. Intent The Flow Control Duration Standard flow control standard assuming historical site conditions is intended to limit the amount of time that erosive flows are at work generating erosion and sedimentation within natural and constructed drainage systems. Such control is effective in preventing development- induced increases in natural erosion rates and reducing existing erosion rates where they may have been increased by past development of the site. This is accomplished by maintaining at historical predevelopment levels the aggregate time that developed flows exceed an erosion-causing threshold(i.e., 2017 City of Renton Surface Water Design Manual 12/12/2016 1-41 SECTION 1.2 CORE REQUIREMENTS 50% of the historical 2-year peak flow). Maintaining natural erosion rates within streams and their tributary areas is important for preventing increases in stream channel erosion and sediment loading that are detrimental to salmonid habitat and production. Effectiveness in Addressing Downstream Drainage Problems While the Flow Control Duration Standard flow control standard assuming historical site conditions provides a reasonable level of protection for preventing most development-induced problems,it does not necessarily prevent increases in existing site conditions 100-year peak flows that can aggravate severe flooding problems as described in Core Requirement 42,nor does it necessarily prevent aggravation of all severe erosion problems. Consequently,if one or more of these problems are identified through offsite analysis per Core Requirement 42, additional onsite flow control and/or offsite improvements will likely be required(see"Drainage Problem-Specific Mitigation Requirements"in Section 1.2.2.2). Target Surfaces Facilities in Flow Control Duration Standard Areas25 must mitigate(either directly or in effect)the runoff from the following target developed surfaces within the threshold discharge area for which the facility is required: 1. New impervious surface that is not fully dispersed per the criteria on Section 1.2.3.2.0 as specified in Appendix C. For individual lots within residential subdivision projects,the extent of new impervious surface shall be assumed as specified in Chapter 3.Note, any new impervious surface such as a bridge or boardwalk that spans the ordinary high water of a stream,pond, or lake may be excluded as a target surface if the runofffrom such span is conveyed to the ordinary high water area in accordance with Criteria(b), (c), (d), and(e)of the `Direct Discharge Exemption" (p 1-39). 2. New pervious surface that is not fully dispersed as specified in Appendix C.For individual lots within residential subdivision projects,the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion and any portion in which native conditions are preserved by covenant,tract, or easement. In addition,the new pervious surface on individual lots shall be assumed to be 100%grass. 3. Replaced impervious surface that is not fully dispersed as specified in Appendix C on a non- redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more, OR new pervious surface is 3/4 acre or more. 4. Replaced impervious surface that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced impervious surface that is not fully dispersed as specified in Appendix C,on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements (including interior improvements and excluding required mitigation improvements) exceeds 50%of the assessed value of the existing site improvements. Exceptions The following exceptions apply only in Flow Control Duration Standard Areas: 1. The historical site conditions exception does not apply to the City. 2. The facility requirement in Flow Control Duration Standard Matching Existing Site Conditions Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference(when modeled using 15 minute time steps)in the sum of developed 100-year peak flows for those target surfaces subject to this requirement and the sum of historical site conditions 100-year 25 Note:Any threshold discharge area that appears to be located within a Flow Control Duration Standard Area according to the Flow Control Applications Map but drains entirely by non-erodible manmade conveyance to a major receiving water(listed on page 1-51)is considered to be located within a Peak Rate Flow Control Standard Area. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-42 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES peak flows(modeled using same time step unit(e.g., 15 minute)used to calculate the developed flow) for the same surface areas. Note:for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A. 3. The facility requirement in Flow Control Duration Standard Matching Forested Site Conditions Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference(when modeled using 15 minute time steps)in the sum of developed 100-year peak flows for those target surfaces subject to this requirement and the sum of forested(historical)site conditions 100-year peak flows(modeled using same time step unit(e.g., 15 minute)used to calculate the developed flow) for the same surface areas.Note:for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A. 4. The facility requirement in Flow Control Duration Standard Areas may be reduced or waived for any threshold discharge area where a plan or study approved by the City and Ecology shows that a lower standard(e.g.,Peak Rate Control Standard or targeting existing site conditions instead of forested conditions)is sufficient or no facility is necessary to protect or allow for restoration of water body beneficial uses and habitat functions essential to salmonids. 5. The regional facilities plan exception does not apply to the City. 6. The facility requirement in Flow Control Duration Standard Areas as applied to replaced impervious surface may be reduced by the CED Manager/designee using the adjustment process detailed in Sections 1.4.3 and 1.4.4 of the adjustment process,if the cost of flow control facilities to mitigate all target surfaces exceeds that necessary to mitigate only for new impervious surface plus new pervious surface and also exceeds 1/3 of the valuation of proposed improvements(including interior improvements)or twice the cost of a facility to mitigate equivalent surfaces on a new development site,whichever is less. The amount of reduction shall be limited such that the cost of flow control facilities is at least equal to that necessary to mitigate only for new impervious surface plus new pervious surface, and beyond this amount,is no greater than 1/3 of the valuation of proposed improvements(including interior improvements)or twice the cost of a facility to mitigate equivalent surfaces on a new development site,whichever is less. C. FLOOD PROBLEM FLOW CONTROL STANDARD AREAS Flood Problem Flow Control Standard Areas are designated by the City of Renton where the City has determined that a higher average level of flow control is needed to prevent aggravation of existing documented flooding problems.At this time,the City has not mapped specific areas,but may apply this standard when a project discharges to a severe flooding or erosion problem. Within Flood Problem Flow Control Standard Areas,or where required by the City to protect aggravation of a downstream problem,required flow control facilities must comply with the following minimum requirements for facility performance and mitigation of targeted surfaces, except where such requirements or the facility requirement itself is waived or reduced by the area-specific exceptions at the end of this subsection. Minimum Required Performance Facilities in Flood Problem Flow Control Standard Areas must comply with the following flow control performance standard and assumptions unless modified by offsite analysis per Core Requirement 42(see Table 1.2.3.A): Flood Problem Flow Control Standard:Apply the Flow Control Duration Standard,AND match the developed 100-year peak discharge rate to the predeveloped 100-year peak discharge rate. If the Flood Problem Flow Control Area is located within a Flow Control Duration Standard Area and does not drain entirely by non-erodible manmade conveyance to a major receiving water(see Table 1.2.3.13),then historical site conditions shall be assumed as the predeveloped condition except 2017 City of Renton Surface Water Design Manual 12/12/2016 1-43 SECTION 1.2 CORE REQUIREMENTS for the purposes of matching 100-year peak discharge rates. For all other situations and for the purposes of matching 100-year peak discharge rates,existing site conditions may be assumed. Intent The Flood Problem Flow Control Standard is intended to prevent significant increases in existing water surface levels for 2-year through 100-year return frequencies. Such increases are expected to occur as the volume of runoff discharging to the water body is increased by upstream development. Because inflow rates to these water bodies are typically much higher than the outflow rates,increased runoff volumes from upstream development are,in effect, stacked on top of existing volumes in the water body,resulting in higher water surface levels.The duration-matching and 100-year peak- matching criteria of the Flood Problem Flow Control Standard counteract this stacking effect by slowing the arrival of additional runoff volumes. Because it can prevent significant aggravation of existing flooding,the Flood Problem Flow Control Standard is also applicable to other flow control areas where severe flooding problems have been identified per Core Requirement 42. Effectiveness in Addressing Downstream Drainage Problems If the Flood Problem Flow Control Standard is implemented onsite,no additional measures are required to prevent aggravation of the three types of downstream drainage problems described in Core Requirement 42. The one exception is for a wetland or lake that is a closed depression with a severe flooding problem,and the proposed project is adding impervious surface area amounting to more than 10% of the 100-year water surface area of the closed depression. In this case, additional onsite flow control or offsite improvements may be necessary as determined by a"point of compliance analysis" (see"Special Provision for Closed Depressions"in Table 1.2.3.A, and see Section 3.3.6, "Point of Compliance Analysis"). Target Surfaces Facilities in Flood Problem Flow Control Standard Areas must mitigate(either directly or in effect)the runoff from the following target developed surfaces within the threshold discharge area for which the facility is required: 1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration Standard Area,then the target surfaces are the same as those required for facilities in Conservation Flow Control Areas(see Section 1.2.3.1.B)-unless otherwise allowed by the area-specific exceptions for Conservation Flow Control Areas.Note:Any Flood Problem Flow Control Standard Area that appears to be located within a Flow Control Duration Standard Area identified on the Flow Control Applications Map, but drains entirely by non-erodible manmade conveyance to a major receiving water(see Table 1.2.3.8), is considered to be located within a Peak Rate Flow Control Standard Area. 2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control Standard Area or drains entirely by non-erodible manmade conveyance to a major receiving water, then the target surfaces are the same as those required for facilities in Peak Rate Flow Control Standard Areas(see Section 1.2.3.1.A). Exceptions The following exceptions apply only in Flood Problem Flow Control Standard Areas: 1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration Standard Matching Existing Site Conditions Area or Peak Rate Flow Control Area,then the facility requirement is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference (when modeled using 15 minute time steps)in the sum of developed 100-year peak flows for the target surfaces subject to this requirement and the sum of historical site conditions 100-year peak flows (modeled using same time step unit(e.g., 15 minute)used to calculate the developed flow) for the same surface areas.Agricultural zoned projects in current agricultural use may use existing site conditions as the predeveloped condition for purposes of this exception calculation. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-44 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES Note:for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A. Also, any Flood Problem Flow Control Standard Area that appears to be located within a Flow Control Duration StandardArea identified on the Flow Control Applications Map, but drains entirely by non-erodible manmade conveyance to a major receiving water(see Table 1.2.3.B), is considered to be located within a Peak Rate Flow Control Standard Area. 2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control Standard Area,then the facility requirement is waived for any threshold discharge area in which the target surfaces subject to this requirement will generate no more than a 0.15-cfs increase(when modeled using 15-minute time steps)in the existing site conditions 100-year peak flow(modeled using same time step unit(e.g., 15-minute)used to calculate the developed flow. Note:for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement 49, Table 1.2.9.A. 3. Any required application of the Flood Problem Flow Control Standard Areas facility requirement to replaced impervious surface may be waived if the City has adopted a plan and implementation schedule approved by the state Department of Ecology for fulfilling this requirement with regional facilities. 4. Any required application of the Flood Problem Flow Control Standard Areas facility requirement to replaced impervious surface may be reduced by CED using the procedures detailed in Sections 1.4.3 and 1.4.4 of the adjustment process,if the cost of flow control facilities to mitigate all target surfaces exceeds that necessary to mitigate only for new impervious surface plus new pervious surface and also exceeds 1/3 of the valuation of proposed improvements(including interior improvements)or twice the cost of a facility to mitigate the same surfaces on a new development site, whichever is less. The amount of reduction allowed by this exception shall be limited such that the cost of flow control facilities is at least equal to that necessary to mitigate only for new impervious surface plus new pervious surface, and beyond this amount,is no greater than 1/3 of the valuation of proposed improvements(including interior improvements) or twice the cost of a facility to mitigate equivalent surfaces on a new development site,whichever is less. 5. Any required application of the Flood Problem Flow Control Standard Areas facility requirement to replaced impervious surface may assume existing site conditions as the predeveloped condition for the purposes of matching the developed 100-year peak discharge rate to the predeveloped 100-year peak discharge rate. 1.2.3.2 FLOW CONTROL FACILITY IMPLEMENTATION REQUIREMENTS Flow control facilities shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. ONSITE VS. OFFSITE IMPLEMENTATION All required flow control facilities must be implemented onsite except where the requirements below can be met by direct discharge to a regional or shared facility constructed to provide flow control for the proposed project. Regional facilities are typically constructed as part of a City-approved plan or study (e.g.,basin plan, stormwater compliance plan, or master drainage plan). Shared facilities may be constructed under a City-developed shared facility drainage plan or under an agreement between two or more private developers. 1. The regional or shared facility must be of adequate size and design to meet the current flow control requirements for the proposed project.Note: the current flow control requirements are those specified by Core Requirement 43 of this manual unless superseded by other adopted area-specific flow control requirements per Special Requirement 41 (see Section 1.3.1). In some cases where the current flow control requirements differ from those used to originally design the regional or shared facility, additional analysis and possible retrofitting of the facility may be required to ensure adequate size and 2017 City of Renton Surface Water Design Manual 12/12/2016 1-45 SECTION 1.2 CORE REQUIREMENTS design. In other cases where the current flow control requirements are not significantly different or are less stringent, adequate size and design may already be documented by an adopted City basin plan or master drainage plan, an approved shared facility drainage plan, or a detailed drainage analysis approved by the City for a separate permitted development. 2. The regional or shared facility must be fully operational at the time of construction of the proposed project. In the case of a shared facility,the proposed project must comply with the terms and conditions of all contracts, agreements, and permits associated with the shared facility. If the offsite facility is an existing City-owned facility,the City may charge a special use fee equal to or based on the property value of the detention capacity being used. 3. The conveyance system between the project site and the regional facility must meet the same criteria specified for direct discharge to a major receiving water except for Criterion(a) (see"Direct Discharge Exemption"in Section 1.2.3.1). In the case of a shared facility,the criteria are the same, except the conveyance system need only have adequate capacity and erosion protection for buildout of the participating portion26 of the contributing drainage area. B. METHODS OF ANALYSIS AND DESIGN Flow control facilities must be analyzed and designed using a continuous flow simulation method such as HSPF (Hydrologic Simulation Program FORTRAN)or the simplified HSPF-based runoff files method. An overview of the runoff files method is found in Chapter 3. Specifications for use of the approved modeling software is provided in the software documentation and augmented with limited SWDM-specific guidance in Reference Section 6-D. Detailed design specifications for flow control facilities are found in Chapter 5. C. SIZING CREDITS FOR FULLY DISPERSED SURFACES A fully dispersed surface(either impervious or nonnative pervious)is one that conforms to the BMP strategy for"full dispersion"detailed in Appendix C, Section C.2.1.This strategy calls for minimizing the area of onsite developed surface relative to native vegetated surface,together with the application of dispersion techniques that utilize the natural retention/detention capacity of the native vegetated surface to mitigate the runoff effects of the developed surfaces. Developed surfaces conforming to this strategy are considered to have a negligible impact downstream, and therefore,may be modeled as forest and are not subject to the area-specific flow control facility requirement(Section 1.2.3.1)or the area-specific water quality facility requirement(Section 1.2.8.1). In order for developed surfaces to qualify as fully dispersed, they must meet the basic criteria listed below and further detailed in Appendix C, Section C.2.1. Criteria for Fully Dispersed Surfaces 1. The total area of impervious surface being fully dispersed must be no more than 15%of the total area of native vegetated surface being preserved by a clearing limit by a City-approved recorded tract, easement, or covenant within the same threshold discharge area. The total area of impervious surface plus nonnative pervious surface27 being fully dispersed must be no more than 35%of a threshold discharge area. 2. The runoff from a fully dispersed surface must be discharged using one of the following dispersion devices in accordance with the design specifications and maximum area of fully dispersed surface for each device set forth in Appendix C, Section C.2.1: a) Splash blocks b) Rock pads c) Gravel filled trenches d) Sheet flow Note: The dispersion device must be situated so as to discharge within the same threshold discharge area of the surface it serves. 26 The participating portion includes those properties that have agreements for use of the shared facility. 27 Nonnative pervious surface means a pervious surface that does not meet the definition of a native vegetated surface. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-46 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES 3. A native vegetated flowpath segment of at least 100 feet in length(25 feet for sheet flow from a nonnative pervious surface)must be available along the flowpath that runoff would follow upon discharge from a dispersion device listed in Minimum Requirement 2 above. The native vegetated flowpath segment must meet all of the following criteria: a) The flowpath segment must be over native vegetated surface. b) The flowpath segment must be onsite or an offsite tract or easement area reserved for such dispersion. c) The average slope of the flowpath segment must be no steeper than 15% for any 20-foot reach of the flowpath segment. d) The flowpath segment must be located between the dispersion device and any downstream drainage feature such as a pipe, ditch, stream,river,pond,lake, or wetland. e) The flowpath segments for adjacent dispersion devices must comply with the minimum spacing requirements in Appendix C, Section C.2.1. These requirements do not allow overlap of flowpath segments, except in the case where sheet flow from a nonnative pervious surface overlaps with the flowpath of any dispersion device listed in Minimum Requirement 2 above. In this case,the longer of the two overlapping flowpath segments must be extended at least 1 foot for every 3 feet of distance along the most representative path that runoff would travel from the upstream end to the discharge end of the nonnative pervious surface. 4. On sites with septic systems,the discharge of runoff from dispersion devices must not be upgradient of the drainfield. This requirement may be waived by CED if site topography clearly prohibits flows from intersecting the drainfield. 5. The dispersion of runoff must not create flooding or erosion impacts as determined by CED. If runoff is proposed to be discharged toward a landslide hazard,erosion hazard area,or steep slope hazard area(i.e., slopes steeper than 20%), CED may require the applicant to have the proposal evaluated by a geotechnical engineer, engineering geologist, or CED. D. SIZING CREDITS FOR USE OF ON-SITE BMPS Projects that implement on-site BMPs as detailed in Core Requirement 49 and Appendix C,whether required or optional,may use the on-site BMP sizing credits as described and allowed in Section 1.2.9.4 and Table 1.2.9.A. E. MITIGATION OF TARGET SURFACES THAT BYPASS FACILITY On some sites,topography may make it difficult or costly to collect all target surface runoff for discharge to the onsite flow control facility. Therefore, some project runoff subject to flow control may bypass required onsite flow control facilities provided that all of the following conditions are met: 1. The point of convergence for runoff discharged from the bypassed target surfaces and from the project's flow control facility must be within a quarter-mile downstream28 of the facility's project site discharge point,AND 2. The increase in the existing site conditions 100-year peak discharge from the area of bypassed target surfaces must not exceed 0.4 cfs,AND 3. Runoff from the bypassed target surfaces must not create a significant adverse impact to downstream drainage systems, salmonid habitat, or properties as determined by CED,AND 4. Water quality requirements applicable to the bypassed target surfaces must be met,AND 28 Note: CED may allow this distance to be extended beyond a half mile to the point where the project site area constitutes less than 15%of the tributary area. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-47 SECTION 1.2 CORE REQUIREMENTS 5. Compensatory mitigation by a flow control facility must be provided so that the net effect at the point of convergence downstream is the same with or without the bypass. This mitigation may be waived if the existing site conditions 100-year peak discharge from the area of bypassed target surfaces is increased by no more than 0.15 cfs(modeled using 15 minute time steps)and on-site BMPs as detailed in Appendix C are applied to all impervious surfaces within the area of bypassed target surfaces. One or combination of the following methods may be used to provide compensatory mitigation by a flow control facility subject to permission/approvals from other parties as deemed necessary by CED: a) Design the project's flow control facility or retrofit an existing offsite flow control facility as needed to achieve the desired effect at the point of convergence,OR b) Design the project's flow control facility or provide/retrofit an offsite flow control facility to mitigate an existing developed area(either onsite or offsite)that has runoff characteristics(i.e., peak flow and volume) equivalent to those of the bypassed target surfaces but is currently not mitigated or required to be mitigated to the same flow control performance requirement as the bypassed target surfaces. Consideration of an offsite area to be mitigated for must take into account the likelihood of that area redeveloping in the future. Those areas determined by the City to have a high likelihood of future redevelopment that will provide its own mitigation may not be used as compensatory mitigation. F. BYPASS OF RUNOFF FROM NON-TARGET SURFACES The performance of flow control facilities can be compromised if the contributing area,beyond that which must be mitigated by the facility,is too large. Therefore,IF the existing 100-year peak flow rate from any upstream area(not targeted for mitigation)is greater than 50% of the 100-year developed peak flow rate (undetained)for the area that must be mitigated,THEN the runoff from the upstream area must bypass the facility. Offsite areas that naturally drain onto the project site must be intercepted at the natural drainage course within the project site and conveyed in a separate conveyance system and must bypass onsite stormwater facilities. The bypass of upstream runoff must be designed so that all of the following conditions are met: 1. Any existing contribution of flows to an onsite wetland must be maintained,AND 2. Upstream flows that are naturally attenuated by natural detention on the project site under predeveloped conditions must remain attenuated, either by natural means or by providing additional onsite detention so that peak flows do not increase,AND 3. Upstream flows that are dispersed or unconcentrated on the project site under predeveloped conditions must be discharged in a safe manner as described in Core Requirement 41 under "Discharge Requirements". 4. Bypasses shall be designed in accordance with standards of Core Requirement 44, Conveyance System G. MITIGATION TRADES A project's flow control facility may be designed to mitigate an existing developed non-target surface area (either onsite or offsite)in trade for not mitigating part or all of the project's target surface area,provided that all of the following conditions are met: 1. The existing developed non-target surface area(i.e., an area of existing impervious surface and/or nonnative pervious surface)must have runoff discharge characteristics(i.e.,peak flow and volume) equivalent to those of the target surface area for which mitigation is being traded and must not be currently mitigated to the same flow control performance requirement as the target surface area,AND 2. Runoff from both the target surface area being traded and the flow control facility must converge prior to discharge of the runoff from the target surface area being traded onto private property without an easement or through any area subject to erosion,AND 12/12/2016 2017 City of Renton Surface Water Design Manual 1-48 1.2.3 CORE REQUIREMENT#3:FLOW CONTROL FACILITIES 3. The net effect in terms of flow control at the point of convergence downstream must be the same with or without the mitigation trade,AND 4. The undetained runoff from the target surface area being traded must not create a significant adverse impact to downstream drainage systems, salmonid habitat, or properties prior to convergence with runoff from the flow control facility. 5. Consideration of an offsite area to be mitigated for must take into account the likelihood of that area redeveloping in the future. Those areas determined by the City to have a high likelihood of future redevelopment that will provide its own mitigation may not be used as a mitigation trade. 6. Mitigation trade proposals must be reviewed and approved with input from the City of Renton. H. MANIFOLD DETENTION FACILITIES A manifold detention facility is a single detention facility designed to take the place of two or more otherwise required detention facilities. It combines the runoff from two or more onsite drainage areas having separate natural discharge locations,and redistributes the runoff back to the natural discharge locations following detention. Because manifold detention facilities divert flows from one natural discharge location to another and then back,they are not allowed except by an approved adjustment(see Section 1.4). I. FACILITY REQUIREMENT IN LANDSLIDE HAZARD DRAINAGE AREAS Proposed projects subject to Discharge Requirement 2 in Core Requirement 41 must provide a tightline system unless the 100-year runoff from the project site can be feasibly infiltrated or one of the other exceptions listed in Section 1.1.2.2.For infiltration to be used as an alternative to the tightline requirement, it must be feasible per the facility design requirements and limitations specified in Section 5.2. When evaluating the feasibility of infiltration,multiple facility locations scattered throughout the project site shall be considered and used where feasible and practical to avoid concentrating infiltrated water in one location. If multiple facilities are not feasible or practical,then a single infiltration facility meeting the minimum setback requirements in Section 5.2 may be used where feasible. Where infiltration is not feasible,it is still possible for a proposed project to qualify for one of the other exceptions to the tightline requirement specified in Core Requirement 41. If such a project is subject to the flow control facility requirement in Core Requirement 43,the required facility must be a detention pond sized to meet, at minimum,the Flow Control Duration Standard Matching Forested site conditions flow control facility standard with a safety factor of 20% applied to the storage volume. The detention pond must be sited and designed so as to maximize the opportunity for infiltration in the pond. To accomplish this, all of the following design requirements must be met: 1. The detention pond must be preceded by either a water quality treatment facility per Core Requirement 48 or a presettling basin per Section 5.2,AND 2. All detention pond side slopes must be 31-1:IV or flatter and must be earthen,AND 3. Detention pond liners that impede infiltration shall not be used,AND 4. The pond bottom shall be at or above the seasonal high groundwater table,AND 5. The detention pond outflow must meet the discharge dispersal requirements specified in Discharge Requirement 1 of Core Requirement 41.29 29 Footnote 29 is not used. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-49 SECTION 1.2 CORE REQUIREMENTS 1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM All engineered conveyance system elements for proposed projects must be analyzed, designed, and constructed to provide a minimum level of protection against overtopping, flooding, erosion, and structural failure as specified in the following groups of requirements: • "Conveyance Requirements for New Systems,"Section 1.2.4.1 • 'Conveyance Requirements for Existing Systems,"Section 1.2.4.2 • "Conveyance System Implementation Requirements,"Section 1.2.4.3 Intent: To ensure proper design and construction of engineered conveyance system elements. Conveyance systems are natural and engineered drainage facilities that provide for the collection and transport of surface water or stormwater runoff. This core requirement applies to the engineered elements of conveyance systems (primarily pipes,culverts, and ditches/channels). 1.2.4.1 CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS All new conveyance system elements,30 both onsite and offsite, shall be analyzed, designed, and constructed according to the following requirements.Also see Section 4.1 for route design and easement requirements. Pipe Systems 1. New pipe systems shall be designed with sufficient capacity to convey and contain(at minimum)the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. Pipe system structures may overtop for runoff events that exceed the 25-year design capacity, provided the overflow from a 100-year runoff event does not create or aggravate a severe flooding problem or severe erosion problem as described in Core Requirement 42, Section 1.2.2.Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions,this overflow must be contained within an onsite drainage easement,tract,covenant, or public right-of-way. 3. The upstream end of a pipe system that receives runoff from an open drainage feature(pond, ditch, etc.)shall be analyzed and sized as a culvert as described below. Culverts 1. New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section 4.3.1 and convey(at minimum)the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. New culverts must also convey as much of the 100-year peak flow as is necessary to preclude creating or aggravating a severe flooding problem or severe erosion problem as described in Core Requirement 42, Section 1.2.2.Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions,this overflow must be contained within an onsite drainage easement,tract,covenant, or public right-of-way. 3. New culverts proposed in streams with salmonids shall be designed to provide for fish passage as detailed in Section 4.3.2.Note: The City's critical areas regulations (RMC 4-3-050)or the state Department ofFish and Wildlife may require a bridge to facilitate fish passage. 30 New conveyance system elements are those that are proposed to be constructed where there are no existing constructed conveyance elements. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-50 1.2.4 CORE REQUIREMENT#4:CONVEYANCE SYSTEM Ditches/Channels 1. New ditches/channels shall be designed with sufficient capacity to convey and contain, at minimum, the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. New ditches/channels must also convey as much of the 100-year peak flow as is necessary to preclude creating or aggravating a severe flooding problem or severe erosion problem as described in Core Requirement 42, Section 1.2.2.Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions, such overflow must be contained within an onsite drainage easement,tract,covenant, or public right-of-way. 3. In both conditions listed above, ditches must be designed with a 6-inch minimum freeboard. Tightline Systems Traversing Steep Slopes New tightline conveyance systems traversing slopes that are steeper than 15% and greater than 20 feet in height, or are within a steep slope hazard area as defined in RMC 4-3-050, shall be designed with sufficient capacity to convey and contain(at minimum)the 100-year peak flow, assuming full build-out conditions31 for all tributary areas,both onsite and offsite. Tightline systems shall be designed as detailed in Section 4.2.2. Bridges New bridges shall be designed to accommodate the 100-year peak flow as specified in Section 4.3.3 and in accordance with the floodplain development standards in RMC 4-3-050. 1.2.4.2 CONVEYANCE REQUIREMENTS FOR EXISTING SYSTEMS The following conveyance requirements for existing systems are less rigorous than those for new systems to allow some salvaging of existing systems that are in useable condition.Existing systems may be utilized if they are capable of providing a minimum level of protection as-is or with minor modifications. Existing Onsite Conveyance Systems No Change in Flow Characteristics: Existing onsite conveyance systems that will not experience a change in flow characteristics(e.g.,peak flows or volume of flows) as a result of the proposed project need not be analyzed for conveyance capacity. Change in Flow Characteristics: Existing onsite conveyance systems that will experience a change in flow characteristics as a result of the proposed project must comply with the following conveyance requirements: 1. The existing system must be analyzed and shown to have sufficient capacity to convey and contain(at minimum)the 25-year peak flow assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. The applicant must demonstrate that the 100-year peak flow to the existing system will not create or aggravate a severe flooding problem or severe erosion problem as described in Core Requirement 42, Section 1.2.2. 3. Minor modifications may be made to the conveyance system to achieve the required capacity stated above. Examples of minor modifications include raising a catch-basin rim,replacing or relaying a section of pipe to match the capacity of other pipes in the system,improving a pipe inlet, or enlarging a short,constricted reach of ditch or channel. 4. Modifications to an existing conveyance system or element that acts to attenuate peak flows, due to the presence of detention storage upstream, shall be made in a manner that does not significantly 31 Full build-out conditions means the tributary area is developed to its full zoning potential except where there are existing sensitive areas,open space tracts,and/or native growth protection easements/covenants. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-51 SECTION 1.2 CORE REQUIREMENTS increase peak flows downstream. For example,if water is detained in a pond upstream of a restrictive road culvert,then installing an overflow system for the culvert should prevent overtopping of the road without significantly reducing existing detention storage. Existing Offsite Conveyance Systems 1. Existing offsite conveyance systems need not be analyzed for conveyance capacity except as required by Core Requirement 42, or if offsite improvements or direct discharge are proposed per Core Requirement 43. 2. Improvements made to existing offsite conveyance systems to address the drainage problem-specific mitigation requirements in Section 1.2.2.2 need only change existing conveyance capacity sufficient to prevent aggravation of the drainage problem(s)being addressed. 3. Existing offsite conveyance systems proposed to be used for direct discharge to a major receiving water per Core Requirement 43 shall meet the same conveyance requirements specified in Section 1.2.4.1 for new systems. 1.2.4.3 CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS Conveyance systems shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. METHODS OF ANALYSIS AND DESIGN Properly sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the return frequencies indicated in Sections 1.2.4.1 and 1.2.4.2. Conveyance capacity shall be demonstrated using the methods of analysis detailed in Chapter 4.Design flows for sizing conveyance systems shall be determined using the appropriate runoff computation method specified in Section 3.2. B. COMPOSITION Where feasible,conveyance systems shall be constructed of vegetation-lined channels,as opposed to pipe systems, except in Zone I of the Aquifer Protection Area where pipe systems are required.Vegetative channels shall generally be considered feasible if all of the following conditions are present: 1. The channel gradient generally does not exceed 5 percent,AND 2. Ditches/roadway section must be approved by the City,AND 3. The channel will be accessible for maintenance(see Section 1.2.6),AND 4. The channel will not be subject to erosion. Exceptions: The following are exceptions to the requirement for vegetative channels: • Conveyance systems proposed under roadways, driveways, or parking areas • Conveyance systems proposed between houses in urban-zoned plats and short plats • Conveyance systems conveying roof runoff only. • Conveyance systems in Zone I of the Aquifer Protection Area. C. INTERFLOW AND INTERCEPTION Interflow is near-surface groundwater that moves laterally through the soil horizon following the hydraulic gradient of underlying relatively impermeable soils. When interflow is expressed on the surface,it is termed a spring or seepage.Any significant springs or seepage areas that impact a roadway or structure proposed by the project must be intercepted and directed into a conveyance system. Where roadways may impede the passage of interflow to downstream wetlands or streams,provision for passage of unconcentrated flows must be made. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-52 1.2.4 CORE REQUIREMENT#4:CONVEYANCE SYSTEM D. PROVISION FOR LOT DRAINAGE WITHIN SUBDIVISIONS Within subdivision projects,32 provision must be made for the safe conveyance of runoff from the discharge location of each lot to the subdivision's main conveyance system or road drainage system. This may include,but is not limited to,provisional stub-outs from an enclosed roadway drainage system to the edge of the road right-of-way at each created lot, or lot-line pipes or ditches that collect lot drainage and convey it to the subdivision's main conveyance system or road drainage system. E. OUTFALLS An outfall is defined as a point where collected and concentrated surface and storm water runoff is discharged from a pipe system or culvert. Energy Dissipation: At a minimum,rock erosion protection is required at outfalls from all drainage systems and elements except where CED determines that erosion protection is being provided by other means or is not needed.Details on outfall structures are included in Section 4.2.2. New Point Discharges Over Steep Slopes:Proposed outfalls that will discharge runoff in a location where the natural(existing)discharge is unconcentrated over a slope steeper than 15% and greater than 20 feet in height, or over a steep slope hazard area(as defined in RMC 4-3-050),must meet the following criteria: • A tightline conveyance system must be constructed to convey the runoff to the bottom of the slope unless other measures are approved by CED based on an evaluation/report by a licensed geotechnical engineer. • The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. • Tightline systems must be designed so that existing baseflow conditions are not significantly changed and adequate energy dissipation is provided at the bottom of the slope. • Where alternative measures(e.g., dispersal trench)to the tightline system are approved upstream of a landslide hazard or steep slope hazard area,they may be placed no closer than 50 feet from the top of the hazard area slope based on an evaluation/report by a licensed geotechnical engineer. F. OUTFALLS TO THE GREEN RIVER New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River between River Mile 6(South Boeing Access Road)and SR 18 are allowed only through the adjustment process. These outfalls must comply with requirements of the Green River Pump Operations Procedure Plan,which establishes storage volumes and release rate criteria for developments proposing to construct or modify outfalls. Copies of the plan are available from DNRP. G. SPILL CONTROL Projects proposing to construct or replace onsite conveyance system elements that receive runoff from non-roof-top pollution-generating impervious surface must provide a spill control device as detailed in Section 4.2.1.1 prior to discharge from the site or into a natural onsite drainage feature.33 More specifically,this requirement applies whenever a proposed project does either of the following: • Constructs a new onsite conveyance system that receives runoff from non-roof-top pollution- generating impervious surface, OR • Removes and replaces an existing onsite conveyance system element that receives runoff from 5,000 square feet or more of non-roof-top pollution-generating impervious surface onsite. 32 For purposes of this requirement,the term subdivision project refers to any project that creates a short plat, plat,or binding site plan. 33 Natural onsite drainage feature means a natural swale,channel,stream,closed depression,wetland,or lake. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-53 SECTION 1.2 CORE REQUIREMENTS The intent of this device is to temporarily detain oil or other floatable pollutants before they enter the downstream drainage system in the event of an accidental spill or illegal dumping. It may consist of a tee section in a manhole or catch basin, or an equivalent alternative as specified in Section 4.2.1.1. Note that in addition to this spill control requirement to protect offsite and natural drainage systems, there are other spill control requirements in this manual for discharges to certain water quality facilities and all infiltration facilities (see the design criteria for water quality facilities in Chapter 6 and the general requirements for infiltration facilities in Section 5.2). The application of these requirements must be such that all stated intents are satisfied. H. GROUNDWATER PROTECTION Any reach of new ditch or channel proposed by a project in which the untreated runoff from 5,000 square feet or more of pollution-generating impervious surface or 3/4 acre or more of pollution-generating pervious surface comes into direct contact with an outwash soil must be lined with either: 1. A low permeability liner or a treatment liner consistent with the specifications for such liners in Section 6.2.4, OR 2. An imported soil compacted till liner meeting the following specifications: • Liner thickness shall be 18 inches after compaction. • Imported soils must meet the gradation listed for compacted till liner in Section 6.2.4. • Soil should be placed in 6-inch lifts. • Soil shall be compacted to no less than 95%minimum dry density,modified proctor method (ASTM D-1557). The intent of this requirement is to reduce the likelihood that pollutants will be discharged to groundwater when untreated runoff is conveyed in ditches or channels constructed in soils with high infiltration rates. I. PUMP SYSTEMS Pump systems may be used to convey water from one location or elevation to another within the project site provided they meet the design criteria specified for such systems in Section 4.2.3 and will be privately owned and maintained. Pump systems discharging flows from the project site that would not have discharged by gravity flow under existing site conditions will require an approved adjustment to Core Requirement 41 (see Section 1.4,"Adjustment Process"). These pump systems will be considered only when there is no other physical gravity alternative and they are necessary to prevent creation or aggravation of a flooding or erosion problem as specified in Section 1.2.2. 1.2.5 CORE REQUIREMENT #5: CONSTRUCTION STORMWATER POLLUTION PREVENTION All proposed projects that will clear,grade, or otherwise disturb the site must provide erosion and sediment controls to prevent,to the maximum extent practicable,the transport of sediment from the project site to downstream drainage facilities,water resources, and adjacent properties.All proposed projects that will conduct construction activities onsite or offsite must provide stormwater pollution prevention and spill controls to prevent,reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses. To prevent sediment transport and pollutant discharges as well as other impacts related to land-disturbing and construction activities,Erosion and Sediment Control(ESC)measures and Stormwater Pollution Prevention and Spill Control(SWPPS)measures that are appropriate to the project site must be applied through a comprehensive Construction Stormwater Pollution Prevention (CSWPP)plan as described in Sections 1.2.5.1 and 1.2.5.3 and shall perform as described in Section 1.2.5.2. In addition,these measures,both temporary and permanent, shall be implemented consistent with the requirements in Section 1.2.5.3 that apply to the proposed project. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-54 1.2.5 CORE REQUIREMENT#5:CONSTRUCTION STORMWATER POLLUTION PREVENTION Intent: • To prevent the transport of sediment and other impacts,like increased runoff,related to land disturbing activities. Erosion of disturbed areas on construction sites can result in excessive sediment transport to adjacent properties and to surface waters. This sediment can result in major adverse impacts, such as flooding from obstructed drainage ways, smothering of salmonid spawning beds, algal blooms in lakes, and exceedances of state water quality standards for turbidity. These impacts can also result from the increased runoff generated by land disturbing activities on construction sites. • To prevent,reduce, or eliminate the discharge of pollutants to onsite or adjacent Stormwater systems or watercourses from construction-related activities such as materials delivery and storage, onsite equipment fueling and maintenance,demolition of existing buildings and disposition of demolition materials and other waste, and concrete handling,washout and disposal. 1.2.5.1 CSWPP MEASURES Construction Stormwater Pollution Prevention (CSWPP)measures include Erosion and Sediment Control(ESC)measures and Stormwater Pollution Prevention and Spill(SWPPS)measures. ESC Measures Each of the following categories of ESC measures must be considered for application to the project site as detailed in the Erosion and Sediment Control(ESC)Standards located in the Construction Stormwater Pollution Prevention Standards adopted as Appendix D of this manual: 1. Clearing Limits 2. Cover Measures 3. Perimeter Protection 4. Traffic Area Stabilization 5. Sediment Retention 6. Surface Water Collection 7. Dewatering Control 8. Dust Control 9. Flow Control 10. Control Pollutants(also see SWPPS Measures below) 11. Protect Existing and Proposed Stormwater Facilities and On-site BMPs 12. Maintain Protective BMPs 13. Manage the Project SWPPS Measures Each of the following categories of SWPPS measures must be considered for application to the project site as detailed in the Stormwater Pollution Prevention and Spill Control(SWPPS)Standards located in the CSWPP Standards adopted as Appendix D of this manual: • Follow effective pollutant handling and disposal procedures. • Provide cover and containment for materials, fuel and other pollutants. • Manage the project site to maximize pollutant control and minimize pollutant sources. • Protect from spills and drips of petroleum products and other pollutants. • Avoid overapplication or untimely application of chemicals and fertilizers. • Prevent or treat contamination of Stormwater runoff by pH modifying sources. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-55 SECTION 1.2 CORE REQUIREMENTS 1.2.5.2 CSWPP PERFORMANCE AND COMPLIANCE PROVISIONS The changing conditions typical of construction sites call for frequent field adjustments of existing ESC and SWPPS measures or additional ESC and SWPPS measures in order to meet required performance. In some cases, strict adherence to specified measures may not be necessary or practicable based on site conditions or project type. In other cases,immediate action may be needed to avoid severe impacts. Therefore,careful attention must be paid to ESC and SWPPS performance and compliance in accordance with the following provisions: A. CSWPP SUPERVISOR For projects in Targeted,Full or Large Project Drainage Review, or projects in Directed Drainage Review as determined by the CED permit reviewer,the applicant must designate a CSWPP supervisor who shall be responsible for the performance,maintenance, and review of ESC and SWPPS measures and for compliance with all permit conditions relating to CSWPP as described in the CSWPP Standards.The applicant's selection of a CSWPP supervisor must be approved by the City. This approval may be rescinded for non-compliance,requiring the applicant to select another CSWPP supervisor and obtain City approval prior to continuing work on the project site.For projects that disturb one acre or more of land,the CSWPP supervisor must be a Certified Professional in Erosion and Sediment Control(see <www.c esg c.net>for more information)or a Certified Erosion and Sediment Control Lead whose certification is recognized by the Department of Ecology or King County.34 The City may also require a certified ESC professional for sites smaller than one acre of disturbance if CED determines that onsite ESC measures are inadequately installed,located, or maintained. For larger,more sensitive sites,the City may require a certified ESC professional with several years of experience in construction supervision/inspection and a background in geology, soil science, or agronomy (See Appendix D, Section D.2.3.1 for more information). B. MONITORING OF DISCHARGES The CSWPP supervisor shall have a turbidity meter onsite and shall use it to monitor surface and storm water discharges from the project site and into onsite wetlands, streams, or lakes whenever runoff occurs from onsite activities and during storm events. If the project site is subject to a NPDES general permit for construction issued by the Washington State Department of Ecology(Ecology),then the project must comply with the monitoring requirements of that permit. The CSWPP supervisor shall also use the specific SWPPS control BMP procedures for monitoring surface and stormwater discharge for pollutants and acceptable discharge levels. The CSWPP supervisor shall keep logs as required by the procedures of all measurements taken onsite and make them available to CED on request. C. ESC PERFORMANCE ESC measures shall be applied/installed and maintained to prevent,to the maximum extent practicable,the transport of sediment from the project site to downstream drainage systems or surface waters or into onsite wetlands, streams, or lakes or onto adjacent properties. This performance is intended to be achieved through proper selection,installation, and operation of the above ESC measures as detailed in the CSWPP Standards (Appendix D) and approved by the City. However,the CSWPP supervisor or the City may determine at any time during construction that the approved measures are not sufficient and that additional action is required based on one of the following criteria: 1. If a turbidity test of surface and storm water discharges leaving the project site is greater than the benchmark value of 25 NTU(nephelometric turbidity units) set by the Washington State Department of Ecology,but less than 250 NTU,the CSWPP Supervisor shall do all of the following: 34 King County recognition of certification means that the individual has taken a King County-approved third party training program and has passed the King County-approved test for that training program. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-56 1.2.5 CORE REQUIREMENT#5:CONSTRUCTION STORMWATER POLLUTION PREVENTION a) Review the ESC plan for compliance and make appropriate revisions within 7 days of the discharge that exceeded the benchmark of 25 NTU,AND b) Fully implement and maintain appropriate ESC measures as soon as possible but no later than 10 days after the discharge that exceeded the benchmark,AND c) Document ESC implementation and maintenance in the site log book. 2. If a turbidity test of surface or storm water entering onsite wetlands, streams, or lakes indicates a turbidity level greater than 5 NTU above background when the background turbidity is 50 NTU or less, or 10%above background when the background turbidity is greater than 50 NTU,then corrective actions and/or additional measures beyond those specified in Section 1.2.5.1 shall be implemented as deemed necessary by the City inspector or onsite CSWPP supervisor. 3. If discharge turbidity is 250 NTU or greater,the CSWPP Supervisor shall do all of the following: a) Notify the City by telephone,AND b) Review the ESC plan for compliance and make appropriate revisions within 7 days of the discharge that exceeded the benchmark of 25 NTU,AND c) Fully implement and maintain appropriate ESC measures as soon as possible but no later than 10 days after the discharge that exceeded the benchmark,AND d) Document ESC implementation and maintenance in the site log book.AND e) Continue to sample discharges until turbidity is 25 NTU or lower, or the turbidity is no more than 10%over background turbidity. 4. If the City determines that the condition of the construction site poses a hazard to adjacent property or may adversely impact drainage facilities or water resources,THEN additional measures beyond those specified in Section 1.2.5.1 may be required by the City. D. SWPPS PERFORMANCE SWPPS measures shall be applied/installed and maintained so as to prevent,reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses or onto adjacent properties. This performance is intended to be achieved through proper selection,installation, and operation of the above SWPPS measures as detailed in the CSWPP Standards (Appendix D)and approved by the City. However,the CSWPP supervisor designated per Section 1.2.5.2.A or the City may determine at any time during construction that such approved measures are not sufficient and additional action is required based on the criteria described in the specific SWPPS BMP standard and/or conditions of an approved adjustment: E. FLEXIBLE COMPLIANCE Some projects may meet the intent of Core Requirement 45 while varying from specific CSWPP requirements contained here and in the CSWPP Standards. If a project is designed and constructed to meet the intent of this core requirement,the City may determine that strict adherence to a specific ESC requirement is unnecessary; an approved adjustment(see Section 1.4)is not required in these circumstances. Certain types of projects are particularly suited to this greater level of flexibility, for instance,projects on relatively flat,well drained soils,projects that are constructed in closed depressions, or projects that only disturb a small percentage of a forested site may meet the intent of this requirement with very few ESC measures. However, SWPPS requirements may actually be emphasized on well- drained soils,particularly in groundwater or well-protection protection areas, or in close proximity to water bodies. More information on intent and general ESC and SWPPS principles is contained in the CSWPP Standards in Appendix D. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-57 SECTION 1.2 CORE REQUIREMENTS F. ROADS AND UTILITIES Road and utility projects often pose difficult erosion control challenges because they frequently cross surface waters and are long and narrow with limited area available to treat and store sediment-laden water. Because of these factors,road and utility projects are allowed greater flexibility in meeting the intent of Core Requirement 45 as described in the CSWPP Standards. G. ALTERNATIVE AND EXPERIMENTAL MEASURES All measures proposed for erosion and sediment control shall conform to the details and specifications in the CSWPP Standards unless an alternative is approved by the City, and if the alternative is a new technology,it must also be approved through Ecology's CTAPE program (see"Alternative and Experimental Measures"in the CSWPP Standards,Appendix D). 1.2.5.3 CSWPP IMPLEMENTATION REQUIREMENTS Proposed projects must identify,install, and maintain required erosion and sediment control and stormwater pollution prevention and spill control measures consistent with the following requirements: A. CSWPP PLAN As specified in Chapter 2, all proposed projects must submit a CSWPP plan for implementing CSWPP measures. The CSWPP plan is comprised of the ESC plan and the SWPPS plan.The ESC plan must show the location and details of all ESC measures as specified in Chapter 2 and the CSWPP Standards and shall include a CSWPP report,which contains additional directions and supporting information like a detailed construction sequence as proposed by the design engineer and any calculations or information necessary to size ESC measures and demonstrate compliance with Core Requirement 45. The CSWPP plan shall also contain plan notes that outline specific permit conditions as outlined in Appendix D Section D.4.2 Standard ESC and SWPPS Plan Notes. The City may require large,complex projects to phase construction and to submit multiple ESC plans for the different stages of construction.New CSWPP plans are not required for changes that are necessary during construction,unless required by the City inspector. B. WET SEASON CONSTRUCTION During the wet season(October 1 to April 30)any site with exposed soils shall be subject to the "Wet Season Requirements"contained in the ESC Standards. In addition to the ESC cover measures,these provisions include covering any newly-seeded areas with mulch and seeding as much disturbed area as possible during the first week of October to provide grass cover for the wet season. Other ESC measures such as baker tanks and portable sand filters may be required for use during the wet season.A separate "Wet Season"ESC plan shall be submitted and approved by the City before continuing work on any site during the wet season. C. CONSTRUCTION WITHIN CRITICAL AREAS AND BUFFERS Any construction that will result in disturbed areas on or within a stream or associated buffer,within a wetland or associated buffer,or within 50 feet of a lake shall be subject to the "Critical Area Restrictions" contained in the CSWPP Standards. These provisions include phasing the project whenever possible so that construction in these areas is limited to the dry season. D. MAINTENANCE All ESC and SWPPS measures shall be maintained and reviewed on a regular basis as prescribed in the CSWPP Standards. E. FINAL STABILIZATION Prior to obtaining final construction approval,the site shall be stabilized, structural ESC and SWPPS measures(such as silt fences, sediment traps and concrete waste collection pits)shall be removed, and 12/12/2016 2017 City of Renton Surface Water Design Manual 1-58 1.2.6 CORE REQUIREMENT#6:MAINTENANCE AND OPERATIONS drainage facilities shall be cleaned as specified in the CSWPP Standards.A separate ESC plan describing final stabilization may be required by the City prior to implementation. F. CONSIDERATION OF OTHER REQUIRED PERMITS Consideration should be given to the requirements and conditions that may be applied by other agencies as part of other permits required for land-disturbing activities. In particular,the following permits may be required and should be considered when implementing CSWPP measures: • A Class IV Special Forest Practices Permit is required by the Washington State Department of Natural Resources for projects that will clear more than two acres of forest or 5,000 board feet of timber.All such clearing is also subject to the State Environmental Policy Act(RCW 43.21C)and will require SEPA review. • A NPDES General Permit for Construction (pursuant to the Washington State Department of Ecology's Construction Stormwater General Permit)is required for projects that will disturb one or more acres for purposes of constructing or allowing for construction of a development, or projects disturbing less than one acre that are part of a larger common plan of sale35 that will ultimately disturb one or more acres. 1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS Maintenance and operation of all drainage facilities is the responsibility of the applicant or property owner, except those facilities for which the City assumes maintenance and operation as described below and in RMC 4-6-030.M. Drainage facilities must be maintained and operated in accordance with the maintenance standards in Appendix A of this manual, or other maintenance standards as approved by the City. Intent: To ensure that the maintenance responsibility for drainage facilities is clearly assigned and that these facilities will be properly maintained and operated in perpetuity. Drainage facilities serving private improvements are not allowed in public right-of-way. On-site BMPs serving private improvements are also not allowed in the public right-of-way.Under certain situations, drainage facilities for single family residential subdivisions with public roads may be allowed in the public right-of-way through the City adjustment/variance process. Examples of conditions in which facilities may be considered for placement in the public way are: • Dead end streets or cul-de-sacs where future extensions of the road is unlikely and where drainage facilities will not conflict with existing utility improvements. • Unimproved right-of-way where future improvements are not anticipated and would not conflict with existing or future utility improvements. Drainage Facilities to be Maintained by the City of Renton The City will assume maintenance and operation of the following drainage facilities36 for any residential subdivision with public streets, except where the City grants an adjustment per Section 1.4, allowing the facilities to be maintained by the homeowners association: • Flow control and water quality treatment facilities within a stormwater tractor right-of-way dedicated to the City. 35 Common plan of development or sale means a site where multiple separate and distinct construction activities may take place at different times or on different schedules,but still under a single plan. Examples include: 1)phased projects and projects with multiple filings or lots,even if the separate phases or filings/lots will be constructed under separate contract or by separate owners(e.g.,a development where lots are sold to separate builders);2)a development plan that may be phased over multiple years, but is still under a consistent plan for long-term development;and 3)projects in a contiguous area that may be unrelated but still under the same contract,such as construction of a building extension and a new parking lot at the same facility. 36 Note:the City of Renton does not assume maintenance of individual lot drainage systems or drainage stub-outs serving single family residential lot downspout,footing, or yard drains. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-59 SECTION 1.2 CORE REQUIREMENTS • On-site BMPs serving more than one lot, and serving public improvements within a stormwater tract. • Bioretention facilities in City right-of-way,mitigating for public improvements. • Where serving public improvements, on-site BMP vegetated flow paths for full dispersion within an easement that includes provisions for access and maintenance. The City maintenance of these vegetated flow paths will be limited to their functionality.All other maintenance shall remain the responsibility of the owner(s). • The conveyance system within a drainage easement,tract or improved public road right-of-way granted to the City. Note: The City may assume maintenance offacilities serving any mix of developments through an adjustment. The City will assume maintenance and operation of these facilities two years after final construction approval by CED and an inspection by the City to ensure the facilities have been properly maintained and are operating as designed. Flow control facilities,water quality treatment facilities,and on-site BMPs to be maintained and operated by the City, along with the required perimeter landscaping(as required per RMC 4-9-150),must be located in a stormwater tract. For drainage facilities requiring perimeter landscaping,the stormwater tract shall be granted and conveyed with all ownership and maintenance obligations(excluding maintenance of the drainage facilities)to the subdivision's lot owners.An easement under and upon said tract shall be dedicated to the City for the purpose of operating,maintaining, and repairing the drainage facilities contained in the stormwater tract. If perimeter landscaping is not required,then the stormwater tract shall be dedicated to the City along with the maintenance of the drainage facility contained therein. Required vegetated flow paths for full dispersion and basic dispersion BMPs require a recorded declaration of covenant that stipulates restrictions on use AND shall be located in an easement that includes provisions for access and maintenance. City maintenance of these vegetated flow paths will be limited to their functionality. All other maintenance shall remain the responsibility of the owner(s).Access roads serving these facilities must also be located in the tract or right-of-way and must be connected to an improved public road right-of-way. Conveyance systems to be maintained and operated by the City must be located in a drainage easement, tract, or right-of-way granted to the City. Note: the City does not normally assume maintenance responsibility for conveyance systems that are outside of improved public road right-of-way. Drainage Facilities to be Maintained by Private Parties For residential subdivisions of nine lots or less with private streets,planned unit developments, and commercial and industrial sites,maintenance and operation of flow control and water quality treatment facilities including on-site BMPs are the responsibility of the property owner(s)and must be located in a tract or easement that identifies each property owner as having equal and undivided interest. Shared facilities shall be maintained jointly by the property owners or users of the facility. Shared facilities must have a City approved maintenance plan or agreement regarding assignment of maintenance and operation. All drainage facilities maintained privately,by the City or by other public agencies must be maintained as specified in Appendix A, "Maintenance Requirements for Stormwater Facilities and On-Site BMPs,"and as further prescribed in Chapter 6 for water quality facilities,unless otherwise approved by the City.A copy of the Operation and Maintenance Manual submitted as part of the permit application for flow control and water quality treatment facilities(see Section 2.3.1)shall be retained on site and shall be transferred with the property to the new owner.A log of maintenance activity indicating when cleaning occurred and where waste was disposed of shall also be kept by the owner and be available for inspection by the City. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-60 1.2.6 CORE REQUIREMENT#6:MAINTENANCE AND OPERATIONS All privately maintained on-site BMPs must be maintained as specified in the sitellot's declaration of covenant and grant of easement per Section 1.2.9. The City shall annually inspect all privately maintained drainage facilities for compliance with these requirements. The City may reduce the inspection frequency based on maintenance records of double the length of time of the proposed inspection frequency. If the property owner(s) fails to maintain their facilities to the acceptable standards,the City shall issue a written notice specifying the required remedial actions and requiring a schedule for timely completion of the actions. If these actions are not performed in a timely manner,the City shall enter the property to perform the actions needed and bill the property owner(s)for the cost of the actions. If a hazard to public safety exists,the City shall perform remedial actions without written notice. If the proposed project is a commercial,industrial, or multifamily development or redevelopment, or a single family residential building permit, a drainage facility declaration of covenant and grant of easement must be recorded at the King County Office of Records and Elections. Whenever a flow control facility, water quality treatment facility, or on-site BMP is proposed to be located on a parcel separate from the parcel or parcels containing the target surfaces mitigated by the facility or BMP,provisions must be made to ensure that the owner or owners of the target surfaces have a perpetual right to operate and maintain the facility. This may be done either by recording an easement granting this right to the owner(s) of the target surfaces, or by conveying the land on which the facility sits (or an interest therein)to the owner(s)of target surfaces. If the proposed project is a residential subdivision development, all privately maintained conveyance systems or other drainage facilities that convey flows through private property must be located in a drainage easement dedicated to convey surface and stormwater. Individual owners of the properties containing these easements must maintain the drainage facilities through their property. The legal instrument creating drainage easements on private property must contain language that requires a private property owner to obtain written approval from the City prior to removing vegetation(except by routine mowing) from any drainage easement containing open,vegetated drainage facilities(such as swales, channels, ditches,ponds, etc.). Maintenance of On-Site BMPs Maintenance and operation of all on-site BMPs are the responsibility of the property owner unless specified above in Section 1.2.6(Drainage Facilities to be Maintained by the City of Renton). On-site BMPs are not allowed in City right-of-way unless constructed to mitigate for public improvements. Maintenance and operation of on-site BMPs constructed in the right-of-way is the responsibility of the adjacent property owner in accordance with RMC 4-6-060. 1.2.7 CORE REQUIREMENT #7: FINANCIAL GUARANTEES AND LIABILITY In accordance with RMC 4-6-030, CED shall require all persons constructing any surface water facilities (including flow control/water quality facilities,conveyance systems, erosion control, and road drainage), to post with the City of Renton a bond, assignment of funds or certified check. The applicant must also maintain liability insurance as described in this Core Requirement 47. Intent: To ensure financial guarantees are posted to sufficiently cover the cost of correcting,if necessary, incomplete or substandard drainage facility construction work, and to warrant for two years the satisfactory performance and maintenance of those newly-constructed drainage facilities. Core Requirement 47 is also intended to ensure that a liability policy is provided that protects the proponent and the City from any damages relating to the construction or maintenance of required drainage facilities by private parties. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-61 SECTION 1.2 CORE REQUIREMENTS Construction Bond for Required Improvements Before a permit,pursuant to the provisions of RMC 4-6-030,may be issued,the applicant may be required to execute to the City a construction bond. In some instances, and at the sole option of the City, a certificate of occupancy, final inspection, or final approval may be issued prior to completion of required public or site improvements if an acceptable form of guarantee is provided by the applicant. Amount of Required Construction Bond: The construction bond shall be for not less than 100 percent of the amount calculated in the bond quantity worksheet(as provided in Reference Section 8-H)of all required drainage improvements associated with the proposed project.The bond quantity worksheet shall be provided by the applicant and is subject to review and acceptance by the City. Utilization of Funds Provided by the Construction Bond: If the required improvements associated with the proposed project are not completed by the termination date of the construction bond,the City shall use the bond to construct the improvements in accordance with the City's standards. Release of Construction Bond: The construction bond will be released when the applicant completes the following: • Correct any defects noted in the final inspection. • Address,to the satisfaction of the City, all deficiencies noted in the final inspection by the City. • Provide to the City as-built drawings, final recorded plat,recorded easements,bill of sale, cost data inventory of public storm system improvements to be owned and maintained by the City, and recorded restricted covenant and grant of easement. • Receive a City Final inspection to ensure the drainage facilities have been properly installed and are operated as desired. • Submission of maintenance bond to the City. Maintenance Bond Prior to acceptance by the City of any newly constructed public improvements to be deeded to the City, or any onsite or offsite private storm drainage improvements,the applicant shall file with the City a construction maintenance bond. The maintenance bond is to be held by the City for a period of two years. Amount of Maintenance Bond: The maintenance bond shall be for 20 percent of the amount calculated in the bond quantity worksheet. Utilization of Funds Provided by the Maintenance Bond: In the event that required improvements are not properly maintained during the required maintenance guarantee period,the City shall notify the developer/owner. If the developer/owner fails to correct the problem within a period of 15 days,the City shall use the maintenance guarantee to perform the maintenance work. Should any failures occur in regard to required improvements associated with a development project within the warranty period,the City shall require the developer/owner to correct all failures. Should the developer/owner fail to perform within a period of 15 days,the City shall use the maintenance and warranty bond to correct any failures. Release of Maintenance Bond: Maintenance bond will be released upon completion of the two-year maintenance bond period following final inspection and correction of any maintenance defects identified in the final inspection by the City. Hold Harmless The permittee shall protect, defend,indemnify, and save harmless the City, its officers, employees, and agents from any and all costs,claims,judgments, or awards of damages, arising out of or in any way resulting from the negligent acts or omissions of the permittee. The permittee agrees that its obligations under this Section extend to any claim, demand, and/or cause of action brought by, or on behalf of, any of its employees or agents. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-62 1.2.8 CORE REQUIREMENT#8:WATER QUALITY FACILITIES Insurance Required Before a permit shall be issued for any construction,insurance will be required as follows: 1. The applicant shall secure and maintain in force throughout the duration of the permit: Commercial General Liability insurance written on an occurrence basis with limits no less than one million dollars ($1,000,000)per occurrence/two million dollars($2,000,000) aggregate. 2. Copies of such insurance policy or policies shall be furnished unto the City with a special endorsement in favor of the City with the City named as a primary and noncontributory additional insured on the insurance policy and an endorsement stating such shall be provided to the City. 3. The policy shall provide that it will not be canceled or reduced without 30 days' advanced written notice to the City. 4. Upon showing of a hardship and at the discretion of the Administrator or his/her designee,the insurance requirements may be reduced or waived for single-family or two-family residential applications. Other Important Information about Core Requirement#7 Other requirements include the following: • Cash Bond Returned:The cash bond will be returned to applicant when work is accepted by the City,less any sums due to the City under the terms of this Core Requirement 47. • Reimbursement of City's Costs Incurred to Obtain Funds Provided by Guarantees: If the City finds it necessary to utilize funds provided for any guarantee, and incurs expenses in obtaining and administering such funds,a portion of these monies shall also be used to reimburse the City for such recovery costs. If the guarantee is not adequate to cover all necessary costs,the developer/owner is required to make up the deficit in cash within 30 days of receipt of written notice from the City. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES All proposed projects,including redevelopment projects,must provide water quality(WQ) facilities to treat the runoff from those new and replaced pollution-generating impervious surfaces and new pollution-generating pervious surfaces targeted for treatment as specified in the following sections. These facilities shall be selected from a menu of water quality facility options specified by the area-specific facility requirements in Section 1.2.8.1 and implemented according to the applicable WQ implementation requirements in Section 1.2.8.2. Intent: To require an efficient,cost-effective level of water quality treatment tailored to the sensitivities and resource protection needs of the downstream receiving water to which the project site drains, or,in the case of infiltration,protection of the receiving groundwater system. Guide to Applying Core Requirement#8 Core Requirement#8 requires that WQ facilities be provided to remove pollutants from runoff discharging , from a project site in accordance with land use-specific WQ facility requirements found in Section 1.2.8.1. For efficient application of CoreRequirement#8,the following steps,are recommended: 1. Check the exemption language in Section 1.2.8 to determine if or which'threshold discharge areas of the project site must provide WQ facilities per Core Requirement#8. 2. Use the Basic WQ treatment areas section(Section 1.2.8.1.A)to determine if basic or enhanced treatment is required. 3. Consult Section 1.2.8.2 for other design requirements,allowances,and flexible compliance provisions related to implementing water quality treatment. 4. Consult Sections 1.2.2,Core Requirement#2: Offsite Analysis, 1.2.2.1,Downstream Analysis,and 1.2.2.1.2,Downstream Water Quality Problems Requiring Special Attention. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-63 SECTION 1.2 CORE REQUIREMENTS Other Important Information about Core Requirement#8 Core Requirement 48 is the primary component of an overall water quality protection strategy required by this manual. Other requirements include the following: • Core Requirement 44: Conveyance System,Spill Control Provisions, Section 1.2.4—This provision generally applies whenever a project constructs or replaces onsite conveyance system elements that receive runoff from pollution-generating impervious surfaces. The provision requires that runoff from such impervious surfaces be routed through a spill control device prior to discharge from the project site or into a natural onsite drainage feature. • Core Requirement 44: Conveyance System,Groundwater Protection, Section 1.2.4—This provision requires that ditches/channels be lined as needed to reduce the risk of groundwater contamination when they convey runoff from pollution-generating impervious surfaces that comes into direct contact with an outwash soil. Open facilities(such as flow control and water quality treatment ponds, stormwater wetlands and infiltration facilities) and/or conveyance systems are not allowed in Zone I of the Aquifer Protection Area. • Special Requirement 44: Source Control, Section 1.3.4—This requirement applies water quality source controls from the King County Stormwater Pollution Prevention Manual to commercial, industrial, and multifamily projects. • Special Requirement 45: Oil Control, Section 1.3.5—This requirement applies special oil controls to those projects proposing to develop or redevelop a high-use site. ❑ EXEMPTIONS FROM CORE REQUIREMENT#8 There are four possible exemptions from the requirement to provide a water quality facility per Core Requirement 48: 1. Surface Area Exemption A proposed project or any threshold discharge area within the site of a project is exempt if it meets all of the following criteria: a) Less than 5,000 square feet of new plus replaced PGIS that is not fully dispersed will be created, AND b) Less than 3/4 acre of new PGPS that is not fully dispersed will be added. 2. Surface Exemption for Transportation Redevelopment Projects A proposed transportation redevelopment project or any threshold discharge area within the site of such a project is exempt if it meets all of the following criteria: a) The total new impervious surface within the project limits is less than 50% of the existing impervious surface,AND b) Less than 5,000 square feet of new PGIS that is not fully dispersed will be added,AND c) Less than 3/4 acre of new PGPS that is not fully dispersed will be added. 3. Cost Exemption for Parcel Redevelopment Projects A proposed redevelopment project on a single or multiple parcel site or any threshold discharge area within the site of such a project is exempt if it meets all of the following criteria: a) The total valuation of the project's proposed improvements(including interior improvements and excluding required mitigation improvements)is less than 50%of the assessed value of the existing site improvements,AND b) Less than 5,000 square feet of new PGIS that is not fully dispersed will be added,AND c) Less than 3/4 acre of new PGPS that is not fully dispersed will be added. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-64 1.2.8 CORE REQUIREMENT#8:WATER QUALITY FACILITIES 4. Soil Treatment Exemption A proposed projector any drainage area within a project is exempt if the runoff from pollution- generating impervious surfaces is infiltrated in soils that meet the "groundwater protection criteria" outlined below. Groundwater Protection Criteria: The first 2 feet or more of the soil beneath an infiltration facility must have a cation exchange capacity greater than 5 (tested using EPA Laboratory Method 908 1)and an organic content greater than 0.5% (measured on a dry weight basis using ASTM D 2974),AND must meet one of the following specifications for general protection of groundwater: a) The soil must have a measured infiltration rate 17 of less than or equal to 9 inches per hour, except in groundwater protection areas where the measured rate must be less than or equal to 2.4 inches per hour,OR a) The soil must be composed of less than 25%gravel by weight with at least 75% of the soil passing the 44 sieve, and the portion passing the 44 sieve must meet one of the following gradations: • At least 50%must pass the 440 sieve and at least 2%must pass the 4100 sieve, OR • At least 25%must pass the 440 sieve and at least 5%must pass the 4200 sieve.37 1.2.8.1 LAND USE-SPECIFIC WATER QUALITY FACILITY REQUIREMENT Projects subject to Core Requirement 48 must provide a water quality facility selected from a menu of water quality facility options identified in the area-specific facility requirements and exceptions for the WQ treatment area in which the proposed project or threshold discharge area of the proposed project is located. These WQ treatment areas are listed below and their requirements and exceptions are detailed in the following subsections: A. Basic WQ Treatment Areas B. Sensitive Lake WQ Treatment Areas C. Sphagnum Bog WQ Treatment Areas. Intent: To apply an appropriate level of water quality treatment based on the sensitivities of receiving waters for the drainage area in which the project lies. These drainage areas are identified as WQ treatment areas on the WQ Applications Map adopted with this manual. In addition to a minimum basic standard, which applies broadly to most geographic areas, special menus are provided for land uses that generate the highest concentrations of metals in stormwater and for sites within the watersheds of sensitive lakes, and sphagnum bog wetlands. A. BASIC WQ TREATMENT AREAS Basic WQ Treatment Areas are designated by the City of Renton where a general,cost-effective level of treatment is sufficient for most land uses. Most direct discharges only require Basic WQ Treatment. Some land uses,however,will need an increased level of treatment(Enhanced Basic WQ Treatment)because they generate high concentrations of metals in stormwater runoff and acute concentrations of metals in streams are toxic to fish. Required Treatment Menu Within Basic WQ Treatment Areas, a water quality facility option from the Basic WQ menu shall be used to treat runoff from the surfaces listed under"Target Surfaces"below, except where such treatment is 37 Measured infiltration rate shall be as measured by the EPA method or the Double Ring Infiltrometer Method(ASTM D3385). For some soils, an infiltration rate of less than 9 inches per hour may be assumed based on a soil texture determination rather than a rate measurement. For more details,see the"Groundwater Protection"requirements in Section 5.4.1. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-65 SECTION 1.2 CORE REQUIREMENTS waived or reduced by the area-specific exceptions at the end of this subsection and except where the Enhanced Basic WQ menu is applicable as follows. If 50% or more of the runoff that drains to any proposed water quality facility is from one or more of the following land uses,then the Enhanced Basic WQ menu shall be used in place of the Basic WQ menu for the design of this facility, except if such treatment is waived or reduced by the area-specific exceptions at the end of this subsection: 1. Commercial,industrial, or multifamily land use. 2. A road with an expected average daily traffic(ADT)count of 7,500 or more vehicles. . Treatment Goal and Options The treatment goal for facility options in the Basic WQ menu is 80%removal of total suspended solids (TSS)for flows or volumes up to and including the WQ design flow or volume for a typical rainfall year, assuming typical pollutant concentrations in urban runoff 38 TSS is the general performance indicator for basic water quality protection because it is the most obvious pollutant of concern.TSS is not a single pollutant; it is a general term for a highly variable mixture of solid pollutants with variable particle size and particle density distributions, and to one degree or another containing a variety of sorbed dissolvable pollutants. The Basic WQ menu includes facilities such as wetponds,combined detention/wetponds, bioswales,vegetated filter strips, and sand filters. See Chapter 6 for specific facility choices and design details.Additional facility designs may appear in Reference Section 14 in the future. The treatment goal for facility options in the Enhanced Basic WQ menu is to accomplish better removal of heavy metals and potentially other toxic materials than can be achieved by basic treatment,while still meeting the basic treatment goal of 80%TSS removal. The specific target performance is>30%reduction of dissolved copper and>60%removal of dissolved zinc. Dissolved copper and zinc are indicators of a wider range of metals typically found in urban runoff that are potentially toxic to fish and other aquatic life. The Enhanced Basic WQ menu includes options for use of a basic-sized stormwater wetland, a large sand filter, or a combination of two facilities in series. See Chapter 6 for specific facility options and designs.Additional facility designs may appear in Reference S 14 in the future. Intent The Basic WQ menu is intended to be applied to both stormwater discharges draining to surface waters and those infiltrating into soils that do not provide adequate groundwater protection(see Exemptions 4 and 5 from Core Requirement 48). Overall,the 80%TSS removal objective,in conjunction with special requirements for source control and high-use site controls, should result in good stormwater quality for all but the most sensitive water bodies. Increased water quality treatment is necessary for developments that generate the highest concentrations of metals and for developments that drain to sensitive lakes and sphagnum bog wetlands. Facility options in the Enhanced Basic WQ menu are intended to remove more metals than expected from those in the Basic WQ menu. Lower metal concentrations reduce the risk to fish from exposure to both chronic and acute toxic concentrations of metals such as copper and zinc, and very low concentration copper deleterious olfactory effects.As the toxicity of metals depends on their concentration,this standard is most effective for project sites with a larger proportion of pollution-generating impervious surface like roadways and medium to high density subdivisions. The Enhanced Basic WQ menu is intended to apply to all such project sites that drain by surface flows to a fish-bearing stream. However,projects that drain entirely by pipe to the major receiving waters listed Table 1.2.3.13 may be excused from the increased treatment and may revert to the Basic WQ menu because concentration effects are of less concern as the overall flow volume increases;however,this exception is not applicable for WQ impaired segments per 38 The influent concentration range for demonstrated pollutant removal is 100 to 200 mg/L. For influent concentrations lower than 100 mg/I the effluent goal is equal to or less than 20 mg/I. For influent concentrations greater than 200 mg/I,the goal is greater than 80%TSS removal. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-66 1.2.8 CORE REQUIREMENT#8:WATER QUALITY FACILITIES Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention,Metals Problem(Type 4). Target Surfaces Facilities in Basic WQ Treatment Areas must treat(either directly or in effect)the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces(see Section 1.2.3.2.C)in Core Requirement 43.For individual lots within residential subdivision projects,the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential subdivision projects,the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant,tract, or easement. 3. Replaced PGIS that is not fully dispersed on a non-redevelopment project. 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements(including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of the existing site improvements. Exceptions The following exceptions apply only in Basic WQ Treatment Areas: 1. Exception 41 does not apply to the City 2. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic WQ menu for treatment of any runoff that is infiltrated per the standards of Section 5.2. This exception is not allowed where infiltrating within one-quarter-mile of a fresh water designated for aquatic life use or that has an existing aquatic life use into soils that do not meet the groundwater protection standards described in Section 5.2.1. 3. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic WQ menu for treatment of any runoff that is discharged directly,via a non-fish-bearing conveyance system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more(at the discharge point of the conveyance system) or a lake that is 300 acres or larger. This exception does not apply where the receiving water is impaired as described in the full description of major receiving water in the Definitions section or is impaired for metals according to Downstream Analysis. 4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use may be reduced to the Basic WQ menu if all of the following criteria are met: a) A facility from the Enhanced Basic WQ menu is not feasible,AND b) No leachable heavy metals are currently used or proposed to be used in areas of the site, exposed to the weather,AND c) A covenant is recorded that prohibits future such use of leachable,heavy metals on the site(use the covenant in Reference Section 8-Q),AND d) Less than 50%of the runoff draining to the proposed water quality facility is from any area of the site comprised of one or both of the following land uses: 2017 City of Renton Surface Water Design Manual 12/12/2016 1-67 SECTION 1.2 CORE REQUIREMENTS • Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square feet of gross building area. • Commercial land use involved with vehicle repair,maintenance, or sales. 5. The facility requirement as applied to replaced PCIS maybe waived if the City has adopted a plan and implementation schedule for fulfilling this requirement using regional facilities. B. SENSITIVE LAKE WQ TREATMENT AREAS There are no Sensitive Lake WQ Treatment Areas in the City at the time this manual was adopted; however,this section has been retained in case of future changes in lake status. Required Treatment Menu Within Sensitive Lake WQ Treatment Areas, a water quality facility option from the Sensitive Lake Protection menu shall be used to treat runoff from the surfaces listed under"Target Surfaces"below, except where such treatment is waived or reduced by the area-specific exceptions at the end of this subsection and except where the Enhanced Basic WQ menu is applicable as follows. If 50% or more of the runoff that drains to any proposed water quality facility is from one or more of the following land uses, then a water quality facility option common to both the Sensitive Lake Protection menu and Enhanced Basic WQ menu shall be used for the design of this facility, except if such treatment is waived or reduced by the area-specific exceptions at the end of this subsection: 1. Commercial,industrial, or multifamily land use. 2. A road with an expected average daily traffic(ADT)count of 7,500 or more vehicles. Treatment Goal and Options The treatment goal for facility options in the Sensitive Lake Protection menu is 50% annual average total phosphorus(TP)removal assuming typical pollutant concentrations in urban runoff 39 This goal was chosen as a realistic and cost-effective level of phosphorus removal. The Sensitive Lake Protection menu includes options for using either Basic WQ facilities of larger size,combinations of two facilities in series,40 or a single facility in combination with land use planning elements that reduce phosphorus. See Chapter 6 for specific facility options and design details. On some developments or portions thereof that have surface uses that generate the highest concentrations of metals in stormwater runoff,the treatment goal is expanded to include>30%reduction of dissolved copper and>60%removal of dissolved zinc. This expanded goal requires use of a water quality facility option that is common to both the Sensitive Lake Protection menu and the Enhanced Basic menu. Intent A project discharging runoff via surface flow contributes phosphorus loading to a sensitive lake regardless of distance from the lake. If discharge is via infiltration through coarse soils,it is also possible that phosphorus would be transported through the ground for some distance without attenuation. This groundwater transport distance is considered to be typically no more than one-quarter mile. Therefore, onsite treatment using the Sensitive Lake Protection menu is required prior to infiltration within one- quarter mile of a sensitive lake. Infiltration through finer soils is expected to provide significant attenuation of TP, so the general groundwater protection criteria specified in Section 1.2.8 under"Soil Treatment Exemption"are considered sufficient for infiltration through finer soils. Where the treatment goal is expanded to include>30%reduction of dissolved copper and>60%removal of dissolved zinc,the facility options common to both the Sensitive Lake Protection menu and the Enhanced Basic WQ menu should meet this goal as well as the lake protection goal of 50%removal of 39 Phosphorus concentrations of between 0.10 and 0.50 mg/L are considered typical of Seattle area runoff(Table 1,"Water Quality Thresholds Decision paper,"King County Surface Water Management Division,April 1994). 40 In series means that the entire treatment water volume flows from one facility to the other in turn. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-68 1.2.8 CORE REQUIREMENT#8:WATER QUALITY FACILITIES annual average total phosphorous. The intent behind the enhanced heavy metals removal goal and why it is applied is described in Section 1.2.8.1. Target Surfaces Facilities in Sensitive Lake WQ Treatment Areas must mitigate (either directly or in effect)the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces(see Section 1.2.3.2.C)in Core Requirement 43.For individual lots within residential subdivision projects,the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential subdivision projects,the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant,tract, or easement.Note:where the runofffrom target PGPS is separated from the runofffrom target PGIS, the Basic WQ menu may be used in place of the Sensitive Lake Protection menu for treatment of runofffrom the target PGPS(see the area-specific exceptions at the end of this subsection). 3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project. 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced PGIS that is not fully dispersed, on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements(including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of the existing site improvements. Exceptions The following exceptions apply only in Sensitive Lake WQ Treatment Areas: 1. The Basic WQ menu may be used in place of the Sensitive Lake Protection menu for treatment of any runoff that is infiltrated according to the standards in Section 5.2.This exception is not allowed where infiltrating within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that receiving water into soils that do not meet the groundwater protection standards described in Section 5.2.1. 2. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be waived for treatment of any runoff that is infiltrated according to the standards in Section 5.2. This exception is not allowed where infiltrating within one-quarter-mile of a fresh water designated for aquatic life use or that has an existing aquatic life use into soils that do not meet the groundwater protection standards described in Section 5.2.1. 3. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be waived for treatment of any runoff that is discharged,via a non-fish-bearing conveyance system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more (at the discharge point of the conveyance system)or a lake that is 300 acres or larger. This exception is not applicable for WQ impaired segments per Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention,Metals Problem (Type 4). 4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use may be waived if the all of the following criteria are met: a) No leachable metals(e.g.,galvanized metals)are currently used or proposed to be used in areas of the site, exposed to the weather,AND 2017 City of Renton Surface Water Design Manual 12/12/2016 1-69 SECTION 1.2 CORE REQUIREMENTS b) A covenant is recorded that prohibits future such use of leachable metals on the site, exposed to the weather(use the covenant in Reference Section 8-Q),AND c) Less than 50%of the runoff draining to the proposed water quality facility is from any area of the site comprised of one or both of the following land uses: • Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square feet of gross building area. • Commercial land use involved with vehicle repair,maintenance, or sales. 5. The Basic WQ menu may be used for treatment of any runoff from target PGPS that is treated separately from the runoff from target PGIS. 6. Exception 46 does not apply in the City. 7. The facility requirement as applied to replaced PGIS may be waived if the City has adopted a plan and implementation schedule for fulfilling this requirement using regional facilities. Note:If a lake management plan has been prepared and adopted by the City, additional treatment and/or other water quality measures may be required as specified in the plan and pursuant to Special Requirement 41, Section 1.2.9. C. SPHAGNUM BOG WQ TREATMENT AREAS There are no Sphagnum Bog WQ Treatment Areas in the City at the time this manual was adopted; however,this section has been retained in case of future changes. Sphagnum Bog WQ Treatment Areas are areas of King County from which runoff drains to or otherwise comes into contact with the vegetation of a sphagnum bog wetland41 larger than 0.25 acres in size.42 These wetlands support unique vegetation communities, and they tend to develop in areas where water movement is minimized.Although sphagnum bog wetlands are typically isolated from significant sources of surface and ground water and receive their main water supply from rainfall,there are instances where they are components of larger wetlands and may be subject to inundation by those wetlands during high intensity or long duration runoff events. Sphagnum bog wetlands are generally uncommon in the Puget Sound area; of all the inventoried wetlands in King County, only a small percentage have sphagnum bog wetland components.43 Only a portion of all sphagnum bog wetlands have been identified and mapped by King County. Consequently,many of these wetlands and their contributing drainage areas must be identified during the wetland identification and delineation for a project site and during offsite analysis as required in Core Requirement 42.A list of identified sphagnum bog wetlands is included on the WQ Applications Map and in the 1997 King County Bog Inventory,updated November 2002;however,if a wetland that meets the definition of a sphagnum bog wetland is found downstream of a project site and runoff from the project site drains to or otherwise comes into contact with the wetland's vegetation,the project site is considered to be within a Sphagnum Bog WQ Treatment Area whether the wetland is listed or not. Note:Any threshold discharge area from which runoff drains to or comes into contact with the vegetation of a sphagnum bog wetland larger than 0.25 acres in size is considered to be within a Sphagnum Bog WQ Treatment Area regardless of the WQ treatment area indicated by the WQ Applications Map. Required Treatment Menu A treatment option from the Sphagnum Bog Protection menu shall be used to treat runoff from the target surfaces specified below, except where this mitigation is waived or reduced by the area-specific exceptions at the end of this subsection. 41 A sphagnum bog wetland is defined as a wetland dominated by sphagnum moss and which has an associated acid-loving plant community.See the"Definitions"section for more details on how King County defines a sphagnum bog wetland. 42 The size of a sphagnum bog wetland is defined by the boundaries of the sphagnum bog plant community. 43 Approximately 3%of wetlands in the 1990 sensitive areas inventory are either sphagnum bog wetlands or include portions of a lake or wetland with sphagnum bog wetland characteristics. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-70 1.2.8 CORE REQUIREMENT#8:WATER QUALITY FACILITIES Treatment Goals and Options The treatment goals for protection of sphagnum bog wetlands include the control of nutrients, alkalinity, and pH.Although these goals may change as additional information about these wetlands becomes available,target pollutant removals for sphagnum bog protection are currently as follows: • Total phosphorus reduction of 50% • Nitrate+nitrite reduction of 40% • pH below 6.5 • Alkalinity below 10 mg CaCO3/L. Facility options to meet these goals are limited;therefore,the City discourages developments from discharging runoff to sphagnum bog wetlands. Where infiltration of developed area runoff is not feasible or applicable per Section 5.2,water quality facility options include a treatment train 44 of two or three facilities in series. One of the facilities in the train must be a sand filter. The order of facilities in the treatment train is important; see Chapter 6 for specific facility options and design details. Intent Sphagnum bog wetlands support unique vegetation communities that are extremely sensitive to changes in alkalinity and nutrients from surface water inputs. The most effective way to prevent these changes is to infiltrate or redirect developed area runoff so it does not come into contact with the vegetation of a sphagnum bog wetland. However,this is not practicable for most development projects due to soil constraints precluding infiltration(see Section 5.2)and the onerous nature of bypassing runoff around a wetland. Therefore,where runoff contact with sphagnum bog vegetation cannot be avoided,the bog protection menu seeks to minimize certain changes in the chemistry of developed area runoff to protect this unique vegetation. This menu applies not only to runoff that drains directly to a sphagnum bog wetland but to runoff that otherwise comes into contact with the bog's vegetation, such as through inundation of the bog by an adjacent water body during high intensity or long duration runoff events. While water quality facility options emphasize reduction of mineral elements(alkalinity) and nutrients in the runoff,little is known about their ability to reduce alkalinity or to actually protect sphagnum-based plant communities. In addition,the effect of frequent water level changes on the sphagnum plant community is also unknown but could be damaging. Hence,it is best to avoid discharge to sphagnum bog wetlands whenever possible.Permeable pavements that are tributary to sphagnum bog wetlands should be types other than Portland cement(PCC)permeable pavement,if feasible. Target Surfaces Facilities in Sphagnum Bog WQ Treatment Areas must mitigate (either directly or in effect)the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces(p. 1-46)in Core Requirement 43.For individual lots within residential subdivision projects,the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential subdivision projects,the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant,tract, or easement. 3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project. 44 A treatment train is a combination of two or more treatment BMPs connected in series(i.e.,the design water volume passes through each facility in turn). 2017 City of Renton Surface Water Design Manual 12/12/2016 1-71 SECTION 1.2 CORE REQUIREMENTS 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50%or more of the existing impervious surface within the project limits. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements(including interior improvements and excluding required mitigation improvements)exceeds 50% of the assessed value of the existing site improvements. Exceptions The following exceptions apply only in Sphagnum Bog WQ Treatment Areas: 1. The Basic WQ menu may be used in place of the Sphagnum Bog Protection menu for treatment of any runoff that is infiltrated in a facility per Section 5.2. This exception is not allowed where infiltrating within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that receiving water into soils that do not meet the groundwater protection standards described in Section 5.2.1. If the infiltration facility is located in soils not meeting the groundwater protection standards described in Section 5.2.1,and within the prescribed distance of a sensitive lake,then the Sensitive Lake Protection menu shall be used. 2. The facility requirement for Sphagnum Bog WQ Treatment Areas may be reduced to that of the surrounding WQ treatment area(i.e., either the Basic WQ Treatment Area or Sensitive Lake Treatment Area,whichever contains the Sphagnum Bog WQ Treatment Area) for treatment of any replaced PGIS runoff. 1.2.8.2 WATER QUALITY IMPLEMENTATION REQUIREMENTS Water quality facilities shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. METHODS OF ANALYSIS AND DESIGN Water quality facilities shall be analyzed and designed as detailed in Chapter 6. B. SITING OF WATER QUALITY FACILITIES Required water quality facilities shall be located so as to treat the runoff from all target surfaces, except as allowed below under"Treatment Trades"and"Untreated Discharges." Any other onsite or offsite runoff draining to a proposed water quality facility must be treated whether it is from a target pollution-generating surface or not and regardless of whether the runoff has already been treated by another facility. The facility must be sized for all flows/volumes entering the facility. This is because treatment effectiveness is determined in part by the total volume of runoff entering the facility. C. TREATMENT TRADES The runoff from target p ollution-gen crating surfaces may be released untreated if an existing non- targeted pollution-generating surface of equivalent size and pollutant characteristics lying within the same watershed or stream reach tributary area is treated on the project site. Such substitution is subject to the following restrictions: 1. The existing non-targeted pollution-generating surface is not currently being treated,is not required to be treated by any phase of the proposed project,is not subject to NPDES or other permit requirements, and is not under a compliance order or other regulatory action,AND 2. The proposal is reviewed and approved by CED. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-72 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS D. UNTREATED DISCHARGES If site topographic constraints are such that runoff from a target pollution-generating surface must be pumped to be treated by the required water quality facility,then CED may allow the area's runoff to be released untreated provided that all of the following conditions are met: 1. Treatment of the constrained area by filter strip,bioswale, or a linear sand filter is not feasible, and a treatment trade as described above is not possible. 2. The untreated target surface is less than 5,000 square feet of new plus replaced PGIS. E. USE OF PROPRIETARY FACILITIES Water quality facilities other than those identified in Chapter 6,Reference Section 14-A, or Reference Section 14-B may be allowed if it can be demonstrated that they are likely to meet the pollutant removal goal for the applicable receiving water.Use of such facilities requires an adjustment,which requires approval by the City according to Section 1.4, "Adjustment Process,"and Section 6.7, "Alternative Facilities."Any new treatment technologies must be approved through the state Department of Ecology's TAPE45 or CTAPE46 program before the technology can be considered by the City. Monitoring will be required,the nature of which will depend on the pre-existing Ecology use-level designation,the number of existing facilities of this design for which monitoring data already exists, and review of the monitoring results from those facilities. When sufficient data on performance and maintenance requirements have been collected and if both are acceptable,the new facility may be added to the appropriate water quality menu for common use through a blanket adjustment or update of this manual. Criteria may be set,which if not met,may require replacement of the facility with a standard facility from Chapter 6. F. OWNER RESPONSIBILITY FOR WATER QUALITY Regardless of the means by which a property owner chooses to meet the water quality requirements of this manual—whether a water quality facility, a train of facilities, or a treatment trade—it is the responsibility of the property owner to ensure that runoff from their site does not create water quality problems or degrade beneficial uses downstream. It is also the responsibility of the property owner to ensure that the discharge from their property is not in violation of state and federal laws. 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS All proposed projects,including redevelopment projects,must provide on-site BMPs to mitigate the impacts of storm and surface water runoff generated by new impervious surface,new pervious surface, existing impervious surfaces, and replaced impervious surface targeted for mitigation as specified in the following sections. On-site BMPs must be selected and applied according to the basic requirements, procedures, and provisions detailed in this section and the design specifications for each BMP in Appendix C, Section C.2. On-site BMPs are methods and designs for dispersing,infiltrating, or otherwise reducing or preventing development-related increases in runoff at or near the sources of those increases. On-site BMPs include, but are not limited to,preservation and use of native vegetated surfaces to fully disperse runoff,use of other pervious surfaces to disperse runoff,roof downspout infiltration;permeable pavements;bioretention; limited infiltration systems; and reduction of development footprint. Intent: To provide mitigation of hydrologic impacts that are not possible/practical to mitigate with a flow control facility. Such impacts include increases in runoff volumes and peak discharges and decreases in groundwater recharge. Increased runoff volume and peak discharges leads to higher and more variable stream velocities at low flows and more frequent water level fluctuations in streams and wetlands. This 45 Ecology W,2011.Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies:Technology Assessment Protocol—Ecology(TAPE), Publication No. 11-10-061,2011 ed.Washington State Department of Ecology, Lacey,WA, pp. 1-73. <https-.//fortress.wa.gov/ecy/publications/summarypages/1110061.html>. 46 Chemical Technology Assessment Protocol—Ecology 2017 City of Renton Surface Water Design Manual 12/12/2016 1-73 SECTION 1.2 CORE REQUIREMENTS causes wash-out and stranding of aquatic species, algal scour and washout of organic matter,loss of vegetation diversity and habitat quality, and disruption of cues for spawning, egg hatching, and migration. Decreased groundwater recharge reduces water supply for human use and summer base flows in streams, which is critical to water temperature, salmonid use of smaller streams, and the habitat quality of mainstem side channels and wetlands used for spawning,rearing, and flood refuge. On-site BMPs seek to reduce runoff volumes and flashiness and increase groundwater recharge by reducing imperviousness and making use of the pervious portions of development sites to maximize infiltration and retention of stormwater onsite. Thus,the goal is to apply on-site BMPs to new impervious surfaces,new pervious surfaces, and replaced impervious surfaces,to the maximum extent feasible without causing flooding or erosion impacts. ❑ EXEMPTION FROM CORE REQUIREMENT#9 There is a single exemption from the on-site BMP provisions of Core Requirement 49: 1. Basic Exemption A proposed project is exempt if it meets the following criteria: a) Less than 2,000 square feet of new plus replaced impervious surface will be created,AND b) Less than 7,000 square feet of land disturbing activity will occur. 2. Flow Control Facility Exemption Any impervious surface served by an infiltration facility designed in accordance with the flow control facility requirement(Section 1.2.3.1),the facility implementation requirements(Section 1.2.3.2), and the design criteria for infiltration facilities(Section 5.2)is exempt from the on-site BMP requirement. 1.2.9.1 ON-SITE BMP REQUIREMENTS OVERVIEW Projects that are subject to Core Requirement 49 must apply on-site BMPs to either supplement the flow mitigation provided by required flow control facilities or provide flow mitigation where flow control facilities are not required.All such on-site BMPs are detailed in Appendix C of this manual. On-site BMPs must be implemented per the requirements and approach detailed in Sections 1.2.9.2 and 1.2.9.3 below for individual lots and subdivisions or road improvement projects,respectively.As described within Sections 1.2.9.2 and 1.2.9.3,there are two methods of satisfying the on-site BMP requirement: (1)application of BMPs to the maximum extent feasible using lists specific to the project location, size, and impervious coverage; or(2)using a continuous runoff model to demonstrate compliance with the Low Impact Development(LID)Performance Standard, described below. Demonstrating compliance with the LID Performance Standard using modeling is an optional method for all projects. A. TARGET SURFACES Target surfaces for application of Core Requirement 49 (On-site BMPs)include new impervious surfaces, new pervious surfaces, and replaced impervious surfaces,not already mitigated with an approved on-site BMP or flow control facility. Projects that trigger Core Requirement 49 by disturbing 7,000 square feet or more of land,but where new plus replaced impervious is less than 2,000 square feet,may consider basic dispersion as an equal choice for treating the target impervious surfaces alongside full infiltration,limited infiltration,bioretention, and permeable pavement. These projects are not required to meet the minimum BMP implementation requirements described in"Small Lot BMP Requirements"and"Large Lot BMP Requirements," (Requirement 45 on both lists), and are not required to comply with Core Requirement 46. Target pervious surfaces must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. Projects or threshold discharge areas of projects qualifying as exempt from the flow control facility requirement using the Direct Discharge Exemption in accordance with Section 1.2.3.1 do not have to 12/12/2016 2017 City of Renton Surface Water Design Manual 1-74 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS achieve the LID performance standard (described below),nor consider bioretention,permeable pavement, and full dispersion.However,target pervious surfaces must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13; and target impervious surfaces must implement full infiltration as detailed in Appendix C, Section C.2.2,Basic Dispersion per Appendix C, Section C.2.4;perforated pipe connection as detailed in Appendix C, Section C.2.11 for roofs,if feasible; and Basic Dispersion per Appendix C, Section C.2.4 for other impervious surfaces,if feasible. B. LOW IMPACT DEVELOPMENT PERFORMANCE STANDARD The LID Performance Standard is defined as follows: For the target surfaces subject to Core Requirement 49,Stormwater discharges shall match developed discharge durations to pre-developed durations for the range ofpre-developed discharge rates from 8%of the 2 yearpeakfow to 50%of the 2 yearpeakfow.Assume historical site conditions as the predeveloped condition. Projects that opt to demonstrate compliance with the LID Performance Standard using a continuous runoff model must protect the soil moisture capacity of new pervious in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. Projects that are required or opt to model compliance with the LID Performance Standard are still subject to meeting applicable area specific flow control requirements as determined in Core Requirement 43 (Section 1.2.3). Note that when demonstrating compliance with the LID Performance Standard, on-site BMPs are modeled explicitly,utilizing design infiltration rates as determined and selected per Section 5.2.1. However,when modeling flow control facility sizing,water quality facility sizing, and the peak flow exceptions from the area-specific flow control facility requirement in Sections 1.2.3.1.A,B, and C,these BMPs are not modeled explicitly,but may use modeling credits as allowed and subject to the limitations described in Section 1.2.9.4 and Table 1.2.9.A. On-site BMPs used to demonstrate compliance with the LID Performance Standard must meet the implementation requirements described in Section 1.2.9.4. C. DEMONSTRATING COMPLIANCE WITH THE LID PERFORMANCE STANDARD Project applicants may opt to use the LID Performance Standard in lieu of the BMP selection and application requirements described in Sections 1.2.9.2 and 1.2.9.3 below. D. IMPLEMENTATION Four kinds of implementation for the on-site BMP requirement are described in this section as follows: 1. For non-subdivision projects making improvements on an individual site/lot,implementation of this requirement shall be in accordance with the "Individual Lot BMP Requirements"in Section 1.2.9.2,which specify the selection of BMPs and the extent of their application on the site/lot. This required implementation of on-site BMPs must occur as part of the proposed project and provisions must be made for their future maintenance as specified in Section 1.2.9.2.As allowed in Sections 1.2.3 and 1.2.8,credits for the application of on-site BMPs per Table 1.2.9.A may be used to reduce the size of a required flow control facility,reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. 2. Subdivision projects and road improvement projects on sites that are 5 acres or larger AND located outside the Urban Growth Area(UGA).This requirement does not apply to the City of Renton. 3. For subdivision projects,implementation of on-site BMPs for associated plat infrastructure improvements(e.g.,roads, sidewalks) shall be done per Section 1.2.9.4 and must occur concurrently and as part of the proposed project,while BMPs associated with the individual lot improvements may be delayed until construction on the lots.As allowed in Sections 1.2.3 and 1.2.8,credits for the 2017 City of Renton Surface Water Design Manual 12/12/2016 1-75 SECTION 1.2 CORE REQUIREMENTS application of on-site BMPs per Table 1.2.9.A.may be used to reduce the size of a required flow control facility,reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. To use these credits, on-site BMPs must be implemented as part of the proposed project and provisions must be made for their future maintenance as specified in Section 1.2.9.4. For subdivision projects proposing to take credit for future implementation of BMPs on individual lots, provisions must be made to ensure their implementation as specified in Section 1.2.9.4. 4. For road improvement projects,implementation of on-site BMPs must occur as part of the proposed project.As allowed in Sections 1.2.3 and 1.2.8,credits for the application of on-site BMPs per Table 1.2.9.A may be used to reduce the size of a required flow control facility,reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. To use these credits, on- site BMPs must be implemented as part of the proposed project and provisions must be made for their future maintenance as specified in Section 1.2.9.4. The information presented in this section is organized as follows: • Section 1.2.9.2, "Individual Lot BMP Requirements" "Small Lot BMP Requirements,"Section 1.2.9.2.1 "Large Lot BMP Requirements,"Section 1.2.9.2.2 "Large Rural Lot BMP Requirements," Section 1.2.9.2.3 "Implementation Requirements for Individual Lot BMPs," Section 1.2.9.2.4 • Section 1.2.9.3, "Subdivision and Road Improvement Projects BMP Requirements" "Small Subdivision Project BMP Requirements,"Section 1.2.9.3.1 "Small Road Improvement and Urban Road Improvement Projects BMP Requirements," Section 1.2.9.3.2 "Large Rural Subdivision and Large Rural Road Improvement Projects BMP Requirements," Section 1.2.9.3.3 • Section 1.2.9.4, "Requirements for Use of BMP Credits" "Use of Credits by Subdivision Projects,"Section 1.2.9.4.1 "Use of Credits by Projects within Rights-of-Way,"Section 1.2.9.4.2 1.2.9.2 INDIVIDUAL LOT BMP REQUIREMENTS For projects on individual sites/lots, on-site BMPs must be selected and applied according to the individual lot BMP requirements in this section. For purposes of applying on-site BMPs to individual sitesllots,three categories of requirements have been established based on the size of sitellot subject to improvements by the project, and the extent of impervious surface coverage resulting from the project on the sitellot. These categories of requirements are as follows: • Small Lot BMP Requirements(for sitesllots<22,000 square feet) • Large Lot BMP Requirements(for sitesllots>_22,000 square feet) On-site BMPs must be applied in the order of preference and to the extent specified for the category of individual lot requirements applicable to the proposed project as described in the following subsections. Note:for lots created by a previous subdivision, some or all of these requirements may have been addressed by on-site BMPs installed on the lots or within common areas, tracts, or road right-of-way.In some cases, the type of BMPs required for a subdivision lot have already been established by a recorded 12/12/2016 2017 City of Renton Surface Water Design Manual 1-76 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS covenant on the lot. See Section 1.2.9.4 for more information on pre-installed or pre-determined BMPs in subdivisions. 1.2.9.2.1 SMALL LOT BMP REQUIREMENTS IF the proposed project is on a sitellot smaller than 22,000 square feet,THEN on-site BMPs must be applied as specified in the requirements below OR the project must demonstrate compliance with the LID Performance Standard (described in Section 1.2.9.1.13)using an approved continuous runoff model. Projects on small lots are typically single family residential improvements (e.g.,homes, outbuildings, etc.) but could be a small commercial development. 1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be evaluated for all target impervious surfaces. If feasible and applicable,full dispersion must be implemented as part of the proposed project. Typically, small lot full dispersion will be applicable only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet the minimum design requirements for full dispersion in Appendix C, Section C.2.1.1 2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause flooding or erosion impacts,the feasibility and applicability of full infiltration as detailed in Appendix C, Section C.2.2 must be evaluated(note, this will require a soils report for the site/lot). If feasible and applicable,full infiltration of roof runoff must be implemented as part of the proposed project. 3. All target impervious surfaces not mitigated by Requirements 1 and 2 above,must be mitigated to the maximum extent feasible using one or more BMPs from the following list.Use of a given BMP is subject to evaluation of its feasibility and applicability as detailed in Appendix C.Feasible BMPs are required to be implemented. The BMPs listed below may be located anywhere on the sitellot subject to the limitations and design specifications for each BMP.These BMPs must be implemented as part of the proposed project. • Full Infiltration per Appendix C, Section C.2.2, or per Section 5.2,whichever is applicable • Limited Infiltration per Appendix C, Section C.2.3, • Rain Gardens per Appendix C, Section C.2.12, sized as follows: o Rain gardens have a maximum contributing area of 5,000 square feet. o Rain gardens must have a minimum horizontal projected surface area below the overflow that is at least 5%of the area draining to it. • Bioretention per Appendix C, Section C.2.6, sized as follows: o SeaTac regional scale factor equals 1.0: In till soils,provide bioretention volume based on 0.6 inches of equivalent storage depth;in outwash soils provide bioretention volume based on 0.1 inches of equivalent storage depth, o SeaTac regional scale factor greater than 1.0: In till soils,provide bioretention volume based on 0.8 inches of equivalent storage depth; in outwash soils,provide bioretention volume based on 0.4 inches of equivalent storage depth, • Permeable Pavement per Appendix C, Section C.2.7 4. All target impervious surfaces not mitigated by Requirements 1,2 and 3 above,must be mitigated to the maximum extent feasible using the Basic Dispersion BMP described below.Use of Basic Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere on the sitellot subject to the limitations and design specifications cited in Appendix C. The BMP must be implemented as part of the proposed project. • Basic Dispersion per Appendix C, Section C.2.4, 5. BMPs must be implemented, at minimum, for an impervious area equal to at least 10%of the sitellot for sitellot sizes up to 11,000 square feet and at least 20%of the sitellot for sitellot sizes between 11,000 and 22,000 square feet.For projects located in Zone I of the Aquifer Protection Area,these impervious area amounts must be doubled. Doubling of the minimum impervious area required for 2017 City of Renton Surface Water Design Manual 12/12/2016 1-77 SECTION 1.2 CORE REQUIREMENTS BMP implementation in Zone I of the Aquifer Protection Area is not required for projects located within 200 feet of a steep slope hazard area,landslide hazard, or erosion hazard area. If these minimum areas are not mitigated using feasible BMPs from Requirements 1,2,3, and 4 above, one or more BMPs from the following list are required to be implemented to achieve compliance. These BMPs must be implemented as part of the proposed project. • Reduced Impervious Surface Credit per Appendix C, Section C.2.9, • Native Growth Retention Credit per Appendix C, Section C.2.10. • Tree Retention Credit per Appendix C, Section C.2.14 6. The soil moisture holding capacity of new pervious surfaces(target pervious surfaces)must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. 7. Any proposed connection of roof downspouts to the local drainage system must be via a perforated pipe connection as detailed in Appendix C, Section C.2.11. 1.2.9.2.2 LARGE LOT BMP REQUIREMENTS IF the proposed project is on a sitellot that is 22,000 square feet or larger,THEN on-site BMPs must be applied as specified in the requirements below OR the project must demonstrate compliance with the LID Performance Standard(described in Section 1.2.9.1.13)using an approved continuous runoff model. 1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be evaluated for all target impervious surfaces. If feasible and applicable for any such surface,then full dispersion must be applied to that surface and implemented as part of the proposed project. Typically, full dispersion will be applicable only on the largest sitesllots where there may be enough forest area available within a threshold discharge area to meet the 15%ratio of fully dispersed impervious area to native vegetated surface. 2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause flooding or erosion impacts,the feasibility and applicability of full infiltration of roof runoff must be evaluated in accordance with Appendix C, Section C.2.2, or Section 5.2,whichever is applicable based on the type of proj ect.47 If feasible and applicable, full infiltration of roof runoff must be implemented as part of the proposed project. 3. All target impervious surfaces not mitigated by Requirements 1 and 2 above,must be mitigated to the maximum extent feasible using one or more BMPs from the following list.Use of a given BMP is subject to evaluation of its feasibility and applicability as detailed in Appendix C.Feasible BMPs are required to be implemented. The BMPs listed below may be located anywhere on the site/lot subject to the limitations and design specifications for each BMP. These BMPs must be implemented as part of the proposed project. • Full Infiltration per Section C.2.2, or per Section 5.2,whichever is applicable • Limited Infiltration per Appendix C, Section C.2.3 • Bioretention per Appendix C, Section C.2.6, sized as follows: o SeaTac regional scale factor equals 1.0: In till soils,provide bioretention volume based on 0.6 inches of equivalent storage depth;in outwash soils provide bioretention volume based on 0.1 inches of equivalent storage depth o SeaTac regional scale factor greater than 1.0: In till soils,provide bioretention volume based on 0.8 inches of equivalent storage depth; in outwash soils,provide bioretention volume based on 0.4 inches of equivalent storage depth, • Permeable Pavement per Appendix C, Section C.2.7 47 For projects subject to Simplified Drainage Review,and for any single family residential project subject to Full or Large Project Drainage Review,the design requirements and specifications in Appendix C,Section C.2.2 may be used for evaluation and design of full infiltration on individual lots. For all other projects,full infiltration must be evaluated and designed in accordance with the infiltration facility standards in Section 5.2. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-78 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS 4. All target impervious surfaces not mitigated by Requirements 1,2, and 3 above,must be mitigated to the maximum extent feasible using the Basic Dispersion BMP described below.Use of Basic Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere on the site/lot subject the limitations and design specifications cited in Appendix C.The BMP must be implemented as part of the proposed project. • Basic Dispersion per Appendix C, Section C.2.4 5. BMPs must be implemented, at minimum, for impervious area amounts defined as follows. • For projects that will result in an impervious surface coverage on the buildable portion of the site/lot of less than 45%, on-site BMPs must be applied to 50% of target impervious surfaces. • For projects that will result in an impervious surface coverage 45-65%on the buildable portion of the site/lot, on-site BMPs must be applied to 50%of target impervious surfaces reduced by 1.5% for each 1%of impervious surface coverage above 45%(e.g.,impervious coverage of 55%results in a requirement of on-site BMPs applied to 35% of target impervious surfaces). • For projects that will result in an impervious surface coverage greater than 65%on the buildable portion of the site/lot, on-site BMPs must be applied to 20% of the target impervious surfaces or to an impervious area equal to at least 10%of the site/lot,whichever is less. The buildable portion of the site/lot is the total area of the site/lot minus any critical areas and minus 200 ft.buffer areas from a steep slope hazard,landslide hazard, or erosion hazard area. If these minimum areas are not mitigated using feasible BMPs from Requirements 1,2,3, and 4 above, one or more BMPs from the following list are required to be implemented to achieve compliance. These BMPs must be implemented as part of the proposed project. • Reduced Impervious Surface Credit per Appendix C, Section C.2.9, • Native Growth Retention Credit per Appendix C, Section C.2.10, • Tree Retention Credit per Appendix C, Section C.2.14. 6. The soil moisture holding capacity of new pervious surfaces(target pervious surfaces)must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. 7. Any proposed connection of roof downspouts to the local drainage system must be via a perforated pipe connection as detailed in Appendix C, Section C.2.11. 1.2.9.2.3 LARGE RURAL LOT BMP REQUIREMENTS This requirement does not apply in the City of Renton. 1.2.9.2.4 481MPLEMENTATION REQUIREMENTS FOR INDIVIDUAL LOT BMPS The on-site BMPs required in Sections 1.2.9.2 above must be implemented in accordance with the following requirements: 1. Implementation Responsibility.All on-site BMPs required for the site/lot must be implemented (installed)by the applicant as part of the proposed project unless they have already implemented as part of a subdivision project that created the lot per Section 1.2.9.4. 2. Maintenance Responsibility.Maintenance of all required on-site BMPs is the responsibility of the owner of the sitellot served by these BMPs.The responsibility for such maintenance must be clearly assigned to the current and future owners of the sitellot through a"declaration of covenant and grant of easement"as described in Requirement 3 below. 3. Declaration of Covenant and Grant of Easement.To ensure future maintenance of on-site BMPs and allow for City inspection of BMPs, a declaration of covenant and grant of easement must be 48 Footnote 48 is not used. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-79 SECTION 1.2 CORE REQUIREMENTS recorded for each site/lot that contains on-site BMPs.A draft of the proposed covenant must be reviewed and approved by CED prior to recording.All required covenants must be recorded prior to final construction approval for the proposed project. If the individual sitellot contains or will contain flow control or water quality facilities,then the drainage facility covenant in Reference Section 8-J(or equivalent)must be used, and is designed to achieve the following: a) Provide notice to future owners of the presence of on-site BMPs on the lot and the responsibility of the owner to retain,uphold, and protect the on-site BMPs, features,pathways,limits, and restrictions. b) Include as an exhibit, a recordable version 49 of the following drainage plan information: • The site plan showing all developed surfaces(impervious and pervious) and the location and dimensions of on-site BMPs,features, flowpaths(if applicable),limits of native growth retention areas(if applicable), and limits of tree retention areas(if applicable). This plan(s) must be to scale and include site topography in accordance with the specifications for such plans in Appendix C, Section C.4.2.Also indicate any areas where City access is excluded (see paragraph 3.d below).Note: CED may waive this element if,for example, the only on-site BMP proposed is a limit on impervious surface(reduced footprint). • The on-site BMP design and maintenance details for each on-site BMP per Appendix C, Section C.4.3. This includes a diagram(if applicable) of each on-site BMP and written maintenance and operation instructions and restrictions for each device, feature, flowpath(if applicable),native growth retention area(if applicable) and impervious surface coverage (if applicable). See Reference Section 8-M for prepared 8-1/2" x 11"maintenance instruction sheets. See City of Renton's Surface Water Design Standards web site: <http://rentonwa.gov/government/default.aspx?1d=7122>for downloadable BMP details. Ensure the exhibits are correctly cross-referenced in the declaration of covenant(the site plan is typically Exhibit A and the design/maintenance details are typically Exhibit B). c) Require that each on-site BMP be operated and maintained at the owner's expense in accordance with the above exhibit. d) Grant the City the right to enter the property at reasonable times for purposes of inspecting the on-site BMPs and to perform any corrective maintenance,repair,restoration, or mitigation work on the on-site BMPs that has not been performed by the property owner within a reasonable time set by CED, and to charge the property owner for the cost of any maintenance,repair,restoration, or mitigation work performed by the City. e) The right to enter typically applies to the entire property,but occasionally accepts areas on the property agreed upon by the City to be excluded from access. Such areas are to be shown on the site plan described above. f) Prohibit any modification or removal of on-site BMPs without written approval from the City. The approval must be obtained from CED and a covenant must be recorded to reflect the changes.Approval will be granted only if equivalent protection in terms of hydrologic performance is provided by other means. 4. Timing of Implementation.All required on-site BMPs must be installed prior to final inspection approval of constructed improvements. For BMPs that rely on vegetation,the vegetation must be planted and starting to grow prior to final construction approval. 49 Recordable version means one that meets King County's"Standard Formatting Requirements for Recording Documents" pursuant to RCW 36.18.010 and 65.04.045,available online at <_http://your.kingcounty.gov/recelec/records/docs/formatting reguirements.pdf>or from the King County Recorder's Office. These requirements include specifications for such things as page size(81/2'x 14"or smaller),font size(at least 8-point),and margin width(1"on all sides of every page if there is a standard cover sheet). 12/12/2016 2017 City of Renton Surface Water Design Manual 1-80 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS 5. Acceptance standards. On-site BMPs may be inspected during and/or following construction. Approval of the constructed BMPs will be based on verification that the materials and placement appear to meet the specifications and that the BMPs appear to function as designed. Onsite observations may be used to verify that materials are as specified and material receipts checked. Performance may be evaluated by a site visit while it is raining or by testing with a bucket of water or garden hose to check pavement permeability or proper connection to BMP devices/features, etc. 6. Drainage concerns.If CED determines that there is a potential for drainage impacts to a neighboring property,then additional measures may be required. Some on-site BMPs may not be appropriate in certain situations, and will not be allowed by CED where they may cause drainage problems. 7. Geotechnical concerns.A geotechnical engineer, engineering geologist, or CED must evaluate and approve on-site BMPs that are proposed: (A)on slopes steeper than 15%; (B)within a setback from the top of slope equal to the total vertical height of the slope area that is steeper than 15%; or (C)within 200 feet of a steep slope hazard area,erosion hazard area, or landslide hazard In addition, CED may require review by a geotechnical engineer or engineering geologist of any proposed BMP that infiltrates, disperses, or directs overflow adjacent to or towards a steep slope hazard area,erosion hazard area, or landslide hazard CED may also require some projects to route flows down or around such slopes using non-perforated pipes. Some on-site BMPs may not be appropriate for these locations, and will not be allowed by CED where flows may cause erosion problems. 8. Sewage system concerns.If CED determines that there is a potential conflict between onsite sewage systems and on-site BMPs, additional measures may be required. Some projects may need to route flows past onsite sewage systems using non-perforated pipes.Also,some on-site BMPs may not be appropriate for these sites, and will not be allowed where sewage systems may be impacted. 9. Engineering Concerns.While most of the on-site BMPs in Appendix C can be implemented by a non-engineer,there are some that have structural components that must be designed or evaluated by a civil engineer or structural engineer. When a BMP is proposed that has such components as identified in Section C.2 in Appendix C,CED may require submittal of engineering plans for that component signed and stamped by a civil engineer or structural engineer. 10. Connection to Subsurface Drains. On-site BMPs should not be connected to subsurface drains(e.g., footing drains) as these connections may adversely affect the performance of the BMPs, and in some cases may cause reverse flow into the footing drains during storm events. 11. Simplified Drainage Plan. The type, size, and placement of proposed on-site BMPs are to be shown on the site plan submitted for the proposed project This plan must be in accordance with the specifications for such plans outlined in Section CA in Appendix C unless otherwise directed by CED. 1.2.9.3 SUBDIVISION AND ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS For subdivision and road improvement projects, on-site BMPs must be selected and applied according to the subdivision and road improvement projects BMP requirements in this section. For purposes of applying on-site BMPs to these projects,two categories of requirements have been established based on the size of sitellot subject to improvements by the project. These categories of requirements are as follows: • Small Subdivision Project BMP Requirements • Small Road Improvement and Urban Road Improvement Projects BMP Requirements On-site BMPs must be applied in the order of preference and to the extent specified for the category of requirements applicable to the proposed project as described in the following subsections. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-81 SECTION 1.2 CORE REQUIREMENTS 1.2.9.3.1 SMALL SUBDIVISION PROJECT BMP REQUIREMENTS On-site BMPs for plat infrastructure improvements(e.g.,road and sidewalk etc.)of these projects shall meet the requirements described in Section 1.2.9.3.2 below for"Small Road Improvement and Urban Road Improvement Project BMP Requirements."Implementation of on-site BMPs required for/on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot and is therefore optional.However,if the applicant wishes to implement or make provision for implementation of BMPs for the lot improvements as part of the subdivision project for purposes of receiving BMP modeling credits,the individual lot BMP requirements described in Section 1.2.9.2 and implementation requirements for subdivision projects described Section 1.2.9.4.1 must be met. 1.2.9.3.2 SMALL ROAD IMPROVEMENT AND URBAN ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS IF the proposed project is a road improvement project that is on a site/parcel less than 5 acres in size, THEN on-site BMPs must be applied as specified in the requirements below. 1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be evaluated for all target impervious surfaces. If feasible and applicable,full dispersion must be implemented as part of the proposed project. Typically, small lot full dispersion will be applicable only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet the minimum design requirements for full dispersion in Appendix C, Section C.2.1.1. 2. All target impervious surfaces not mitigated by Requirement 1 above,must be mitigated to the maximum extent feasible using one or more BMPs from the following list.Use of a given BMP is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs are not required to be implemented. The BMPs listed below may be located anywhere on the site/lot subject to the limitations and design specifications for each BMP. These BMPs must be implemented as part of the proposed project. • Full Infiltration per Section C.2.2, or per Section 5.2,whichever is applicable • Limited Infiltration per Appendix C, Section C.2.3, • Bioretention per Appendix C, Section C.2.6, sized as follows: o SeaTac regional scale factor equals 1.0 : In till soils,provide bioretention volume based on 0.6 inches of equivalent storage depth;in outwash soils provide bioretention volume based on 0.1 inches of equivalent storage depth, o SeaTac regional scale factor greater than 1.0: In till soils,provide bioretention volume based on 0.8 inches of equivalent storage depth; in outwash soils,provide bioretention volume based on 0.4 inches of equivalent storage depth, • Permeable Pavement per Appendix C, Section C.2.7, 3. All target impervious surfaces not mitigated by Requirements 1 and 2 above,must be mitigated to the maximum extent feasible using the Basic Dispersion BMP described below.Use of Basic Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs are not required to be implemented. Basic Dispersion BMPs may be located anywhere on the site/lot subject to the limitations and design specifications cited in Appendix C. The BMPs must be implemented as part of the proposed project. • Basic Dispersion per Appendix C, Section C.2.4, 4. The soil moisture holding capacity of new pervious surfaces(target pervious surfaces)must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. 1.2.9.3.3 LARGE RURAL SUBDIVISION AND LARGE RURAL ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS This requirement does not apply in the City of Renton. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-82 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS 1.2.9.4 REQUIREMENTS FOR USE OF BMP CREDITS Projects that implement on-site BMPs,whether required or optional,may use the on-site BMP credits described in this section subject to the implementation requirements in Section 1.2.9.2.4(for Individual Lots), Section 1.2.9.4.1 (for Subdivision Projects), and Section 1.2.9.4.2(for Right-of-Way Projects).An alternative approach is to perform continuous runoff modeling per Chapter 3 instead of applying the on- site BMP credits in Table 1.2.9.A. Two kinds of credits are available. First, any impervious surface served by an on-site BMP that meets the design specifications for that BMP in Appendix C may be modeled as indicated in Table 1.2.9.A. Such credits may be used in the following situations: 1. To compute post-development runoff time series when sizing required flow control facilities. 2. To compute post-development 100-year peak flows when assessing any of the peak flow exceptions from the area-specific flow control facility requirement in Sections 1.2.3.1.A,B, and C. 3. To compute post-development runoff time series when sizing required flow rate based water quality facilities(e.g.,bioswales) and to re-characterize post developed land types when sizing volume based water quality facilities(e.g.,wetponds,wetvaults). Use of credits for water quality facility sizing as described above is limited to BMPs that are treating flows downstream from the BMP and tributary to a required water quality facility. Second, any impervious or nonnative pervious surface that is fully dispersed per the full dispersion criteria in Section 1.2.3.2.0 is not considered a target surface of the area-specific flow control facility requirement (Section 1.2.3.1) or the area-specific water quality facility requirement(Section 1.2.8.1). 2017 City of Renton Surface Water Design Manual 12/12/2016 1-83 SECTION 1.2 CORE REQUIREMENTS TABLE 1.2.9!A ON-SITE BMP SIZING CREDITS(') On-Site BMP Type Sizing Credit Full dispersion Model fully dispersed surface as forest(2) Full infiltration(3) Subtract impervious area that is fully infiltrated Limited infiltration Model tributary impervious surface as 90% impervious, 10%grass Basic dispersion Model dispersed impervious surface as 90%impervious, 10%grass Bioretention Model tributary impervious surface as 90% impervious, 10%grass Permeable pavement(unlined with Model permeable pavement area as 50%impervious, 50%grass. no underdrain) Run-on from other impervious surfaces does not receive a credit. Grassed modular grid pavement Model modular grid pavement as all grass Rainwater harvesting Credit only allowed via, and as specified in, an approved drainage adjustment that details conditions of use Vegetated roof with 3 to 8 inches of Model vegetated roof area as 50% impervious, 50%grass growing medium Vegetated roof with greater than Model vegetated roof area as 50% impervious, 50% pasture 8 inches of growing medium Restricted footprint Model footprint as restricted (Appendix C, Section C.2.9.2) Wheel strip driveways Model credited area as 50% impervious, 50%grass Minimum disturbance foundation Model foundation area as 50% impervious, 50%grass Open grid decking over pervious Model deck area as 50% impervious, 50%grass area Native growth retention credit Model mitigated impervious area as 50% impervious, 50%grass Perforated pipe connection None Notes: These credits do not apply when determining eligibility for exemptions from Core Requirement#3,Core Requirement#8, or exceptions from the flow control or water quality facility requirements unless otherwise noted in the exemption or exception.When applying modeling credits for flow control facility sizing, infiltrative BMPs tributary to the facility that are included in the modeling scenario(including the permeable pavement element with area reduced to 50%impervious area fraction,or other BMPs(e.g., bioretention,trenches,drywells)treating upstream runoff)must have the infiltration option turned off during the flow routing analysis for facility sizing to avoid double-counting the BMP infiltration benefit. Alternatively, permeable pavement with infiltration turned off may be represented by an impervious area land use element of equivalent area. (2) Surface shall be modeled using the soil type found at that location on the site. (3) For any project subject to Simplified Drainage Review,and for any single family residential project subject to Directed, Full or Large Project Drainage Review,the design requirements and specifications in Appendix C, Section C.2.2 may be used for design of full infiltration on individual lots. For all other projects,including any project where full infiltration is proposed to serve more than one lot,full infiltration must be designed in accordance with infiltration facility standards in Section 5.2. 1.2.9.4.1 USE OF CREDITS BY SUBDIVISION PROJECTS If a proposed project is a subdivision project,50 implementation of on-site BMPs for plat infrastructure improvements(e.g.,road, sidewalk, or other non-lot improvements)is required concurrent with the subdivision improvements. Implementation of on-site BMPs on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot and is therefore optional as part of the subdivision project. In order to receive the modeling credits(noted above)for on-site BMPs required for plat infrastructure improvements(e.g.,road, sidewalk, or other non-lot improvements), and/or for individual lot BMPs where the applicant elects to implement or make provision for implementation of individual lot BMPs as part of 50 For purposes of applying on-site BMPs,the term subdivision or subdivision project refers to any project that is a short plat, plat, or binding site plan. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-84 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS the subdivision project,the following requirements must be met depending on where the BMPs are located on the site. A. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN ROAD RIGHT-OF-WAY These are on-site BMPs installed within public or private road right-of-way as part of the construction of street and drainage improvements for the subdivision. To receive credit for these BMPs,the subdivision project must meet all of the following requirements: 1. The BMPs must serve impervious surface located only within the road right-of-way. 2. The BMPs must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. 3. If the road right-of-way will be privately maintained,provision must be made for future maintenance of the BMPs in accordance with Core Requirement 46, Section 1.2.6.As specified in Core Requirement 46,the City will assume maintenance of such BMPs in certain cases. 4. If the City will be assuming maintenance of the BMPs,the BMPs must comply with the drainage facility financial guarantee and liability requirements in Core Requirement 47, Section 1.2.7. B. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN DEDICATED TRACTS These are on-site BMPs installed on or associated with the features(e.g., forest) of common area tracts dedicated by the subdivision. Such BMPs may serve future improvements on lots,common area improvements, or road right-of-way improvements. To receive credit for these BMPs,the subdivision project must meet all of the following requirements: 1. The BMPs must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. 2. Provision must be made for future maintenance of the BMPs in accordance with Core Requirement 46, Section 1.2.6. When maintenance by the City is specified by Core Requirement 46, the City will assume maintenance of BMP devices(e.g., dispersion trenches)that are within a tract dedicated to the City for drainage purposes. The City will not assume maintenance of BMPs located in common areas dedicated for purposes other than just drainage(e.g.,play areas,parks, etc.). Where City maintenance is specified by Core Requirement 46,the City will assume maintenance for on-site BMP vegetated flow paths that are within an easement that allows for inspection and maintenance by the City. The City maintenance of these vegetated flow paths will be limited to their on-site BMP functionality.All other maintenance shall remain the responsibility of the owner(s). 3. BMPs to be maintained by the City in accordance with Core Requirement 46 must comply with the drainage facility financial guarantee and liability requirements in Core Requirement 47, Section 1.2.7. 4. If the BMPs installed within a dedicated tract satisfy some or all of the BMP requirements for individual lots per Section 1.2.9.2,then a note must be placed on the recorded documents for the subdivision indicating those lots for which BMPs have been provided. C. SUBDIVISION IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS These are on-site BMPs installed on a subdivision's proposed lots as part of the subdivision project. For example,the subdivision developer may elect to pre-install some or all of the on-site BMPs required by the individual lot BMP requirements in Section 1.2.9.2.To receive credits for these BMPs,the subdivision project must meet all of the following requirements: 1. The on-site BMPs must be installed and implemented in accordance with the individual lot BMP requirements in Section 1.2.9.2. This includes recording a declaration of covenant and grant of easement for each lot with BMPs as specified in Implementation Requirement 3 of Section 1.2.9.2.4. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-85 SECTION 1.2 CORE REQUIREMENTS If not all of the required BMPs are installed on a lot as part of the subdivision project,language must be included in the covenant notifying the future lot owner of additional required BMPs. 2. BMPs to be installed on individual lots as part of the subdivision project must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. D. SUBDIVISION FUTURE IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS These are on-site BMPs stipulated to be installed on some or all of a subdivision's proposed lots by a declaration of covenant recorded for each such lot. To receive credits for these BMPs,the subdivision project must meet all of the following requirements: 1. Demonstrate through a lot-specific assessment that the on-site BMPs stipulated for each lot are feasible and applicable according to the individual lot BMP requirements in Section 1.2.9.2 and the BMP design specifications in Appendix C. This lot-specific assessment must be included in the TIR submitted with engineering plans for the subdivision. The assessment shall include any soils reports, calculations, or other information necessary to select and properly apply BMPs. 2. Record a declaration of covenant and grant of casement for each lot stipulating the type or types of BMP being proposed for credit. This covenant must be as specified in Implementation Requirement 3 of Section 1.2.9.2.4, except as follows: a) The site plan requirement may be waived depending on the BMPs proposed or may be conceptual, showing only the information necessary to stipulate the type or types of BMP being proposed for credit.For example,if the BMP is full dispersion,the approximate location of future impervious surface and the limits of the"native vegetated flowpath segment"(see Appendix C, Section C.2.1)must be shown. If the BMP is full infiltration,the approximate location of future impervious surface, septic drain field(if applicable), and infiltration devices must be shown.For all other BMPs,the"design and maintenance details"(see Item b below)for each proposed BMP per Appendix C may be sufficient as determined by CED. b) The on-site BMP design and maintenance details must include the dimensions of all proposed devices, features, and flowpaths, expressed as unit amounts per square foot of impervious surface served or as a percentage of the lot size or impervious surface created. c) The notice to future lot owners must indicate that they are responsible to install the on-site BMP or BMPs stipulated for the lot prior to final inspection approval of constructed lot improvements. Alternative BMPs that provide equivalent performance may be proposed at the time of permit application for proposed lot improvements. In any case, a revised covenant will need to be recorded to reflect the final approved BMPs and site improvement plan(s). 3. If single family residential lots are being created, a note must be placed on the recorded documents for the subdivision indicating the following: "Single family residences and other improvements constructed on the lots created by this subdivision must implement the flow control best management practices(BMPs) stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the small project drainage plan submitted for drainage review when application is made for a single family residential building permit for the lot." 4. If commercial lots are being created, a note must be placed on the recorded documents for the subdivision indicating the following: "Improvements constructed on the lots created by this subdivision must implement the flow control best management practices(BMPs)stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the engineering plans submitted for drainage review when application is made for a permit to make improvements to the lot." 12/12/2016 2017 City of Renton Surface Water Design Manual 1-86 1.2.9 CORE REQUIREMENT#9:ON-SITE BMPS 5. If a binding site plan is being created, a note must be placed on the recorded documents for the subdivision indicating the following: "Improvements constructed on the lots created by this binding site plan must implement the flow control best management practices(BMPs)stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the engineering plans submitted for drainage review when application is made for a permit to make improvements to the lot." 1.2.9.4.2 USE OF CREDITS BY PROJECTS WITHIN RIGHTS-OF-WAY If a proposed project is located primarily within an established public or private right-of-way, implementation of on-site BMPs is as required per Section 1.2.9.3. To receive credit for these BMPs,the project must meet all of the following requirements: 1. The BMPs must serve impervious surface located only within the right-of-way. 2. If the right-of-way will be privately maintained,provision must be made for future maintenance of the BMPs in accordance with Core Requirement 46, Section 1.2.6. 3. If the City will be assuming maintenance of the BMPs,the BMPs must comply with the drainage facility financial guarantee and liability requirements in Core Requirement 47, Section 1.2.7. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-87 SECTION 1.2 CORE REQUIREMENTS (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 1-88 CITY OF RENTON SURFACE WATER DESIGN MANUAL 1.3 SPECIAL REQUIREMENTS This section details the following six special drainage requirements that may apply to the proposed project depending on its location or site-specific characteristics: • "Special Requirement 41: Other Adopted Area-Specific Requirements,"Section 1.3.1 • "Special Requirement 42:Flood Hazard Area Delineation," Section 1.3.2 • "Special Requirement 43:Flood Protection Facilities,"Section 1.3.3 • "Special Requirement 44: Source Control,"Section 1.3.4 • "Special Requirement 45: Oil Control," Section 1.3.5 • "Special Requirement 46:Aquifer Protection Area,"Section 1.3.6 1.3.1 SPECIAL REQUIREMENT #1: OTHER ADOPTED AREA-SPECIFIC REQUIREMENTS This manual is one of several adopted regulations in the City of Renton that apply requirements for controlling drainage on an area-specific basis. Other adopted area-specific regulations include requirements that have a more direct bearing on the drainage design of a proposed project. These regulations include the following: • Master Drainage Plans(MDPs): MDPs are comprehensive drainage plans prepared for urban planned developments(UPDs)or other large,complex projects(described in Section 1.1.2.5).Projects covered by a MDP must meet any adopted requirements specific to that plan. • Basin Plans(BPs): The City of Renton adopts basin plans to provide for the comprehensive assessment of resources and to accommodate growth while controlling adverse impacts to the environment.A basin plan may recommend specific land uses,regional capital projects, and special drainage requirements for future development within the basin area it covers. • Salmon Conservation Plans(SCPs): Salmon conservation plans are comprehensive, ecosystem- based plans intended to identify and assess the means to protect and restore salmon habitat through mechanisms such as habitat improvements,regulations,incentives,BMPs,land acquisition, and public education activities. These plans are developed in collaboration with other jurisdictions within a water resource inventory area(WRIA) designated by the state under WAC 173-500-040 and spanning several basins or subbasins. • Lake Management Plans: The City of Renton may adopt lake management plans to provide for comprehensive assessment of resources and to accommodate growth while controlling adverse impacts from nutrient loading to selected lakes.A lake management plan may recommend nutrient control through special drainage and source control requirements for proposed projects within the area it covers. • Hazard Mitigation Plan: The City's Hazard Mitigation Plan prepared in accordance with RCW 86.12.200 includes information on reducing flood risks. • Shared Facility Drainage Plans(SFDPs): SFDPs are approved by the City of Renton to allow two or more projects to share drainage facilities required by this manual.Projects covered by a SFDP must meet any specific requirements of that plan. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-89 SECTION 1.3 SPECIAL REQUIREMENTS Threshold Requirement IF a proposed project is in an area THEN the proposed project shall included in an adopted master drainage comply with the drainage requirements plan, basin plan, salmon conservation of the master drainage plan, basin plan, plan, stormwater compliance plan, salmon conservation plan, stormwater hazard mitigation plan, lake compliance plan, hazard mitigation management plan, or shared facility plan, lake management plan, or shared drainage plan ... facility drainage plan, respectively. Application of this Requirement The drainage requirements of adopted MDPs,BPs, SCPs,Hazard Mitigation Plan,lake management plans, and SFDPs shall be applied in addition to the drainage requirements of this manual unless otherwise specified in the adopted regulation. Where conflicts occur between the two,the drainage requirements of the adopted area-specific regulation shall supersede those in this manual. Examples of drainage requirements found in other adopted area-specific regulations include the following: • More or less stringent flow control • More extensive water quality controls • Forest retention requirements • Infiltration restrictions • Groundwater recharge provisions • Discharge to a constructed regional flow control or conveyance facility. Adjustments to vary from the specific drainage requirements mandated by BPs, SCPs,FHMPs, and lake management plans may be pursued through the adjustment process described in Section 1.4 of this manual. Copies of all adopted basin plans, SCPs,Hazard Mitigation Plan, and lake management plans are available from the City of Renton. Projects covered by SFDPs shall demonstrate that the shared facility will be available by the time the project is constructed and that all onsite requirements are met. Projects covered by a SFDP are still required to provide any onsite controls necessary to comply with drainage requirements not addressed by the shared facility. 1.3.2 SPECIAL REQUIREMENT #2: FLOOD HAZARD AREA DELINEATION Flood hazard areas are composed of the 100-year floodplain,zero-rise flood fringe,zero-rise floodway, and FEMA floodway. If a proposed project contains or is adjacent to a flood hazard area as determined by CED,this special requirement requires the project to determine those components that are applicable and delineate them on the project's site improvement plans and recorded maps. Floodplains are subject to inundation during extreme events. The 100-year floodplain, and floodway if applicable,is delineated in order to minimize flooding impacts to new development and to prevent aggravation of existing flooding problems by new development. Regulations and restrictions concerning development within a 100-year floodplain are found in the critical areas code,RMC 4-3-050. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-90 1.3.3 SPECIAL REQUIREMENT#3:FLOOD PROTECTION FACILITIES Threshold Requirement IF a proposed project contains or is THEN the 100-year floodplain, and adjacent to a flood hazard area for a river, applicable floodway, shall be determined stream, lake, wetland, closed depression, , and their boundaries, together with the or if other City of Renton regulations boundaries of the severe and moderate require study of flood hazards related to channel migration hazard area (if the proposed project ... applicable), shall be delineated on the site improvement plans and profiles, and on any final subdivision maps prepared for the proposed project. Application of this Requirement The applicant is required to use the best available floodplain/floodway data when delineating the 100-year floodplain and floodway boundaries on site improvement plans and profiles, and on any final subdivision maps. The floodplain/floodway delineation used by the applicant shall be in accordance with RMC 4-3-050 and associated public rules. If floodplain/floodway data and delineation does not exist,then a floodplain/floodway analysis shall be prepared by the applicant as described in Section 4.4.2, "Floodplain/Floodway Analysis." 1.3.3 SPECIAL REQUIREMENT #3: FLOOD PROTECTION FACILITIES Flood protection facilities, such as levees and revetments require a high level of confidence in their structural integrity and performance. Proper analysis, design, and construction are necessary to protect against the potentially catastrophic consequences if such facilities should fail. Threshold Requirement IF a proposed project will: THEN the applicant shall demonstrate • Rely on an existing flood protection that the flood protection facility, as facility(such as a levee or revetment) determined by a licensed professional for protection against hazards posed engineer, conforms with siting, structural by erosion or inundation, OR stability, environmental, and all other • Modify or construct a new flood relevant standards cited in the following protection facility ... regulations and documents: • Washington State Integrated Streambank Protection Guidelines, • Corps of Engineers Manual for Design and Construction of Levees (EM 1110-2-1913), • RMC 4-3-050 and • Special Requirement#1 (specifically the City Hazard Mitigation Plan) AND, flood containment levees shall meet or exceed the professional engineering standards summarized in FEMA National Flood Insurance mapping regulations ... (44 CFR, subsection 65.10)or FEMA's Analysis and Mapping Procedures for non-Accredited Levee Systems. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-91 SECTION 1.3 SPECIAL REQUIREMENTS Application of this Requirement Conformance with the requirements listed above shall be addressed in the Technical Information Report submitted with the project's engineering plans(see Section 2.3.1.1). Conformance also requires that certain casement requirements(outlined in Section 4.1)be met in order to allow County access to the facility. If the proposed project contains an existing City of Renton flood protection facility or proposes to rely on a City of Renton flood protection facility,the applicant shall provide an easement to the City of Renton consistent with the river protection easement requirements outlined in Section 4.1. 1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS Water quality source controls prevent rainfall and runoff water from coming into contact with pollutants, thereby reducing the likelihood that pollutants will enter public waterways and violate water quality standards or City stormwater discharge permit limits.A Stormwater Pollution Prevention Manual was prepared for citizens,businesses, and industries to identify and implement source controls for activities that often pollute water bodies. The City of Renton provides advice about source control implementation upon request. The City may,however,require mandatory source controls at any time through formal code enforcement if complaints or studies reveal water quality violations or problems. Threshold Requirement IF a proposed project requires a THEN water quality source controls commercial building or commercial site applicable to the proposed project shall development permit ... be applied as described below in accordance with the King County Stormwater Pollution Prevention Manual and Renton Municipal Code, Title IV. Application of this Requirement When applicable per the Stormwater Pollution Prevention Manual, structural source control measures, such as car wash pads or dumpster area roofing, shall be applied to the entire site containing the proposed project,not just the project site. If the applicant is a tenant or lessee for only a portion of the site, CED may limit the entire site application of structural source controls to only that portion of the site occupied or leased by the applicant.All applicable structural source control measures shall be shown on the site improvement plans submitted for engineering review and approval. Other, nonstructural source control measures, such as covering storage piles with plastic or isolating areas where pollutants are used or stored, are to be implemented after occupancy and need not be addressed during the plan review process. All commercial,industrial, and multifamily projects(irrespective of size)undergoing drainage review are required to implement applicable source controls. Activities That May Result In Structural Improvements There are a number of activities that may require structures and/or specific drainage configurations in order to protect stormwater and maintain compliance with county code. Roof structures,wheel washes, cement pads, shutoff valves,containment berms and indoor mop sinks are all examples of things that need to be in place prior to commencing the activity. These may require building permits and other approvals prior to construction. Below are some highlighted activities and the numbered BMP activity sheets in the Stormwater Pollution Prevention Manual that provide more detail: 12/12/2016 2017 City of Renton Surface Water Design Manual 1-92 1.3.4 SPECIAL REQUIREMENT#4: SOURCE CONTROLS Commercial Composting Structural improvements: paved composting and storage pads,leachate collection system,lined collection ponds,wheel wash system • A-24 Commercial Composting Fueling of Equipment and Vehicles Structural improvements: Portland cement pads,roofs, spill control devices,trench drains, oil/water separators • A-17 Fueling Operations • A-48 Older Fueling Operations Horse Stables Structural improvements: Wash racks connected to sanitary sewer or separate infiltration area,manure containment areas • A-35 Livestock Mining of Sand or Gravel Structural improvements: Wheel wash system and track-out control,catch basin inserts • A-41 Wheel Wash System Painting,Finishing, and Coating of Vehicles and Equipment Structural improvements: Permitted, enclosed paint booths • A-22 Painting,Finishing, &Coating of Vehicles,Products, &Equipment Restaurants and Food Trucks Structural improvements: Indoor sinks format and rack washing and mop and wastewater disposal. • A-8 Storage of Solid and Food Wastes • A-12 Cleaning of Cooking Equipment Outdoor Storage of Erodible Materials (e.g.,compost,bark, sand, etc.) Structural improvements: Wheel wash system and track-out control,berms,containment areas,covering, catch basin inserts • A-41 Wheel Wash and Tire Bath Track-Out Control Outdoor Storage or Processing of Galvanized Materials Structural improvements: Roofs or other covering, stormwater collection and treatment system • A-21 Manufacturing and Post-Processing of Metal Products Storage of Liquid Materials Structural improvements: Secondary containment,roofed structures, spill control devices • A-2 Storage of Liquid Materials in Stationary Tanks • A-3 Storage of Any Liquid Materials in Portable Containers Utility Corridor Maintenance Structural improvements: Road stabilization • A-45 Maintenance of Public&Private Utility Corridors&Facilities 2017 City of Renton Surface Water Design Manual 12/12/2016 1-93 SECTION 1.3 SPECIAL REQUIREMENTS Washing of Cars, Trucks, and Equipment(not just commercial car washes) Structural improvements: Dedicated wash pads,sewer connection,holding tanks,catch basin inserts • A-13 Vehicle washing Wood Treatment and Preserving Structural improvements: Paved,contained and covered storage and processing areas • A-23 Wood Treatment&Preserving 1.3.5 SPECIAL REQUIREMENT #5: OIL CONTROL Projects proposing to develop or redevelop a high-use site must provide oil controls in addition to any other water quality controls required by this manual. Such sites typically generate high concentrations of oil due to high traffic turnover, onsite vehicle or heavy or stationary equipment use, some business operations, e.g., automotive recycling, or the frequent transfer of liquid petroleum or coal derivative products. The traffic threshold in the definition above focuses on vehicle turnover per square foot of building area (trip generation)rather than ADT alone because oil leakage is greatest when engines are idling or cooling. In general, all-day parking areas are not intended to be captured by these thresholds except those for diesel vehicles,which tend to leak oil more than non-diesel vehicles. The petroleum storage and transfer stipulation is intended to address regular transfer operations like service stations,not occasional filling of heating oil tanks. Threshold Requirement IF a proposed project: THEN the project must treat runoff from • Develops a site that will have high- the high-use portion of the site using oil use site characteristics, OR control treatment options from the High- * Is a redevelopment project Use menu (described below and detailed proposing $100,000 or more of in Chapter 6). improvements to an existing high-use site, OR • Is a redevelopment project that results in new plus replaced pollution generating impervious surfaces of 5,000 square feet or more or new pollution generating pervious surface of 3/ acre or more improvements to an existing high-use site ... High-Use Menu High-use oil control options are selected to capture and detain oil and associated pollutants. The goal of this treatment is no visible sheen on runoff leaving the facility, or less than 10 mg/L total petroleum hydrocarbons (TPH)in the runoff, depending on the facility option used. Oil control options include facilities that are small,handle only a limited tributary area, and require frequent maintenance,as well as facilities that treat larger areas and generally have less frequent maintenance needs.Facility choices include catch basin inserts,linear sand filters, and oil/water separators. See Chapter 6 for specific facility choices and design details. Application of this Requirement For high-use sites located within a larger commercial center, only the impervious surface associated with the high-use portion of the site is subject to treatment requirements. If common parking for multiple businesses is provided,treatment shall be applied to the number of parking stalls required for the high-use 12/12/2016 2017 City of Renton Surface Water Design Manual 1-94 1.3.4 SPECIAL REQUIREMENT#4: SOURCE CONTROLS business only. However,if the treatment collection area also receives runoff from other areas,the water quality facility must be sized to treat all water passing through it. High-use roadway intersections shall treat the intersection itself,plus lanes where vehicles accumulate during the signal cycle,including all lanes, from the beginning of the left tum pocket(see Figure 1.3.5.A below). If no left tum pocket exists,the treatable area shall begin at a distance of 75 feet from the stop line. If runoff from the intersection drains to more than two collection areas that do not combine within the intersection,treatment may be limited to any two of the collection areas. Oil control facilities shall be designed for all flows tributary to the oil control facility including flow from otherwise exempt areas that are not bypassed around the facility. Note:For oil control facilities to be located in public road right-of-way and maintained by the City of Renton, only coalescing plate or baffle oil/water separators shall be used unless otherwise approved through an adjustment. Catch basin inserts are not allowed for oil control. Methods of Analysis The traffic threshold for the High-Use menu shall be estimated using information from Trip Generation, published by the Institute of Transportation Engineers, from a traffic study prepared by a professional engineer or transportation specialist with experience in traffic estimation, or from documented data from the City. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-95 SECTION 1.3 SPECIAL REQUIREMENTS FIGURE 1.3.5.A TREATABLE AREAS FOR HIGH-USE ROAD INTERSECTIONS High use area of intersection i --------------- ------------ ------------ Ii 12/12/2016 2017 City of Renton Surface Water Design Manual 1-96 1.3.6 SPECIAL REQUIREMENT#6:AQUIFER PROTECTION AREA 1.3.6 SPECIAL REQUIREMENT #6: AQUIFER PROTECTION AREA Aquifer Protection Area(s) (APA)are identified in the RMC 4-3-050. If a proposed project is located within the APA,this special requirement requires the project to determine those components that are applicable and delineate them on the project's site improvements plans. Reference Section 15-B includes a map of the City's Aquifer Protection Area,Zones 1 and 2. Threshold Requirement IF a proposed project is in Zone 1 of the THEN the following drainage facilities are APA ... prohibited: a. Open facilities such as flow control and water quality treatment ponds, stormwater wetlands and infiltration facilities b. On-site BMPs that rely on infiltration. (On-site BMPs that rely on dispersion are allowed for non- PGIS.) c. Open conveyance systems such as ditches and channels If a proposed project is in Zone 2 of the THEN the proposed project must comply APA ... with the following requirements a. Open facilities such as flow control and water quality treatment ponds, stormwater wetlands and infiltration facilities may require a liner in accordance with the design criteria in Section 6.2.4. b. Open conveyance systems such as ditches and channels may require a liner in accordance with Section 1.2.4.3. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-97 SECTION 1.3 SPECIAL REQUIREMENTS (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 1-98 CITY OF RENTON SURFACE WATER DESIGN MANUAL 1.4 ADJUSTMENT PROCESS For proposed projects subject to drainage review by the City of Renton this process is provided for the occasions when a project proponent desires to vary from one of the core or special requirements, or any other specific requirement or standard contained in this manual.Proposed adjustments should be approved prior to final permit approval,but they may be accepted up to the time the City approves final construction or accepts drainage facilities for maintenance. Types of Adjustments To facilitate the adjustment process and timely review of adjustment proposals,the following types of adjustments are provided: • Standard Adjustments: These are adjustments of the standards and requirements contained in the following chapters and sections of this manual: o Chapter 1,"Drainage Review and Requirements" o Chapter 2,"Drainage Plan Submittal" o Chapter 3,"Hydrologic Analysis and Design" o Chapter 4,"Conveyance System Analysis and Design" o Chapter 5,"Flow Control Design" o Chapter 6,"Water Quality Design" o Appendix A, "Maintenance Requirements for Stormwater Facilities and On-Site BMPs" o Appendix B, "Master Drainage Plans" o Appendix C, "Simplified Drainage Requirements" o Appendix D, "Construction Stormwater Pollution Prevention Standards" Requests for standard adjustments will be accepted only for permits pending approval or approved permits that have not yet expired. • Blanket Adjustments: This type of adjustment may be established by the City based on approval of any of the above-mentioned adjustments. Blanket adjustments are usually based on previously approved adjustments that can be applied routinely or globally to all projects where appropriate. Blanket adjustments are also used to effect minor changes or corrections to manual design requirements or to add new designs and methodologies to this manual. 1.4.1 ADJUSTMENT AUTHORITY CED shall have full authority to determine if and what type of adjustment is required for any proposed project subject to drainage review by CED. The authority to grant adjustments for such projects is distributed as follows: • CED shall have full authority to approve or deny adjustments, except those involving outfalls or pump discharges to the Green River between River Mile 6 and SR 18 per Section 1.2.4.21 and 1.2.4.2.I. CED decisions on those adjustments are subject to approval by the King County Flood Control District. 1.4.2 CRITERIA FOR GRANTING ADJUSTMENTS Adjustments to the requirements in this manual may be granted provided that granting the adjustment will achieve the following: 1. Produce a compensating or comparable result that is in the public interest,AND 2. Meet the objectives of safety, function, appearance, environmental protection, and maintainability based on sound engineering judgment. Also,the granting of any adjustment that would be in conflict with the requirements of any other City department will require review and concurrence with that department. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-99 SECTION 1.4 ADJUSTMENT PROCESS Criteria Exception If it can be demonstrated that meeting the above criteria for producing a compensating or comparable result will deny reasonable use of a property, approval of the adjustment will require an adjustment criteria exception to be approved by the City.An adjustment that requires a criteria exception may be granted following legal public notice of the adjustment request,the proposed decision on the request, and a written finding of fact that documents the following: 1. There are special physical circumstances or conditions affecting the property such that strict application of the criteria for producing a compensating or comparable result would deprive the applicant of all reasonable use of the parcel of land in question, and every effort has been made to find creative ways to meet the intent of the requirement for which the adjustment is sought,AND 2. Granting the adjustment for the individual property in question will not create a significant adverse impact to public health,welfare,water quality, and properties downstream or nearby,AND 3. The adjustment requires the best practicable alternative for achieving the spirit and intent of the requirement in question. In addition,the written finding of fact must include the following information: • The current(pre-project)use of the site. • How application of the requirement for which an adjustment is being requested denies reasonable use of the site compared to the restrictions that existed under the 2009 King County Surface Water Design Manual and City ofRenton Amendments to the 2009 King County Surface Water Design Manual. • The possible remaining uses of the site if the criteria exception were not granted. • The uses of the site that would have been allowed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual. • A comparison of the estimated amount and percentage of value loss as a result of the requirements of this manual versus the estimated amount and percentage of value loss as a result of requirements that existed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual. • The feasibility for the owner to alter the project to apply the requirements of this manual.51,52 1.4.3 ADJUSTMENT APPLICATION PROCESS Standard Adjustments The application process for standard adjustments is as follows: • Requests for standard adjustments will be accepted only for permits pending approval or approved permits that have not yet expired. • The completed adjustment request application forms must be submitted to CED along with sufficient engineering information(described in Chapter 2)to evaluate the request. The application shall note the specific requirement for which the adjustment is sought. • If the adjustment request involves use of a previously unapproved construction material or construction practice,the applicant should submit documentation that includes,but is not limited to, a record of successful use by other agencies and/or evidence of meeting criteria for quality and performance, such as that for the American Association of State Highway and Transportation Officials (AASHTO) and the American Society of Testing and Materials(ASTM). • If the adjustment requires a criteria exception, additional engineering or other information may be required by CED to document that denial of reasonable use would occur,that every effort was made to achieve compliance, and that the best practicable alternative will not cause significant adverse impact. • A fee reduction may be requested if it is demonstrated that the adjustment request requires little or no engineering review. Blanket Adjustments There is no application process for establishing blanket adjustments because they are initiated and issued solely by the City. 51 Footnote 51 is not used. 52 Footnote 52 is not used. 12/12/2016 2017 City of Renton Surface Water Design Manual 1-100 1.4.4 ADJUSTMENT REVIEW PROCESS 1.4.4 ADJUSTMENT REVIEW PROCESS All adjustments are governed by the review procedures and time lines set forth by the City. Consistent with these procedures,the general steps of the review process for specific types of adjustments are presented as follows. Standard Adjustments • CED will review the adjustment request application forms and documentation for completeness and inform the applicant in writing as to whether additional information is required from the applicant in order to complete the review. The applicant will also be informed if CED determines that special technical support is required in cases where the adjustment involves a major policy issue or potentially impacts a City drainage facility. • The CED Development Review Engineer will review and either approve or deny the adjustment request following determination that all necessary information has been received from the applicant. • If a criteria exception is required for the adjustment, CED will issue a legal public notice of the adjustment request that indicates the director's proposed decision on the request,including the written finding of fact specified in Section 1.4.2. The public notice will include a 15-working-day public comment period within which a request for reconsideration may be made to the CED director as described in Section 1.4.5.Absent a request for reconsideration,the director's decision becomes final after the two week public comment period. • Approvals of standard adjustments will expire upon expiration of the permit to which they apply. Blanket Adjustments Blanket adjustments may be established at the discretion of CED.Blanket adjustments are established by memorandum based on: 1. Previously approved adjustments and supporting documentation,AND 2. Monitoring results in conjunction with any TAPE or CTAPE results AND 3. Information presenting the need for the blanket adjustment. Typically,blanket adjustments should apply globally to design or procedural requirements and be independent of site conditions. CED must approve creation of a blanket adjustment. Applicants may use any approved blanket adjustment listed in Reference Section 14,by submitting the form titled"Surface Water Design Manual Requirements/Standards Blanket Adjustment No. "to the CED plan reviewer currently reviewing the specific project proposal,but no further approval is required. 1.4.5 APPEALS Any appeals from administrative determinations for variances or adjustments related to the Storm Drainage regulations and codes shall be filed in writing to the Hearing Examiner by any person aggrieved, or by any officer, department,board or bureau of the City affected by such determination per RMC 4-8-110. 2017 City of Renton Surface Water Design Manual 12/12/2016 1-101 SECTION 1.4 ADJUSTMENT PROCESS (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 1-102 Section Page CHAPTER 2 2.1 Plans for Permits and Drainage Review 2-3 DRAINAGE PLAN 2.1.1 Plans Required for Pre-Application Submittal 2-3 SUBMITTAL 2.1.2 Site Plans Required for Drainage Review 2-3 2.2 Plans Required with Construction Permit Application 2-5 k acv 1 ' r 2.2.1 Subdivision, PUD, and Binding Site Plans 2-6 2.2.2 Short Subdivisions 2-7 2.2.3 Commercial Site Development 2-7 2.2.4 Single-Family Residential 2-7 9 2.2.5 Other Permits 2-7 2.3 Drainage Review Plan Specifications 2-9 2.3.1 Engineering Plan Specifications 2-10 2.3.2 Projects in Targeted Drainage Review � (TDR) 2-34 2.4 Plans Required After Drainage Review 2-35 CITY OF RENTON 2.4.1 Plan Changes After Permit Issuance 2-35 SURFACE WATER 2.4.2 Final Corrected Plan Submittal 2-35 2.4.3 Final Plat, Short Plat, and Binding Site DESIGN MANUAL Plan Submittals 2-36 2017 Surface Water Design Manual 12/12/2016 CITY OF RENTON SURFACE WATER DESIGN MANUAL (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL CHAPTER 2 DRAINAGE PLAN SUBMITTAL This chapter details the drainage related submittal requirements for engineering design plans as part of a permit application to the City of Renton Community and Economic Development(CED)Department. The intent of these requirements is to present consistent formats for design plans and the technical support data required to develop the plans. These conventions are necessary to review engineering designs for compliance with City of Renton ordinances and regulations, and to ensure the intent of the plan is easily understood and implemented in the field.Properly drafted design plans and supporting information also facilitate the construction, operation, and maintenance of the proposed system long after its review and approval. When plans comply with the formats and specifications contained herein,they facilitate review and approval with a minimum of time-consuming corrections and resubmittals. Note that this chapter primarily describes how to submit drainage plans for review—what must be submitted,in what formats, at what times and to what offices. The basic drainage requirements that these plans must address are contained in Chapter 1, "Drainage Review and Requirements."The specific design methods and criteria to be used are contained in Chapters 3,4, 5, and 6. Several key forms used in the plan review process are reproduced in Reference Section 8, "Forms and Worksheets."The drainage submittal requirements for different types of developments are contained in this chapter with the exception of Master Drainage Plans,which if required,the scope of the requirements will be determined by the Surface Water Utility and will generally follow King County's Master Drainage Planning for Large or Complex Site Development and requirements. For information on general requirements for any permit type,refer to the City of Renton website or customer information counter at CED. Chapter Organization The information presented in this chapter is organized into four main sections as follows: • Section 2.1,"Plans for Permits and Drainage Review" • Section 2.2,"Plans Required with Construction Permit Application" • Section 2.3,"Drainage Review Plan Specifications" • Section 2.4,"Plans Required After Drainage Review" These sections begin on odd pages so the user can insert tabs if desired for quicker reference. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-1 CHAPTER 2 DRAINAGE PLAN SUBMITTAL (This page intentionally left blank) 12/12/2016 2017 City of Renton Surface Water Design Manual 2-2 CITY OF RENTON SURFACE WATER DESIGN MANUAL 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW CED is responsible for the review of all engineering aspects of private development proposals. Drainage review is a primary concern of engineering design. This section describes the types of engineered drainage plans required for engineering review at various permit review stages.Refer to the City website for other details or requirements, such as the submittal and expiration periods set for each type ofpermit application, review fees, right-of-way use requirements, and other code requirements. 2.1.1 PLANS REQUIRED FOR PRE-APPLICATION SUBMITTAL Most projects require some degree of drainage plans or analysis to be submitted for drainage review;the extent of which will depend upon the type of permit, size and type of project, and project vicinity to any critical areas.All proposed developments within the City of Renton have the option to request a free pre- application meeting to gain feedback on development requirements and feasibility prior to formal submittal of any required permit application materials. Subdivisions,PUDs, short plats and binding site plans require conceptual plans(may be engineered or non-engineered)be submitted with the pre- application meeting request.Preliminary plans provide general information on the proposal,including location of critical areas,road alignments and right-of-way,site topography,building locations,land use information, and lot dimensions. They are used to determine the appropriate drainage conditions and requirements to be applied to the proposal during the drainage review process. For more information refer to the permit submittal requirements documents that are applicable to the development proposal(available on the City's website and from staff in the pre-application meeting,if held). 2.1.2 SITE PLANS REQUIRED FOR DRAINAGE REVIEW For drainage review purposes, engineering plans consist of the following: 1. Site improvement plans (see Section 2.3.1.2),which include all plans,profiles, details,notes, and specifications necessary to construct road, drainage,utilities, and off-street parking improvements. 2. A construction stormwater pollution prevention (CSWPP)plan,which identifies the measures and BMPs required to prevent the discharge of sediment-laden water and other pollutants associated with construction/land disturbing activities. The CSWPP plan includes two component plans: an erosion and sediment control(ESC)plan (see Section 2.3.1.3),which addresses prevention of sediment- laden discharges; and a stormwater pollution prevention and spill(SWPPS)plan (see Section 2.3.1.4),which addresses prevention of other pollutant discharges. 3. A technical information report(TIR)(see Section 2.3.1.1),which contains all the technical information and analysis necessary to develop the site improvement plan and CSWPP plan. Projects Under Targeted Drainage Review usually require engineering plans, except that only certain sections of the technical information report are required to be completed and the site improvement plan may have a limited scope depending upon the characteristics of the proposed project. The scope of these plans should be confirmed during the pre-application meeting with CED.For other permits, such as single-family residential permits,the scope of the targeted engineering analysis is usually determined during CED engineering review. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-3 SECTION 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW Plans Required for Simplified Drainage Review Simplified drainage plans are a simplified form of site improvement and CSWPP plans(without a TIR or a separate SWPPS plan)that may be prepared by a non-engineer from a set of pre-engineered design details. Simplified drainage plans are only allowed for single family in Simplified Drainage Review but may be required for individual lots created by a subdivision project to show how required on-site BMPs,ESC and SWPPS measures will be applied to future lot construction. For single-family residential permits,the level and scope of drainage plan requirements are determined by CED during drainage review. Some projects subject to Simplified Drainage Review may also require Targeted Drainage Review. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-4 CITY OF RENTON SURFACE WATER DESIGN MANUAL 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION This section describes the submittal requirements for construction permit applications at the City of Renton. Required plans for each permit or project type described in Section 2.2.1 through 2.2.5 are summarized in Table 2.2.A.The timing for submittal of engineering plans will vary depending on permit type.For commercial, subdivisions, short plats, and other types of construction permits,this submittal usually follows the City's approval of plans described in Section 2.2.For commercial building permits, engineering plans must be submitted as part of the construction permit application,unless otherwise approved by CED.For other permit types the drainage plan requirements are determined during the permit review process. Note:If engineering plans are required to be submitted with the construction permit application, they must be accompanied by the appropriate supporting documents (e.g., required application forms, reports, etc). For more details, see the City's website. Design Plan Certification All preliminary plans and engineering plans must be stamped by a civil engineer. All land boundary surveys and legal descriptions used for preliminary and engineering plans must be stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping prepared specifically for a proposed project may be performed by the civil engineer stamping the engineering plans as allowed by the Washington State Board of Registration for Professional Engineers and Land Surveyors. TABLE 2.2.A DRAINAGE PLAN SUBMITTALS Plans Required with Type of Permit Construction Permit Type of Plans Required for or Project Application Drainage Review Drainage Review SUBDIVISIONS, Plat Map(5) Full or Targeted Drainage • Preliminary Plans(5) PUDs,AND Engineering Plans('),(2) Review(2) . Engineering Plans(') BINDING SITE Large Project Drainage PreliminaryPlans(5) PLANS Review . Master Drainage Plan(4)or Special Study • Engineering Plans(') SHORT PLATS Site Plan(5) Simplified Drainage Review Simplified Drainage Plans(3) Site Plan(5) Simplified Drainage Review • Simplified Drainage Preliminary Reports AND Targeted Drainage Plans(3) Review(2) . Engineering Plans(') Full or Targeted Drainage Engineering Plans(') Review(2) COMMERCIAL Engineering Plans('),(2) Full or Targeted Drainage Engineering Plans(') PERMITS Review SINGLE- Site Plan(5)for Single-Family Simplified Drainage Review Simplified Drainage FAMILY I Residential Building Permits Plans(3) 2017 City of Renton Surface Water Design Manual 12/12/2016 2-5 SECTION 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION TABLE 2.2.A DRAINAGE PLAN SUBNHTTALS Plans Required with Type of Permit Construction Permit Type of Plans Required for or Project Application Drainage Review Drainage Review RESIDENTIAL Simplified Drainage Review • Simplified Drainage BUILDING AND Targeted Drainage Plans(3) PERMITS Review(2)AND Directed . Engineering Drainage Review(6) Plans(l)(6) Full or Targeted Drainage Engineering Plans0) Review(2) OTHER Project-specific(contact CED Full or Targeted Drainage Engineering Plans0) PROJECTS OR or the City's website) Review(2) PERMITS Notes: (1) Submittal specifications for engineering plans are detailed in Section 2.3.1. (2) Submittal specifications for Targeted Drainage Review are found in Section 2.3.2. (3) Specifications for submittal of Simplified drainage plans are found in Appendix C, Simplified Drainage Requirements. (4) Specifications for submittal of master drainage plans or special studies are found in the King County publication titled Master Drainage Planning for Large or Complex Site Developments. (5) Submittal specifications for these plans are found on the City's website and/or from CED staff in the pre-application meeting. (6) Scope of submittals for Directed Drainage Review is determined by CED staff at the City's Permit Counter and/or during the plan review process.Submittal specifications per Notes 1,2,and 3. 2.2.1 SUBDIVISION, PUD, AND BINDING SITE PLANS Applications for proposed subdivision,PUD, and binding site plan projects must include engineered preliminary plans,which are used to help determine engieening plan requirements to recommend to the Hearing Examiner. Preliminary plans shall include the following: 1. A conceptual drainage plan prepared, stamped, and signed by a civil engineer. This plan must show the location and type of the following: a) Existing and proposed flow control facilities b) Existing and proposed water quality facilities c) Existing and proposed conveyance systems. The level of detail of the plan should correspond to the complexity of the project. 2. A Level I Downstream Analysis as required in Core Requirement 42 and outlined under"TIR Section 3,Offsite Analysis."This offsite analysis shall be submitted in order to assess potential offsite drainage and water quality impacts associated with development of the project, and to help propose appropriate mitigation of those impacts.A higher level of offsite analysis may be requested by the City prior to preliminary approval, or as a condition of engineering plan submittal. The offsite analysis must be prepared, stamped, and signed by a civil engineer. 3. Survey/topographic information.The submitted site plan and conceptual drainage plan shall include the following: a) Field topographic base map to accompany application(aerial topography allowed with CED permission) b) Name and address of surveyor and surveyor's seal and signature 12/12/2016 2017 City of Renton Surface Water Design Manual 2-6 2.2.5 OTHER PERMITS c) Notation for field or aerial survey d) Datum and benchmark/location and basis of elevation e) Location of all critical areas f) Contour intervals per the following chart: Zoning Designation Contour Intervals Densities of developed area 2 feet at less than 15% slope of over 4 DU per acre 5 feet at 15%slope or more Densities of developed area 5 feet of 4 DU or less per acre 2.2.2 SHORT SUBDIVISIONS Applications for proposed short plats' require a proposed site plan drawn to scale showing geographic features such as adjacent streets, existing buildings, and critical areas if any are known to be present; and a Level l Downstream Analysis. Site plans are usually engineered, except for projects exempt from drainage review or projects subject to Simplified Drainage Review for the entire project. The specifications for submittal of site plans are outlined on the City's website. The Level 1 Downstream Analysis is required for all short plats except those meeting the exemptions outlined in Section 1.2.2 or those subject to Simplified Drainage Review for the entire project.A higher level of offsite analysis may be requested by CED prior to preliminary approval, or as a condition of engineering plan submittal. 2.2.3 COMMERCIAL SITE DEVELOPMENT Applications for commercial permits require that engineering plans be submitted as part of the building permit application,unless otherwise approved by CED. Most commercial projects will go through Full Drainage Review and require complete engineering plans. Projects that qualify for limited scope engineering design should request Targeted Drainage Review during the pre-application meeting with CED. 2.2.4 SINGLE-FAMILY RESIDENTIAL Applications for single-family residential permits' require a non-engineered site plan to be submitted. The specifications for site plans are outlined on the City's website. 2.2.5 OTHER PERMITS Other permit applications' will require project-specific information. Submittal requirements can be obtained by contacting CED or consulting the City's website. The specific level of required drainage analysis and design is usually determined during the preliminary drainage review of the plans submitted with the application.The overall plan review process may be expedited if the project is submitted with the appropriate level of detail. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-7 SECTION 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 2-8 CITY OF RENTON SURFACE WATER DESIGN MANUAL 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS This section presents the specifications and contents required of plans to facilitate drainage review. Most projects subject to Full Drainage Review will require engineering plans that include a"technical information report(TIR),""site improvement plans,"and a"construction stormwater pollution prevention (CSWPP)plan,"which includes an"erosion and sediment control(ESC)plan"and a"stormwater pollution prevention and spill(SWPPS)plan."For more information on the types of projects subject to Full Drainage Review, see Section 1.1.2.4.Additional information is available at the City's website and from the staff in the pre-application meeting,if held. Small projects with specific drainage concerns that are subject to Targeted Drainage Review also require engineering plans that include the same elements, except that the TIR may be of limited scope.The site improvement plans,ESC and SWPPS plans may also be of limited scope,but must meet all applicable specifications.For more information on the types of projects subject to Targeted Drainage Review, see Section 1.1.2.2. Projects subject to Simplified Drainage Review may be required to submit"Simplified drainage plans." These are simplified drainage and erosion control plans that may be prepared by a non-engineer from a set of pre-engineered design details, and which do not require a TIR or a separate SWPPS plan.The Simplified Drainage Requirements booklet available at King County Department of Permitting and Environmental Review and appended to this manual(Appendix C)contains the specifications for Simplified drainage plans and details on the Simplified Drainage Review process. Note:Projects in Simplified Drainage Review may be required to submit engineering plans if they are also subject to Targeted Drainage Review as determined in Section 1.1.2.2 and Appendix C.Also, short plats in Simplified Drainage Review will be required to submit engineering plans if roadway construction is a condition ofpreliminary approval. Single-family residential projects that do not qualify for Simplified Drainage Review may qualify for Directed Drainage Review,which requires a specialized list of submittals(plans,technical reports, etc.) and engineering requirements determined by CED review staff that ensures compliance with all core and special requirements of the SWDM. The scope of the submittal requirement is determined during the initial review of the application. Specifications for the plans and TIR generally follow those described for the other review types but may be reduced in scope or complexity in accordance with CED's determination. Design Plan Certification All preliminary plans and engineering plans must be stamped by a civil engineer. All land boundary surveys, and legal descriptions used for preliminary and engineering plans must be stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping prepared specifically for a proposed project may be performed by the civil engineer stamping the engineering plans as allowed by the Washington State Board of Registration for Professional Engineers and Land Surveyors. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-9 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 2.3.1 ENGINEERING PLAN SPECIFICATIONS For drainage review purposes, engineering plans must consist of the following: 1. A TIR as detailed in Section 2.3.1.1,AND 2. Site improvement plans as detailed in Section 2.3.1.2,AND 3. A CSWPP plan,which includes an ESC plan as detailed in Section 2.3.1.3 and a SWPPS plan as detailed in Section 2.3.1.4. Projects in Targeted Drainage Review require a limited scope TIR with site improvement plans and a CSWPP plan, as detailed in Section 2.3.2. CED may allow a modified site improvement plan for some projects in Targeted Drainage Review(see Section 2.3.2)or where major improvements(e.g., detention facilities,conveyance systems,bridges,road right-of-way improvements, etc.)are not proposed. 2.3.1.1 TECHNICAL INFORMATION REPORT (TIR) The full TIR is a comprehensive supplemental report containing all technical information and analysis necessary to develop the site improvement plan. This report shall contain all calculations,conceptual design analysis,reports, and studies required and used to construct a complete site improvement plan based on sound engineering practices and careful geotechnical and hydrological design. The TIR must be stamped and dated by a civil engineer. The TIR shall contain the following ten sections,preceded by a table of contents: 1. Project Overview 2. Conditions and Requirements Summary 3. Offsite Analysis 4. Flow Control,Low Impact Development(LID)and Water Quality Facility Analysis and Design 5. Conveyance System Analysis and Design 6. Special Reports and Studies 7. Other Permits 8. CSWPP Analysis and Design 9. Bond Quantities, Facility Summaries, and Declaration of Covenant 10. Operations and Maintenance Manual. Every TIR must contain each of these sections;however,if a section does not apply,the applicant may simply mark "N/A"and a brief explanation shall be provided. This standardized format allows a quicker, more efficient review of information required to supplement the site improvement plan. The table of contents should include a list of the ten section headings and their respective page numbers, a list of tables with page numbers, and a list of numbered references, attachments, and appendices. When the TIR package requires revisions,the revisions must be submitted in a complete TIR package. ❑ TIR SECTION 1 PROJECT OVERVIEW The project overview must provide a general description of the proposal,predeveloped and developed site conditions,site and project site area, size of the improvements, and the disposition of stormwater runoff before and after development. The overview shall identify and discuss difficult site parameters,the natural drainage system, and drainage to and from adjacent property,including bypass flows. The following figures are required: 12/12/2016 2017 City of Renton Surface Water Design Manual 2-10 2.3.1 ENGINEERING PLAN SPECIFICATIONS Figure 1. TIR Worksheet Include a copy of the TIR Worksheet(see Reference Section 8-A). Figure 2. Site Location Provide a map that shows the general location of the site. Identify all roads that border the site and all significant geographic features and critical areas(lakes, streams, steep slopes, etc.). Figure 3. Drainage Basins, Subbasins, and Site Characteristics This figure shall display the following: 1. Show acreage of subbasins. 2. Identify all site characteristics. 3. Show existing discharge points to and from the site. 4. Show routes of existing,construction, and future flows at all discharge points and downstream hydraulic structures. 5. Use a minimum USGS 1:2400 topographic map as a base for the figure. 6. Show(and cite)the length of travel from the farthest upstream end of a proposed storm system in the development to any proposed flow control facility. Figure 4. Soils Show the soils within the following areas: 1. The project site 2. The area draining to the site 3. The drainage system downstream of the site for the distance of the downstream analysis(see Section 1.2.2). Copies of King County Soil Survey maps may be used;however,if the maps do not accurately represent the soils for a proposed project(including offsite areas of concern),it is the design engineer's responsibility to ensure that the actual soil types are properly mapped. Soil classification symbols that conform to the SCS Soil Survey for King County shall be used; and the equivalent soil type(till, outwash, or wetlands)per the approved stormwater model shall be indicated(see Table 3.2.2.13). Projects will need to evaluate the soils on each lot for applicability of the full infiltration and other low impact on-site BMPs as specified in Core Requirement 49. This soils report, as well as geotechnical investigations necessary for proposed infiltration facilities, shall be referenced in the TIR Overview and submitted under Special Reports and Studies,TIR Section VI.A figure in the required geotechnical report that meets the above requirements may be referenced to satisfy 1,2, and 3 above. Projects located in outwash soils may need to provide a low-permeability liner or a treatment liner for water quality facilities and upstream conveyance ditches,consistent with the specifications for such liners in Section 6.2.4. ❑ TIR SECTION 2 CONDITIONS AND REQUIREMENTS SUMMARY The intent of this section is to ensure all preliminary approval conditions and applicable requirements pertaining to site engineering issues have been addressed in the site improvement plan. All conditions and requirements for the proposed project shall be included. In addition to the core requirements of this manual, adopted basin plans and other plans as listed in Special Requirement 41 should be reviewed and applicable requirements noted.Additionally,critical area requirements,conditions of plat approval, and conditions associated with development requirements(e.g., conditional use permits,rezones,variances and adjustments, SEPA mitigations, etc.)shall be included. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-11 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS ❑ TIR SECTION 3 OFFSITE ANALYSIS All projects in engineering review shall complete, at a minimum, an Offsite Analysis, except for projects meeting the exemptions outlined in Section 1.2.2. The Offsite Analysis is usually completed as part of the initial permit application and review process, and is to be included in the TIR.Note:If offsite conditions have been altered since the initial submittal, a new offsite analysis may be required. The primary component of the offsite analysis is the downstream analysis described in detail below. Upstream areas are included in this component to the extent they are expected to be affected by backwater effects from the proposed project. Other components of the offsite analysis could include,but are not limited to, evaluation of impacts to fish habitat,groundwater levels,groundwater quality, or other environmental features expected to be significantly impacted by the proposed project due to its size or proximity to such features. Levels of Analysis The offsite analysis report requirements vary depending on the specific site and downstream conditions. Each project submittal shall include at least a Level 1 downstream analysis.Upon review of the Level 1 analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional,more detailed analysis may be required.Note:Potential impacts upstream of the proposal shall also be evaluated. Level 1 Analysis The Level 1 analysis is a qualitative survey of each downstream system leaving a site. This analysis is required for all proposed projects and shall be submitted with the initial permit application. Depending on the findings of the Level 1 analysis, a Level 2 or 3 analysis may need to be completed or additional information may be required. If further analysis is required,the applicant may schedule a meeting with CED staff. Level 2 or 3 Analysis If drainage problems are identified in the Level 1 analysis, a Level 2(rough quantitative) analysis or a Level 3 (more precise quantitative)analysis may be required to further evaluate proposed mitigation for the problem. CED staff will determine whether a Level 2 or 3 analysis is required based on the evidence of existing or potential drainage problems identified in the Level 1 analysis and on the proposed design of onsite drainage facilities. The Level 3 analysis is required when results need to be as accurate as possible: for example,if the site is flat;if the system is affected by downstream controls;if minor changes in the drainage system could flood roads or buildings; or if the proposed project will contribute more than 15 percent of the total peak flow to the drainage problem location. The Level 2 or 3 analysis may not be required if CED determines from the Level 1 analysis that adequate mitigation will be provided. Additional Analysis Additional,more detailed hydrologic analysis may be required if CED determines that the downstream analysis has not been sufficient to accurately determine the impacts of a proposed project on an existing or potential drainage problem. This more detailed analysis may include a point of compliance analysis as detailed in Section 3.3.6. Scope of Analysis Regardless of the level of downstream analysis required,the applicant shall define and map the study area (Task 1),review resources(Task 2),inspect the study area(Task 3), describe the drainage system and problems (Task 4), and propose mitigation measures(Task 5)as described below. Task 1. Study Area Definition and Maps For the purposes of Task 2 below,the study area shall extend downstream one mile (minimum flowpath distance)from the proposed project discharge location and shall extend upstream as necessary to encompass the offsite drainage area tributary to the proposed project site. For the 12/12/2016 2017 City of Renton Surface Water Design Manual 2-12 2.3.1 ENGINEERING PLAN SPECIFICATIONS purposes of Tasks 3,4,and 5,the study area shall extend downstream to a point on the drainage system where the proposed project site constitutes less than 15 percent of the total tributary drainage area,but not less than one-quarter mile (minimum flowpath distance). The study area shall also extend upstream of the project site a distance sufficient to preclude any backwater effects from the proposed project. The offsite analysis shall include a site map showing property lines, and the best available topographical map (e.g., from CED and Renton topographic map)with the study area boundaries, site boundaries, downstream flowpath for a distance of one mile, and potential/existing problems (Task 4) shown. Other maps, diagrams,photographs and aerial photos may be helpful in describing the study area. Task 2.Resource Review To assist the design engineer in preparing an offsite analysis,Renton has gathered information regarding existing and potential flooding, erosion, and water quality problems. For all levels of analysis, all of the resources described below shall be reviewed for existing/potential problems in the study area(upstream and one mile downstream of the project site): • Adopted basin plans available at King County DPER,King County DNRP, and CED.For areas where there is no adopted basin plan,Basin Reconnaissance Summary Reports may be useful. • Floodplain/floodway(FEMA) maps available at CED and the library. • Other offsite analysis reports in the same subbasin,if available(check with CED staff). • Sensitive Areas Folio available at King County DPER,King County DNRP, and the library(see also the City's website:<httD://rentonwa.gov/government/default.aspx?id=29885>)must be used to document the distance downstream from the proposed project to the nearest critical areas. • 2U.S.Department of Agriculture,King County Soils Survey available at King County DPER and the library. • Wetlands Inventory maps available at CED. • Washington State Department of Ecology's latest published Clean Water Act Section 303d list of polluted waters posted at the following website: <htti)://www.ecy.wa.gov/programs/wg/303d/currentassessmt.html>. • City of Renton Erosion Maps and Landslide Maps. Potential/existing problems identified in the above documents shall be documented in the Drainage System Table(see Reference Section 8-13) as well as described in the text of the Level I Downstream Analysis Report. If a document is not available for the site,note in the report that the information was not available as of a particular date. If necessary, additional resources are available from King County,the Washington State Department of Fisheries and Wildlife(WDFW),the State Department of Ecology(Ecology),the United States Army Corps of Engineers(Corps), and the public works departments of other municipalities in the vicinity of the proposed project site. Task 3.Field Inspection The design engineer shall physically inspect the existing on-and offsite drainage systems of the study area for each discharge location. Specifically,he/she shall investigate any evidence of the following existing or potential problems and drainage features: Level 1 Inspection: 1. Investigate any problems reported or observed during the resource review. 2. Locate all existing/potential constrictions or lack of capacity in the existing drainage system. 3. Identify all existing/potential downstream drainage problems as defined in Section 1.2.2.1. 4. Identify existing/potential overtopping, scouring,bank sloughing, or sedimentation. 2 Footnote 2 is not used. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-13 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 5. Identify significant destruction of aquatic habitat or organisms(e.g., severe siltation,bank erosion, or incision in a stream). 6. Collect qualitative data on features such as land use,impervious surfaces,topography, and soil types. 7. Collect information on pipe sizes,channel characteristics, drainage structures, and relevant critical areas(e.g.,wetlands, streams, steep slopes). 8. Verify tributary basins delineated in Task 1. 9. Contact neighboring property owners or residents in the area about past or existing drainage problems, and describe these in the report(optional). 10. Note the date and weather conditions at the time of the inspection. Level 2 or 3 Inspection: 1. Perform a Level 1 Inspection. 2. Document existing site conditions(approved drainage systems or pre-1979 aerial photographs)as defined in Core Requirement 43. 3. Collect quantitative field data.For Level 2,conduct rough field survey using hand tape,hand level, and rod; for Level 3,collect field survey profile and cross-section topographic data prepared by an experienced surveyor. Task 4.Drainage System Description and Problem Descriptions Each drainage system component and problem shall be addressed in the offsite analysis report in three places: on a map (Task 1),in the narrative (Task 4), and in the Offsite Analysis Drainage System Table (see Reference Section 8-13). Drainage System Descriptions: The following information about drainage system components such as pipes,culverts,bridges, outfalls,ponds,tanks, and vaults shall be included in the report: 1. Location(corresponding map label and distance downstream/upstream from site discharge) 2. Physical description(type, size,length, slope,vegetation, and land cover) 3. Problems including copies of any relevant drainage complaints 4. Field observations. Problem Descriptions: All existing or potential drainage and water quality problems (e.g.,ponding water,high/low flows, siltation, erosion,listed water bodies, etc.)identified in the resource review or field inspection shall be described in the offsite analysis. These descriptions will help in determining if such problems require special attention per Core Requirement 42(see Section 1.2.2.1)because they are one of three defined drainage problem types or one of seven defined water quality problem types. Special attention may include more analysis, additional flow control, or other onsite or offsite mitigation measures as specified by the problem-specific mitigation requirements set forth in Sections 1.2.2.2 and 1.2.2.3. The following information shall be provided for each existing or potential drainage problem: 1. Description of the problem(ponding water,high or low flows, siltation, erosion, slides, etc.). 2. Magnitude of or damage caused by the drainage problem(siltation of ponds, dried-up ornamental ponds,road inundation, flooded property, flooded building, flooded septic system, significant destruction of aquatic habitat or organisms). 3. General frequency and duration of drainage problem(dates and times the problem occurred,if available). 12/12/2016 2017 City of Renton Surface Water Design Manual 2-14 2.3.1 ENGINEERING PLAN SPECIFICATIONS 4. Return frequency of storm or flow(cfs)of the water when the problem occurs(optional for Level 1 and required for Levels 2 and 3).Note:A Level 2 or 3 analysis may be required to accurately idents the return frequency of a particular downstream problem;see Section 3.3.3. 5. Water surface elevation when the problem occurs(e.g., elevation of building foundation,crest of roadway, elevation of septic drainfields, or wetland/stream high water mark). 6. Names and concerns of involved parties(optional for all levels of analysis). 7. Current mitigation of the drainage problem. 8. Possible cause of the drainage problem. 9. Whether the proposed project is likely to aggravate (increase the frequency or severity of)the existing drainage problem or create a new one based on the above information. For example, an existing erosion problem should not be aggravated if Durational flow control is already required in the region for the design of onsite flow control facilities. Conversely, a downstream flooding problem inundating a home every 2 to 5 years will likely be aggravated if only Peak Flow Control is being applied in the region. See Section 1.2.3.1 for more details on the effectiveness of flow control standards in addressing downstream problems. The following information shall be provided for each existing or potential water quality problem: 1. Description of the problem as documented by the State, County, or City in the problem's listing. This should include the pollutant or pollutants of concern,the nature or category of the listing, and any other background information provided in the listing. 2. Flow path distance downstream of the project site and percentage of area draining to the problem that the project site occupies. 3. Possible or probable cause of the water quality problem. 4. Any current mitigation of the water quality problem. Task 5.Mitigation of Existing or Potential Problems For any existing or potential offsite drainage problem determined to be one of the three defined problem types in Section 1.2.2.1,the design engineer must demonstrate that the proposed project neither aggravates(if existing)nor creates the problem as specified in the drainage problem-specific mitigation requirements set forth in Section 1.2.2.2.The engineer must review each relevant drainage complaint found and include a narrative explaining how each complaint problems is addressed or mitigated.Actual copies of the relevant complaints must be included in the Analysis. To meet these requirements,the proposed project may need to provide additional onsite flow control as specified in Table 1.2.3.A(see also Section 3.3.5), or other onsite or offsite mitigation measures as described in Section 3.3.5. For any existing or potential water quality problem determined to be one of the seven defined water quality problem types in Section 1.2.2.1,the design engineer must document how the applicable water quality problem-specific mitigation requirement in Section 1.2.2.3 will be met. ❑ TIR SECTION 4 FLOW CONTROL, LOW IMPACT DEVELOPMENT(LID)AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Existing Site Hydrology (Part A) This section of the TIR shall include a discussion of assumptions and site parameters used in analyzing the existing site hydrology. The acreage, soil types, and land covers used to determine existing flow characteristics, along with basin maps,graphics, and exhibits for each subbasin affected by the development, shall be included. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-15 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS The following information must be provided on a topographical map: 1. Delineation and acreage of areas contributing runoff to the site 2. Flow control facility and BMP location(s) 3. Outfall(s) 4. Overflow route(s). The scale of the map and the contour intervals must be sufficient to determine the basin and subbasin boundaries accurately. The direction of flow,the acreage of areas contributing drainage, and the limits of development shall all be indicated on the map. Each subbasin contained within or flowing through the site shall be individually labeled and parameters for the approved stormwater model referenced to that subbasin. All natural streams and drainage features,including wetlands and depressions,must be shown. Rivers, closed depressions,streams, lakes, and wetlands must have the 100-year floodplain(and floodway where applicable)delineated as required in Special Requirement 42(see Section 1.3.2)and by the critical areas requirements in RMC 4-3-050. Developed Site Hydrology (Part B) This section shall provide narrative,mathematical, and graphical presentations of parameters selected and values used for the developed site conditions,including acreage, soil types and land covers,roadway layouts, and all constructed drainage facilities and any required on-site BMPs. Developed subbasin areas and flows shall be clearly depicted on a map and cross-referenced to computer printouts or calculation sheets. Relevant portions of the calculations shall be highlighted and tabulated in a listing of all developed subbasin flows. All maps, exhibits,graphics, and references used to determine developed site hydrology must be included, maintaining the same subbasin labeling as used for the existing site hydrology whenever possible. If the boundaries of the subbasin have been modified under the developed condition,the labeling should be modified accordingly(e.g., Subbasin"Am"is a modified version of existing Subbasin"A"). Performance Standards (Part C) The design engineer shall include brief discussions of the following: • The applicable area-specific flow control facility standard determined from the Flow Control Applications Map per Section 1.2.3.1, any modifications to the standard to address onsite or offsite drainage conditions, and applicable on-site BMP requirements determined from Section 1.2.3.3 and Core Requirement 49; • The applicable conveyance system capacity standards per Section 1.2.4; and • The applicable area-specific water quality treatment menu determined from the Water Quality Applications Map per Section 1.2.8.1, and any applicable special requirements for source control or oil control determined from Sections 1.3.4 and 1.3.5. Flow Control System (Part D) This section requires: • An illustrative sketch of the flow control facility(or facilities),required on-site BMPs, and appurtenances. The facility sketch(or sketches)must show basic measurements necessary to calculate the storage volumes available from zero to the maximum head, all orifice/restrictor sizes and head relationships,control structure/restrictor orientation to the facility, and facility orientation on the site. The on-site BMP sketch(or sketches)must show basic measurements and dimensions, orientation on the site, flowpath lengths, etc. • The applicant shall include all supporting documentation such as computer printouts,calculations, equations,references, storage/volume tables,graphs, soils data,geotechnical reports and any other 12/12/2016 2017 City of Renton Surface Water Design Manual 2-16 2.3.1 ENGINEERING PLAN SPECIFICATIONS aides necessary to clearly show results and methodology used to determine the storage facility volumes and on-site BMP applications. • Facility documentation files, flow duration comparison files,peaks files,return frequency or duration curves, etc., developed with the approved model shall be included to verify the facility meets the performance standards indicated in Part C. • The volumetric safety factor used in the design shall be clearly identified, as well as the reasoning used by the design engineer in selecting the safety factor for this project. • If on-site BMP credits are used as allowed in Core Requirement 49, documentation must be provided, explaining how the credits will be used and how the criteria for use of credits will be met. • If the flow control system is an infiltration facility,the soils data,groundwater mounding analysis, and other calculations used to determine the design infiltration rate shall be provided. • On-site BMP infeasibility discussion and supporting documentation shall also be included in Part D. Water Quality System (Part E) This section requires an illustrative sketch of the proposed water quality facility(or facilities), source controls, oil controls, and appurtenances.This sketch(or sketches) of the facility, source controls, and oil controls must show basic measurements and dimensions, orientation on the site,location of inflow, bypass, and discharge systems, etc. The applicant shall also include all supporting documentation such as computer printouts,calculations, equations,references, and graphs necessary to show the facility was designed and sized in accordance with the specifications and requirements in Chapter 6. If the water quality credit option is used as allowed in Section 6.1.3, documentation must be provided,identifying the actions that will be taken to acquire the requisite credits. ❑ TIR SECTION 5 CONVEYANCE SYSTEM ANALYSIS AND DESIGN This section shall present a detailed analysis of any existing conveyance systems, and the analysis and design of the proposed stormwater collection and conveyance system for the development. This section also includes any analysis required for the design of bridges to convey flows and pass sediments and debris per Section 4.4.3.Analysis information should be presented in a clear,concise manner that can be easily followed,checked, and verified.All pipes,culverts,catch basins,channels, swales, and other stormwater conveyance appurtenances must be clearly labeled and correspond directly to the engineering plans. The minimum information included shall be pipe flow tables, flow profile computation tables, nomographs,charts,graphs,detail drawings, and other tabular or graphic aides used to design and confirm performance of the conveyance system. Verification of capacity and performance must be provided for each element of the conveyance system. The analysis must show design velocities and flows for all drainage facilities within the development, as well as those offsite that are affected by the development. If the final design results are on a computer printout, a separate summary tabulation of conveyance system performance shall also be provided. ❑ TIR SECTION 6 SPECIAL REPORTS AND STUDIES Some site characteristics, such as steep slopes or wetlands,pose unique road and drainage design problems that are particularly sensitive to stormwater runoff.As a result, CED may require the preparation of special reports and studies that further address the site characteristics,the potential for impacts associated with the development, and the measures that would be implemented to mitigate impacts. Special reports shall be prepared by people with expertise in the particular area of analysis. Topics of special reports may include any of the following: 2017 City of Renton Surface Water Design Manual 12/12/2016 2-17 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS • Floodplain delineation in accordance with Section 1.3.2 • Flood protection facility conformance in accordance with Section 1.3.3 • Critical areas analysis and delineation • Geotechnical/soils(soils documentation supporting on-site BMP design,infiltration rate determination and infeasibility conclusions may also be located in TIR Section 6) • Groundwater,including groundwater mounding analyses required for infiltration design • Slope protection/stability • Erosion and deposition • Geology • Hydrology • Fluvial geomorphology • Anadromous fisheries impacts • Water quality • Structural design • Structural fill • Aquifer Protection Areas ❑ TIR SECTION 7 OTHER PERMITS Construction of road and drainage facilities may require additional permits from other agencies for some projects. These additional permits may contain more restrictive drainage plan requirements. This section of the TIR should provide the titles of any other permits,the agencies requiring the other permits, and the permit requirements that affect the drainage plan. Examples of other permits are listed in Section 1.1.3. If a UIC well registration is required, a copy must be provided. ❑ TIR SECTION 8 CSWPP PLAN ANALYSIS AND DESIGN This section of the TIR should include the analysis and design information used to prepare the required construction stormwater pollution prevention (CSWPP)plan.This information should be presented in two parts associated with the CSWPP plan's two component plans,the erosion sediment control(ESC) plan(Part A) and the stormwater pollution prevention and spill control(SWPPS)plan(Part B). See Sections 2.3.1.3 and 2.3.1.4 for plan specifications and contents. ESC Plan Analysis and Design (Part A) This section must include all hydrologic and hydraulic information used to analyze and design the erosion and sediment control measures,including final site stabilization measures. The TIR shall explain how proposed ESC measures comply with the Erosion and Sediment Control Standards in Appendix D and show compliance with the implementation requirements of Core Requirement 45, Section 1.2.5. Part A must include the following: 1. Provide sufficient information to justify the overall ESC plan and the choice of individual ESC measures.At a minimum,there shall be a discussion of each of the measures specified in Section 1.2.5 and their applicability to the proposed project. 2. Include all hydrologic and hydraulic information used to analyze and size the ESC facilities shown in the engineering plans.Describe the methodology, and attach any graphics or sketches used to size the facilities. 3. Identify areas with a particularly high susceptibility to erosion because of slopes or soils, as well as areas to be protected for existing and proposed on-site BMPs. Discuss any special measures taken to protect these areas as well as any special measures proposed to protect water resources on or near the site. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-18 2.3.1 ENGINEERING PLAN SPECIFICATIONS 4. Identify any ESC recommendations in any of the special reports prepared for the project. In the project geotechnical report supporting on-site BMP design,provide recommendations to address mitigation of on-site BMP areas impacted by erosion and/or sedimentation during construction. If these special reports' recommendations are not included in the ESC plan,provide justification. 5. If proposing exceptions or modifications to the standards detailed in the Erosion and Sediment Control Standards in Appendix D,clearly present the rationale. If proposing techniques or products different from those detailed in the ESC Standards,provide supporting documentation so the City can determine if the proposed alternatives provide similar protection. SWPPS Plan Design (Part B) The stormwater pollution prevention and spill control plan must identify all activities that could contribute pollutants to surface and storm water during construction. This section of the TIR must provide sufficient information to justify the selection of specific stormwater pollution prevention(SWPPS)BMPs proposed to be applied to the pollution-generating activities that will occur with construction of the proposed project. BMPs applicable to such activities are found in the Construction Stormwater Pollution Prevention and Spill Control(CSWPP)Standards (Appendix D)and the King County Stormwater Pollution Prevention Manual(viewable at<http://www.kin eg ounty.gov/environment/water-and- land/stormwater/documents/pollution-prevention-manual.aspx>)adopted pursuant to RMC 4-6-030. At a minimum, there shall be a discussion of each anticipatedpollution-generating activity and the pollution prevention BMPs selected to address it. If there are any calculations required for the selected BMP,include those in the discussion. If an alternative BMP or major modification to one of the City's standard BMPs will be used,a written request must be submitted for review and approval, detailing how the alternative will work.An"Alternative BMP Request Form"is available in the Stormwater Pollution Prevention Manual. Updates or revisions to the SWPPS plan may be requested by CED at any time during project construction if CED determines that pollutants generated on the construction site have the potential to contaminate surface, storm, or ground water. The SWPPS plan shall also discuss the receiving waters, especially if the receiving water body is listed on the 303d list. Information must be provided that shows the plan meets TMDL requirements. Discuss the 303(d)listed pollutant generated or used onsite and any special handling requirements or BMPs. ❑ TIR SECTION 9 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT Bond Quantities Worksheet Each plan submittal requires a construction quantity summary to establish appropriate bond amounts. Using the Site Improvement Bond Quantities Worksheet furnished by CED (see the City's website),the design engineer shall separate existing right-of-way and erosion control quantities from other onsite improvements. In addition,the design engineer shall total the amounts based on the unit prices listed on the form. Drainage facilities for single-family residential building permits,which are normally not bonded, shall be constructed and approved prior to finalization of the permit. Flow Control and Water Quality Facility Summary Sheet and Sketch Following approval of the plans, a Flow Control and Water Quality Facility Summary Sheet and Sketch (see Reference Section 8-D)shall be submitted along with an 8i/2" x I I"plan sketch for each facility proposed for construction.The plan shall show a north arrow,the tract,the facility access road,the extent of the facility, and the control structure location. The approximate street address shall be noted.At project completion,the Summary Sheet and Sketch shall be updated in the Final Corrected TIR to reflect the completed project(see Section 2.4.2). 2017 City of Renton Surface Water Design Manual 12/12/2016 2-19 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS Declaration of Covenant for Privately Maintained Flow Control and WQ Facilities Any declaration of covenant and grant of easement required for proposed flow control and water quality facilities per Section 1.2.6 must be included here for review and approval before recording. The necessary covenant exhibits, and maintenance instructions associated with the facility type (see Reference Section 5), shall be included with the declaration of covenant. After approval by CED,the declaration of covenant and grant of easement must be signed and recorded at the office of King County Records and Elections before finalization of any permit.A copy of the recorded document shall be included in the Final Corrected TIR(see Section 2.4.2). Declaration of Covenant for Privately Maintained On-Site BMPs Any declarations of covenant and grant of easement required for proposed on-site BMPs per Core Requirement 49 must be included here for review and approval before recording. The necessary covenant exhibits, and maintenance instructions associated with the on-site BMP type(see Reference Section 5), shall be included with the declaration of covenant. After approval by CED, all such documents must be signed and recorded at the office of King County Records and Elections before finalization of any permit.A copy of the recorded document shall be included in the Final Corrected TIR (see Section 2.4.2) or otherwise provided to the CED if no TIR was required. ❑ TIR SECTION 10 OPERATIONS AND MAINTENANCE MANUAL For each flow control and water quality facility and/or BMP that is to be privately maintained, and for those that have special non-standard features,the design engineer shall prepare an operations and maintenance manual. The manual should be simply written and should contain a brief description of the facility or BMP,what it does, and how it works. In addition,the manual shall include a copy of the Maintenance Requirements for Flow Control, Conveyance, and WQ Facilities (see Appendix A)and provide an outline of maintenance tasks and the recommended frequency each task should be performed. This is especially important for on-site BMP and water quality facilities where proper maintenance is critical to facility performance.For this reason,most of the flow control facility designs in Chapter 5 and the water quality facility designs in Chapter 6 include "maintenance considerations"important to the performance of each facility. BMP maintenance instructions by BMP type,prepared in 8-1/2"x 11" size for inclusion in TIRs and declarations of covenant, are also provided in Reference Section 5. 2.3.1.2 SITE IMPROVEMENT PLAN Site improvement plans shall portray design concepts in a clear and concise manner. The plans must present all the information necessary for persons trained in engineering to review the plans, as well as those persons skilled in construction work to build the project according to the design engineer's intent. Supporting documentation for the site improvement plans must also be presented in an orderly and concise format that can be systematically reviewed and understood by others. Survey Datum and Precision (RC) The horizontal component of all surveys shall have as its coordinate base: The North American Datum of 1983/91.All horizontal control for projects must be referenced to or in conjunction with a minimum of two of the City of Renton's Survey Control Network monuments. The source of the coordinate values used will be shown on the survey drawing per RCW 58.09.070. The horizontal component of all surveys shall meet or exceed the closure requirements of WAC 332-130-060. The control base lines for all surveys shall meet or exceed the requirements for a Class A survey revealed in Table 2 of the Minimum Standard Detail Requirements for ALTA/AGSM Land Title Surveys jointly established and adopted by ALTA and ACSM in 1992 or comparable classification in future editions of said document. The angular and linear closure and precision ratio of traverses used for survey control shall be revealed on the face of the survey drawing, as shall the method of adjustment. The horizontal component of the control system for surveys using global positioning system methodology shall exhibit at least 1 part in 50,000 precision in line length dependent error analysis at a 95 percent confidence level and performed pursuant to Federal Geodetic Control Subcommittee Standards for GPS control surveys as defined in Geometric Geodetic Accuracy Standards &Specifications for Using GPS 12/12/2016 2017 City of Renton Surface Water Design Manual 2-20 2.3.1 ENGINEERING PLAN SPECIFICATIONS Relative Positioning Techniques dated August 1, 1989 or comparable classification in future editions of said document. The vertical component of all surveys shall be based on NAVD 1988,the North American Vertical Datum of 1988, and tied to at least one of the City of Renton Survey Control Network benchmarks. If there are two such benchmarks within 3000 feet of the project site a tie to both shall be made. The benchmark(s)used will be shown on the drawing. If a City of Renton benchmark does not exist within 3,000 feet of a project, one must be set on or near the project in a permanent manner that will remain intact throughout the duration of the project. Source of elevations(benchmark)will be shown on the drawing, as well as a description of any bench marks established. See the City ofRenton Drafting Standards. The site improvement plans consist of all the plans,profiles, details,notes, and specifications necessary to construct road, drainage structure, and off-street parking improvements. Site improvement plans include the following: • A base map (described below), and • Site plan and profiles(described below). Note:Site improvement plans must also include grading plans if onsite grading extends beyond the roadway. Modified Site Improvement Plan CED may allow a modified site improvement plan for some projects in Targeted Drainage Review(see Section 2.3.2) or Directed Drainage Review, or where major improvements(e.g., detention facilities, conveyance systems,bridges,road right-of-way improvements, etc.)are not proposed. The modified site improvement plan must: 1. Be drawn on a 11"x 17" or larger sheet, 2. Accurately locate structure(s)and access,showing observance of the setback requirements given in this manual,the critical areas code(RMC 4-3-050), or other applicable documents, 3. Provide enough information(datum,topography, details,notes, etc.)to address issues as determined by CED. ❑ GENERAL PLAN FORMAT Site improvement plans should use City of Renton Drafting Standards as appropriate, and must include Standard Plan Notes (see Reference Section 7).Each plan must follow the general format detailed below: 1. Plan sheets and profile sheets, or combined plan and profile sheets, specifications, and detail sheets as required shall be on 22-inch by 34-inch sheets(22"x 34").Right-of-way improvements must be on 22-inch by 34-inch sheets(22"x 34"). Original sheets shall be archive quality reproducibles,Mylar, or equal. 2. Drafting details shall generally conform to King County Standard Map Symbols (see <http://www.kingeounty.gov//media/transportation/kedot/roads/engineering/documents/CADDFiles/ 2014KCDOTCADDStandardsManual.ashx>)with standard text height of 0.125" (1/8"). Existing features shall be shown with dashed lines or as half-toned(screened)in order to clearly distinguish existing features from proposed improvements. Hatch patterns shall not be used. 3. Each submittal shall contain a project information/cover sheet with the following: a) Title:Project name and CED file number(s) b) Table of contents(if more than three pages) C) Vicinity map d) Name and phone number of utility field contacts(e.g.,water, sanitary sewer,gas,power, telephone, and TV)and the One-Call number(811 or 1-800-424-5555) e) The City's preconstruction/inspection notification requirements f) Name and phone number of the erosion control/CSWPP supervisor 2017 City of Renton Surface Water Design Manual 12/12/2016 2-21 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS g) Name and phone number of the surveyor h) Name and phone number of the owner/agent i) Name and phone number of the applicant j) Legal description k) Plan approval signature block for CED 1) Name and phone number of the engineering firm preparing the plans(company logos acceptable) m) Renton Regional Fire Authority's' approval stamp (if required) n) Statement that mailbox locations have been designated or approved by the U.S.Postal Service (where required) o) List of conditions of preliminary approval and conditions of approved adjustments and variances on all site improvements 4. An overall site plan shall be included if more than three plan sheets are used. The overall plan shall be indexed to the detail plan sheets and include the following: a) The complete property area development b) Right-of-way information c) Street names and road classification d) All project phasing and proposed division boundaries e) All natural and proposed drainage collection and conveyance systems with catch basin numbers shown 5. Each sheet of the plan set shall be stamped, signed, and dated by a civil engineer.At least one sheet showing all boundary survey information and tied to two City of Renton monuments must be provided and stamped by a land surveyor licensed in the State of Washington. 6. Detail sheets shall provide sufficient information to construct complex elements of the plan. Details may be provided on plan and profile sheets if space allows. 7. The City of Renton title block shall be provided on each plan sheet. Title block can be obtained at the City's website.At a minimum,the title block shall list the following: a) Development title b) Name, address, and phone number of the firm or individual prepaning the plan c) A revision block d) Page (of pages)numbering e) Sheet title (e.g.,road and drainage,grading, erosion and sediment control, stormwater pollution prevention and spill control) 8. A blank CED approval block(included on the City's title block)shall be provided on each plan sheet. 9. The location and label for each section or other detail shall be provided. 10. Critical areas,critical area buffers, and critical area building setbacks as required by RMC 4-3-050 shall be delineated and labeled. 11. All match lines with matched sheet number shall be provided. 12. All division or phase lines and the proposed limits of construction under the permit application shall be indicated. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-22 2.3.1 ENGINEERING PLAN SPECIFICATIONS 13. Wetlands shall be labeled with the number from the City's wetland inventory, or shall be labeled as "uninventoried"if not listed on the wetland inventory. 14. The standard plan notes that apply to the project shall be provided on the plans(see Reference Section 7-B). 15. Commercial building permit applications shall include the designated zoning for all properties adjacent to the development site(s). ❑ BASE MAP A site improvement plan base map provides a common base and reference in the development and design of any project.A base map helps ensure that the engineering plans,grading plans,and CSWPP plans are all developed from the same background information. This base map shall include the information listed in Table 2.3.1.A. TABLE 2.3.1.A BASE MAP REQUIREMENTS Feature Requirements Ground Surface Provide topography within the site and extending beyond the property lines. Topography Contour lines must be shown as described in "Plan View: Site Plan and Roadway Elements." Surface Water Provide ground surface elevations for a reasonable "fan" around points of Discharge discharge extending at least 50 feet downstream of all point discharge outlets. Hydrologic Provide spot elevations in addition to contour lines to aid in delineating the Features boundaries and depth of all existing floodplains, wetlands, channels, swales, streams, storm drainage systems and BMPs, roads (low spots), bogs, depressions, springs, seeps, swales, ditches, pipes, groundwater, and seasonal standing water. Other Natural Show the location and relative sizes of other natural features such as rock Features outcroppings, existing vegetation, and trees 12 inches in diameter and greater that could be disturbed by the project improvements and construction activities (within tree canopy), noting species. Flows Provide arrows that indicate the direction of surface flow on all public and private property and for all existing conveyance systems. Floodplains/ Show the floodplain/floodways as required by the flood hazard portion of the Floodways critical areas code (RMC 4-3-050)and Section 4.4.2. General Show the location and limits of all existing: Background • Property boundaries Information • Structures • Easements (including dimensions) • Total property(including dimensions) • Roads and right-of-way • Sanitary sewers and water utilities • Common open space • Public dedications • Other manmade features affecting existing topography/proposed improvements. Development Delineate limitations to the development that may occur as identified on the Limitations TIR worksheet, Part 11 (see Reference Section 8-A). 2017 City of Renton Surface Water Design Manual 12/12/2016 2-23 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS ❑ SITE PLAN AND PROFILES The design engineer shall provide plans and profiles for all construction,including but not limited to the following information: Plan View: Site Plan and Roadway Elements 1. Provide property lines, right-of-way lines, and widths for proposed roads and intersecting roads. Note:the condition of all public right-of-way and the right to use it as proposed must be verified. 2. Provide all existing and proposed roadway features, such as centerlines, edges of pavement and shoulders, ditchlines,curbs,and sidewalks. In addition, show points of access to abutting properties and roadway continuations. 3. Show existing and proposed topography contours at 2-foot intervals(5-foot intervals for slopes greater than 15 percent, 10-foot intervals for slopes greater than 40 percent). Contours may be extrapolated from USGS mapping, aerial photos, or other topography map resources. However, contours shall be field verified for roadway and stream centerlines, steep slopes, floodplains, drainage tracts easements, and conveyance systems. Contours shall extend 20 feet beyond property lines to resolve questions of setback,cut and fill slopes, drainage swales, ditches, and access or drainage to adjacent property. 4. Show the location of all existing utilities and proposed utilities(except those designed by the utility and not currently available)to the extent that these will be affected by the proposed project. Clearly identify all existing utility poles. 5. Identify all roads and adjoining subdivisions. 6. Show right-of-way for all proposed roadways,using sufficient dimensioning to clearly show exact locations on all sections of existing and proposed dedicated public roadway. 7. Clearly differentiate areas of existing pavement and areas of new pavement. If the project is a redevelopment project, delineate areas of replaced impervious surface. 8. For subdivision projects,generally use drawing scales of 1"=20;however, 1"=50'is optional for development of lots one acre or larger.For commercial,multi-family, or other projects,generally use scales of 1"=20;however, 1"=10', 1"=30', 1"=40'and 1"=50'are acceptable. Show details for clarification,including those for intersections and existing driveways, on a larger scale. Plan View: Drainage Conveyance 1. Sequentially number all catch basins and curb inlets starting with the structure farthest downstream. 2. Represent existing storm drainage facilities and BMPs in dashed lines and label with`Existing." 3. Clearly label existing storm drainage facilities to be removed with`Existing to be removed." 4. Show the length,diameter, and material for all pipes, culverts, and stub-outs. Include the slope if not provided on the profile view. Material may be noted in the plan notes. Where open conveyance is provided, and a low-permeability liner or treatment liner is required per Section 6.2.4,indicate the limits of such liner(s). 5. Clearly label catch basins as to size and type (or indicate in the plan notes). 6. Clearly label stub-out locations for footing drains and other lot-specific connections to the storm drainage system. Locate all stub-outs to allow gravity flow from the lowest comer of the lot to the connecting catch basin. 7. Show datum,benchmark locations, and elevations on each plan sheet. 8. Clearly label all stub-out locations for any future pipe connections. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-24 2.3.1 ENGINEERING PLAN SPECIFICATIONS 9. Clearly show on the plans all drainage easements,tracts,access easements,Native Growth Retention Areas, Critical Area Tracts, Critical Area Setback Areas,and building setback lines. Show dimensions,type of restriction, and use. 10. Using arrows,indicate the drainage direction of hydraulic conveyance systems. Plan View: Other 1. Show the location,identification, and dimensions of all buildings,property lines, streets, alleys, and easements. 2. Show the locations of structures on abutting properties within 50 feet of the proposed project site. 3. Show the location of all proposed drainage facility fencing,together with a typical section view of each fencing type. 4. Provide section details of all retaining walls and rockeries,including sections through critical portions of the rockeries or retaining walls. 5. Show all existing and proposed buildings with projections and overhangs. 6. Show the location of all wells on site and within 100 feet of the site.Note wells to be abandoned. 7. Show the location and dimensions of proposed on-site BMPs, features,pathways,limits, and set- asides. 8. Show the location and dimensions of structural source control BMPs required by the SWPPS Standards in Appendix D and the King County Stormwater Pollution Prevention Manual. Profiles: Roadway and Drainage 1. Provide existing centerline ground profile at minimum of 50-foot stations and at significant ground breaks and topographic features,with average accuracy to within 0.1 feet on unpaved surface and 0.02 feet on paved surface. 2. For publicly maintained roadways,provide final road and storm drain profile with the same stationing as the horizontal plan,to show stationing of points of curve,tangent, and intersection of vertical curves,with elevation of 0.01 feet. Include tie-in with intersecting pipe runs. 3. On a grid of numbered lines,provide a continuous plot of vertical positioning against horizontal. 4. Show finished road grade and vertical curve data (road data measured at centerline or edge of pavement). Include stopping sight distance. 5. Show all roadway drainage,including drainage facilities and BMPs that are within the right-of-way or easement. 6. On the profile, show slope,length, size, and type (in plan notes or on a detail sheet) for all pipes and detention tanks in public right-of-way. 7. Indicate the inverts of all pipes and culverts and the elevations of catch basin grates or lids. It is also desirable,but not required,to show invert elevations and grate elevations on plan sheets. 8. For pipes that are proposed to be within 2.0 feet of finished grade,indicate the minimum cover dimensions. 9. Indicate roadway stationing and offset for all catch basins. 10. Indicate vertical and horizontal scale. 11. Clearly label all profiles with respective street names and plan sheet reference numbers, and indicate all profile sheet reference numbers on plan sheets,if drawn on separate sheets. 12. Locate match points with existing pavements, and show elevations. 13. Show all property boundaries. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-25 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 14. Label all match line locations. 15. Provide profiles for all 12-inch and larger pipes and for channels(that are not roadside ditches). 16. Show the location of all existing and proposed(if available or critical for clearance) gas,water, and sanitary sewer crossings. 17. Show energy dissipater locations. 18. Identify datum used and all benchmarks (may be shown on plan view instead). Datum and benchmarks must refer to established control when available. 19. Use a vertical scale of 1"=5'.As an exception,vertical scale shall be 1"=10'if the optional 1"=50' horizontal scale is used on projects with lots one acre or larger. Clarifying details,including those for intersections and existing driveways, should use a larger scale. 20. Split sheets,with the profile aligned underneath the plan view, are preferred but not required. ❑ DETAILS The design engineer shall provide details for all construction,including but not limited to the following. Flow Control, Water Quality, and Infiltration Facility and BMP Details 1. Provide a scaled drawing and supporting details of each detention pond or vault, on-site BMP, and water quality facility,including the tract boundaries. 2. Show predeveloped and finished grade contours at 2-foot intervals. Show and label maximum design water elevation. 3. Dimension all berm widths. 4. Show and label at least two cross sections through a pond or water quality facility, or any BMP large enough to require design elements of ponds and/or water quality facilities. One cross section must include the restrictor when included in the design. 5. Specify soils and compaction requirements for pond construction and on-site BMP construction. Specify low-permeability liners or treatment liners as required for ponds and ditches per Section 6.2.4. 6. Show the location and detail of emergency overflows, spillways, and bypasses. 7. Specify rock protection/energy dissipation requirements and details. 8. Provide inverts of all pipes,grates,inlets,tanks, and vaults, and spot elevations of the pond bottom. 9. Show the location of access roads to control manholes and pond/forebay bottoms. 10. Provide plan and section views of all energy dissipaters,including rock splash pads. Specify the size of rock and thickness. 11. Show bollard locations on plans. Typically,bollards are located at the entrance to drainage facility access roads. 12. On the pond or water quality facility detail, show the size,type(or in plan notes), slope, and length of all pipes. 13. Show to scale the section and plan view of restrictor and control structures. The plan view must show the location and orientation of all inlet pipes, outlet pipes, and flow restrictors. 14. Draw details at one of the following scales: 1"=1', 1"=2', 1"=4', 1"=5', 1"=10', or 1"=20'. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-26 2.3.1 ENGINEERING PLAN SPECIFICATIONS Structural Plan Details Any submittal that proposes a structure (e.g.,bridge crossing,reinforced concrete footings,walls, or vaults) shall include plan sheets that include complete working drawings showing dimensions, steel placement, and specifications for construction. Structures may require a design prepared and stamped by a professional structural engineer licensed in the State of Washington, and an application for a separate commercial building permit. 2.3.1.3 EROSION AND SEDIMENT CONTROL (ESC) PLAN This section details the specifications and contents for ESC plans.Note that an ESC plan includes the plan's drawings plus an ESC report,which provides all supporting information and any additional direction necessary for implementing ESC measures and meeting ESC implementation requirements. The ESC plan's drawings may be simplified by the use of the symbols and codes provided for each ESC measure in the Erosion and Sediment Control Standards in Appendix D. In general,the ESC plan's drawings shall be submitted as a separate plan sheet(s). However,there may be some relatively simple projects where providing separate grading and ESC plan drawings is unnecessary. ❑ GENERAL SPECIFICATIONS The site improvement plan shall be used as the base of the ESC plan. Certain detailed information that is not relevant(e.g.,pipe/catch basin size, stub-out locations, etc.)may be omitted to make the ESC plan easier to read.At a minimum,the ESC plan shall include all of the information required for the base map (see Table 2.3.1.A), as well as existing and proposed roads, driveways,parking areas,buildings, drainage facilities and BMPs,utility corridors not associated with roadways,relevant critical areas3 and critical area buffers, and proposed final topography.A smaller scale may be used to provide better comprehension and understanding. The ESC plan shall generally be designed for proposed topography,not existing topography, since rough grading is usually the first step in site disturbance. The ESC plan shall address all phases of construction (e.g.,clearing,grading,installation of utilities, surfacing, and final stabilization). If construction is being phased, separate ESC plans may need to be prepared to address the specific needs for each phase of construction. The ESC plan outlines the minimum requirements for anticipated site conditions. During construction, ESC plans shall be revised as necessary by the CSWPP supervisor or as directed by CED to address changing site conditions,unexpected storm events, or non-compliance with the ESC performance criteria in Core Requirement 45. The ESC plan shall be consistent with the information provided in Section 8 of the TIR and shall address the following: 1. Identify areas with a high susceptibility to erosion. 2. Provide all details necessary to clearly illustrate the intent of the ESC design. 3. Include ESC measures for all on-and offsite utility construction included in the project. 4. Specify the construction sequence. The construction sequence shall be specifically written for the proposed project.An example construction sequence is provided in Appendix D. 5. Include ESC standard plan notes(see Reference Section 7-13). 6. Include an inspection and maintenance program for ESC measures,including designation of a CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24-hour contact. 7. Include the basis and calculations for selection and sizing of ESC measures. 3 Relevant critical areas,for the purposes of drainage review, include aquatic areas,wetlands, flood hazard areas, erosion hazard areas,landslide hazards,steep slope hazard areas,and critical aquifer recharge areas. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-27 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS ❑ MEASURE-SPECIFIC INFORMATION ESC plan drawings must include the following information specific to applicable ESC measures and implementation requirements.As noted above,this information may need to be updated or revised during the life of the project by the CSWPP supervisor or as directed by CED. Clearing Limits 1. Delineate clearing limits. 2. Provide details sufficient to install and maintain the clearing limits. Cover Measures 1. Specify the type and location of temporary cover measures to be used onsite. 2. If more than one type of cover measure is to be used onsite,indicate the areas where the different measures will be used,including steep cut and fill slopes. 3. If the type of cover measures to be used will vary depending on the time of year, soil type,gradient, or some other factor, specify the conditions that control the use of the different measures. 4. Specify the nature and location of permanent cover measures. If a landscaping plan is prepared,this may not be necessary. 5. Specify the approximate amount of cover measures necessary to cover all disturbed areas. 6. If netting,blankets,or plastic sheeting are specified,provide typical detail sufficient for installation and maintenance. 7. Specify the mulch types, seed mixes,fertilizers, and soil amendments to be used, as well as the application rate for each item. 8. For surface roughening, describe methods, equipment and areas where surface roughening will be use. 9. If PAM is used, show location(s)and describe application method. 10. When compost blankets are used, show location, application rates, and the name of the supplier to document that compost meets quality specifications per Reference Section 11-C. Perimeter Protection 1. Specify the location and type of perimeter protection to be used. 2. Provide typical details sufficient to install and maintain the perimeter protection. 3. If silt fence is to be used, specify the type of fabric to be used. 4. If compost berms or socks are used, documentation must be provided to ensure the supplier meets the criteria and compost meets quality standards per Reference Section 11-C. Traffic Area Stabilization 1. Locate the construction entrance(s). 2. Provide typical details sufficient to install and maintain the construction entrance. 3. Locate the construction roads and parking areas. 4. Specify the measure(s)that will be used to create stabilized construction roads and parking areas. Provide sufficient detail to install and maintain. 5. If a wheel wash or tire bath system will be installed,provide location,typical details for installation and maintenance. 6. Provide a list of dust control products that will be used onsite and the location of potential application areas. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-28 2.3.1 ENGINEERING PLAN SPECIFICATIONS Sediment Retention 1. Show the locations of all sediment ponds and traps. 2. Dimension pond berm widths and all inside and outside pond slopes. 3. Indicate the trap/pond storage required and the depth,length, and width dimensions. 4. Provide typical section views through pond and outlet structures. 5. If chemical or electrocoagulation treatment of sediment-laden waters will be used, approval documentation from Ecology must be included(see SWPPS plan requirements for chemical storage). 6. Provide details for disposal of contaminated or chemically treated waters(e.g.,where Chitosan or CO2 have been used) (see SWPPS plan requirements for chemical storage). 7. Include appropriate approval documentation from local sewer districts if contaminated or chemically treated water will be discharged to the sanitary sewer. 8. Provide typical details of the control structure and dewatering mechanism. 9. Detail stabilization techniques for outlet/inlet protection. 10. Provide details sufficient to install cell dividers. 11. Specify mulch or recommended cover of berms and slopes. 12. Indicate the required depth gage with a prominent mark at 1-foot depth for sediment removal. 13. Indicate catch basins that are to be protected. 14. Indicate existing and proposed on-site BMP areas that are to be protected. 15. Provide details of the catch basin and on-site BMP protection sufficient to install and maintain. 16. Provide sediment retention prior to any discharge to the City sewer or local sewer district system. Surface Water Control 1. Locate all pipes, ditches,interceptor ditches, dikes,and swales that will be used to convey stormwater. 2. Provide details sufficient to install and maintain all conveyances. 3. Indicate locations of outlet protection and provide detail of protections. 4. Indicate locations and outlets of any possible dewatering systems. Provide details of alternative discharge methods from dewatering systems if adequate infiltration rates cannot be achieved. Do not route dewatering water,clean or untreated,through stormwater sediment ponds. 5. Indicate the location of any level spreaders and provide details sufficient to install and maintain. 6. Show all temporary pipe inverts. 7. Provide location and specifications for the interception of runoff from disturbed areas and the conveyance of the runoff to a non-erosive discharge point. 8. Provide locations of rock check dams. 9. Provide details,including front and side sections, of typical rock check dams. Protection of Existing and Proposed On-site BMP Areas 1. Provide perimeter protection at existing and proposed on-site BMP locations 2. Provide cautionary plan notes emphasizing avoidance of negative impacts to receptor soils and existing vegetation to remain. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-29 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS BMP Maintenance 1. Provide adequate plan notes for guidance of BMP maintenance methods and schedules. 2. Include an inspection and maintenance program for ESC measures. Management of the Project 1. Provide plan notes to clarify and emphasize the management responsibilities for the project. 2. Include an inspection and maintenance program for ESC measures,including designation of a CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24-hour contact. Wet Season Requirements 1. Provide a list of all applicable wet season requirements. 2. Clearly identify that from October Ist through April 30th,no soils shall be exposed for more than two consecutive working days.Also note that this two-day requirement may be applied at other times of the year if storm events warrant more conservative measures. 3. Clearly identify that exposed soils shall be stabilized at the end of the workday prior to a weekend, holiday, or predicted rain event. 4. Weekly maintenance report is required to be submitted to CED. Critical Areas Restrictions 1. Delineate and label the following critical areas, and any applicable buffers,that are on or adjacent to the project site: aquatic areas,wetlands,flood hazard areas,erosion hazard areas,landslide hazards, steep slope hazard areas, and aquifer protection areas per RMC 4-3-050. 2. If construction creates disturbed areas within any of the above listed critical areas or associated buffers, specify the type,locations, and details of any measures or other provisions necessary to comply with the critical area restrictions in Appendix D and protect surface waters and steep slopes. 2.3.1.4 STORMWATER POLLUTION PREVENTION AND SPILL (SWPPS) PLAN This section details the specifications and contents for SWPPS plans,which together with ESC plans, comprise the construction stormwater pollution prevention(CSWPP)plan that must be submitted as part of the engineering plans required for drainage review. Additional guidance for developing the SWPPS plan can be found in the SWPPS Standards in Appendix D, Construction Stormwater Pollution Prevention Standards,in the King County Stormwater Pollution Prevention Manual and in the Stormwater Management Manual for Western Washington (SWMMWW)published by the Washington State Department of Ecology(Ecology). The SWPPS plan must be kept on site during all phases of construction and shall address the construction-related pollution-generating activities outlined in Subsection A below. The plan must include a description of the methods the general contractor will use to ensure sub-contractors are aware of the SWPPS plan.A form or record must be provided that states all sub-contractors have read and agree to the SWPPS plan. A SWPPS plan consists of the following three elements,which are further described in Subsections B,C, and D below: 1. A site plan showing the location and description of BMPs required to prevent pollution and control spills from construction activities and from chemicals and other materials used and stored on the construction site. See Subsection B below for more specifics on the SWPPS site plan. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-30 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2. A pollution prevention report listing the potential sources of pollution and identifying the operational, source control, and treatment BMPs necessary to prevent/mitigate pollution from these sources. See Subsection C below for more specifics on the SWPPS pollution prevention report. 3. A spill prevention and cleanup report describing the procedures and BMPs for spill prevention and including provisions for cleanup of spills should they occur. See Subsection D below for more specifics on the SWPPS spill prevention and cleanup report. A. ACTIVITY-SPECIFIC INFORMATION REQUIRED At a minimum,the SWPPS plan shall address,if applicable,the following pollution-generating activities typically associated with construction and include the information specified below for each activity. If other pollution-generating activities associated with construction of the proposed project are identified,the SWPPS plan must address those activities in a similar manner. Storage and Handling of Liquids 1. Identify liquids that will be handled or stored onsite,including but not limited to petroleum products, fuel, solvents, detergents,paint,pesticides,concrete admixtures, and form oils. 2. Specify types and sizes of containers of liquids that will be stored/handled onsite. Show locations on the SWPPS site plan. 3. Describe secondary containment methods adequately sized to provide containment for all liquids stored onsite. Show the locations of containment areas on the SWPPS site plan. Storage and Stockpiling of Construction Materials and Wastes 1. Identify construction materials and wastes that may be generated or stockpiled onsite. Show the locations where these materials and wastes will be generated and stockpiled on the SWPPS site plan. 2. Specify type of cover measures to be used to keep rainwater from contacting construction materials and wastes that can contribute pollutants to storm, surface, and ground water. 3. If wastes are kept in containers, describe how rainwater will be kept out of the containers. Fueling 1. Specify method of onsite fueling for construction equipment(i.e., stationary tanks,truck mounted tanks,wet hosing, etc.). If stationary tanks will be used, show their location on the SWPPS site plan. 2. Describe type and size of tanks. 3. Describe containment methods for fuel spills and make reference to the SWPPS site plan for location information. 4. If fueling occurs during evening hours, describe lighting and signage plan. Make reference to the SWPPS site plan for location information. Maintenance, Repairs, and Storage of Vehicles and Equipment 1. Identify maintenance and repair areas and show their locations on the SWPPS site plan.Use of drip pans or plastic beneath vehicles is required.A note to this effect must be shown on the SWPPS site plan. 2. Describe method for collection, storage, and disposal of vehicle fluids. 3. If an area is designated for vehicle maintenance,signs must be posted that state no vehicle washing may occur in the area.A note to this effect must be shown on the SWPPS site plan. Concrete Saw Cutting, Slurry, and Washwater Disposal 1. Ensure that washout of concrete trucks is performed off-site or in designated concrete washout areas only. Identify truck washout areas to ensure such areas are not within a critical aquifer recharge area. If they are,the washout area must be lined with an impervious membrane. Show location information on the SWPPS site plan. Locate washout area at least 50 feet from sensitive areas such as storm drains, open ditches, or water bodies,including wetlands. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-31 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 2. Specify size of sumps needed to collect and contain slurry and washwater. Show location information on the SWPPS site plan. 3. Identify areas for rinsing hand tools including but not limited to screeds, shovels,rakes, floats and trowels. Show the locations of these areas on the SWPPS site plan. 4. Describe methods for collecting,treating,and disposal of waste water from exposed aggregate processes,concrete grinding and saw cutting, and new concrete washing and curing water. Handling of pH Elevated Water New concrete vaults/structures may cause collected water to have an elevated pH. This water cannot be discharged to storm or surface water until neutralized. 1. Provide details on treating/neutralizing water when pH is not within neutral parameters.Written approval from Ecology is required before using chemical treatment other than CO2 or dry ice to adjust pH. 2. Provide details on disposal of water with elevated pH or of the treated water. Application of Chemicals including Pesticides and Fertilizers 1. Provide a list of chemicals that may be used on the project site and the application rates. 2. Describe where and how chemicals will be applied. Show location information on the SWPPS site plan. 3. Describe where and how chemicals will be stored. Show location information on the SWPPS site plan. B. SWPPS SITE PLAN The site plan element of the SWPPS plan shall include all of the information required for the base map (see Table 2.3.1.A), as well as existing and proposed roads,driveways,parking areas,buildings, drainage facilities,utility corridors not associated with roadways,relevant critical areas4 and associated buffers, and proposed final topography.A smaller scale may be used to provide more comprehensive details on specific locations of each activity and specific prevention measure. In addition to this information,the following items, at a minimum, shall be provided as applicable: 1. Identify locations where liquids will be stored and delineate secondary containment areas that will be provided. 2. Identify locations where construction materials and wastes will be generated and stockpiled. 3. Identify location of fueling for vehicles and equipment if stationary tanks will be used. 4. Delineate containment areas for fuel spills. 5. Show location of lighting and signage for fueling during evening hours. 6. Delineate maintenance and repair areas and clearly note that drip pans or plastic shall be used beneath vehicles.Also,clearly note that signs must be posted that state no vehicle washing may occur in the area. 7. Delineate truck washout areas and identify the location of slurry/washwater sumps and rinsing areas for tools. 8. Delineate where chemicals will be applied and identify where they will be stored. 9. Identify where spill response materials will be stored. 4 Relevant critical areas,for the purposes of drainage review, include aquatic areas,wetlands, flood hazard areas,erosion hazard areas,landslide hazards,steep slope hazard areas,and aquifer protection areas as described in RMC 4-3-050. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-32 2.3.1 ENGINEERING PLAN SPECIFICATIONS C. POLLUTION PREVENTION REPORT This report provides the specifics on pollution prevention and must include the following information in addition to the activity-specific information specified in Subsection A above: 1. List the possible sources of pollution per Subsection A above and identify the BMPs to be used for each source to prevent pollution. Include any supporting information (site conditions,calculations, etc.)for the selection and sizing of pollution prevention BMPs. 2. Identify the personnel responsible for pollution prevention and clearly list the responsibilities of each person identified. Contact information for these personnel must be clearly identified in the report and on the SWPPS site plan. 3. Describe the procedures to be used for monitoring pollution prevention BMPs and for responding to a BMP that needs attention,including keeping records/reports of all inspections of pollution prevention BMPs(see Reference Section 8-E for examples of worksheets that may be used). D. SPILL PREVENTION AND CLEANUP REPORT This report provides the specifics on spill prevention and cleanup and must include the following information in addition to any activity-specific information in Subsection A above related to spill prevention: 1. List the possible sources of a spill and identify the BMPs to be used for each source to prevent a spill. 2. Identify personnel responsible for spill prevention and cleanup and clearly list the responsibilities of each person identified. Contact information for these personnel must be clearly identified in the report and on the SWPPS site plan. (On typical projects,the primary contact for SWPPS issues will be the CSWPP supervisor,who may be managing other spill responders to ensure compliance;complex projects may warrant specialist personnel for specific site applications.) 3. Describe the procedures to be used for monitoring spill prevention BMPs and for responding to a spill incident,including keeping records/reports of all inspections and spills(see Reference Section 8-E for examples of worksheets that may be used). 4. Identify where spill response materials will be stored. Make reference to the SWPPS site plan for location information. 5. Identify disposal methods for contaminated water and soil after a spill. 2.3.1.5 LANDSCAPE MANAGEMENT PLANS (IF APPLICABLE) The City of Renton does not allow landscape management plans to be used as an alternative to the requirement to formally treat(with a facility)the runoff from pollution generating pervious surfaces subject to Core Requirement 48 (see Section 1.2.8).A landscape management plan is an approved plan for defining the layout and long-term maintenance of landscaping features to minimize the use of pesticides and fertilizers, and reduce the discharge of suspended solids and other pollutants. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-33 SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 2.3.2 PROJECTS IN TARGETED DRAINAGE REVIEW (TDR) This section outlines the specifications and contents of limited scope engineering plans allowed for projects in Targeted Drainage Review. Table 2.3.2.A specifies the minimum required elements of the targeted technical information report based on the type of permit or project, and on the three categories of project characteristics subject to Targeted Drainage Review per Section 1.1.2.2. TABLE 2.3.2.A MINIMUM ENGINEERING PLAN ELEMENTSM FOR PROJECTS IN TARGETED DRAINAGE REVIEW Project Category 1(2) Projects in TDR that Project contain or are adjacent Category 3(2) to a flood, erosion, or Project Category 2(2) Redevelopment steep slope hazard Projects in TDR that projects in TDR area; or are within a propose to construct or that propose Landslide Hazard modify a 12"or larger $100,000 or more of Type of Drainage Drainage Area or pipe/ditch, or receive improvements to an Permit or Review Aquifer Protection runoff from a 12"or larger existing high-use Project Type Area pipe/ditch site SINGLE- Targeted . TIR Sections 1,2, . TIR Sections 1,2, 3, 5, N/A FAMILY Drainage and 6 (minimum) 6, 7, and 8(minimum) RESIDENTIAL Review . Simplified ESC . Simplified ESC Plan(3) (SFR) ONLY Plan(3)and SWPPS and SWPPS Plan BUILDING PERMITS Plan ESC Plan(4)for SHORT • Site Improvement conveyance work PLATS Plan(5) . Site Improvement Plan(5) Targeted . TIR Sections 1, 2, • TIR Sections 1,2, 3, 5, N/A Drainage and 6 (minimum) 6, 7, and 8(minimum) Review . Simplified ESC . Simplified ESC Plan(3) COMBINE Plan(3)and SWPPS and SWPPS Plan D WITH Simplified Plan ESC Plan(4)for Drainage • Site Improvement conveyance work Review Plan(5) . Site Improvement Plan(5) OTHER Targeted . TIR Sections 1,2, 6, • TIR Sections 1,2, 3, 5, • TIR Sections 1, 2, PROJECTS Drainage and 8(minimum) 6, 7, and 8(minimum) 4, 8, and 10 OR PERMITS Review . ESC Plan(4)and • ESC Plan(4)and (minimum) ONLY SWPPS Plan for any SWPPS Plan for any • ESC Plan(4)and site disturbance work site disturbance work SWPPS Plan for • Site Improvement • Site Improvement any site Plan(5) Plan(5) disturbance work • Site Improvement Plan(5) Notes: (1)The above plan elements are considered the recommended minimum for most development cases in Targeted Drainage Review.CED may add to these elements if deemed necessary for proper drainage review. Predesign meetings with CED are recommended to identify all required elements. (2) For more detailed descriptions of project categories,see Section 1.1.2.2. If the proposed project has the characteristics of more than one category, the plan elements under each applicable category shall apply. (3) Simplified ESC plans are an element of the Simplified drainage plan as explained in the Simplified Drainage Requirements booklet(Appendix C). (4) ESC plans shall meet the applicable specifications detailed in Section 2.3.1.3 (5) Site improvement plans shall meet the applicable specifications detailed in Section 2.3.1.2. CED may allow modified site improvement plans as described in Section 2.3.1.2. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-34 CITY OF RENTON SURFACE WATER DESIGN MANUAL 2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW This section includes the specifications and contents required of those plans submitted at the end of the permit review process or after a permit has been issued. 2.4.1 PLAN CHANGES AFTER PERMIT ISSUANCE If changes or revisions to the originally approved engineering plans require additional review,the revised plans shall be submitted to CED for approval prior to construction. The plan change submittals shall include all of the following: 1. The appropriate Plan Change form(s) 2. One (1)revised TIR or addendum 3. One (1) set of the engineering plans 4. One (1)Complete Electronic copy of all submittal items 5. Other information needed for review. 2.4.2 FINAL CORRECTED PLAN SUBMITTAL During the course of construction,changes to the approved engineering plans are often required to address unforeseen field conditions or design improvements. Once construction is completed,it is the applicant's responsibility to submit to CED a final corrected plan (or record drawings). These corrected drawings must be professionally drafted revisions applied to the original approved plan, excluding the CSWPP plan, and must include all changes made during the course of construction. The final as-built plans must be stamped, signed, and dated by a civil engineer or land surveyor.A CAD drawing file (.dwg) of the final as-built plan set must be submitted along with Mylars. The CAD file must contain all the pages of the plan set for road and drainage infrastructure,but need not contain other sheets.A final corrected TIR,updated to include all changes made to the originally approved TIR during the course of construction,must be submitted with the final corrected plan. In addition to any design changes and supporting calculations and documentation,the final corrected TIR shall include a final updated Stormwater Facility Summary Sheet (see Reference Section 8-D)and signed/recorded copies of all required easements and declarations of covenant. The electronic copy of the final corrected TIR may be in .pdf or other approved format. Additional information regarding the final corrected plan submittal can be found on the City's website. Disposition of Approved Engineering Plans for Subdivisions CED will retain the Mylar set,utilizing it to make copies for public inspection, distribution,base reference, and permanent public record as required. 2017 City of Renton Surface Water Design Manual 12/12/2016 2-35 SECTION 2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW 2.4.3 FINAL PLAT, SHORT PLAT, AND BINDING SITE PLAN SUBMITTALS Any subdivision to be finalized,thereby completing the subdivision process and legally forming new lots, requires a final submittal for approval and recording. Binding site plans and short plats also require a final submittal for approval and recording.The final plat or map page shall contain the elements summarized and specified in detail on the City's website. Submittals shall be accompanied by appropriate fees as prescribed by Renton Municipal Code. Final submittals will be allowed only after the approval of preliminary plans(for subdivisions only)and any required engineering plans. All final map sheets and pages shall be prepared by a land surveyor licensed in the State of Washington and shall conform to all state and local statutes. The final submittal for recording only applies to subdivisions(plats),binding site plans, and short plats. This plan is required by state and local statutes. In addition to the requirements described on the City's website and in the City of Renton Municipal Code, submittals for final recording of subdivisions, short plats, and binding site plans must include the following information: 1. Indicate dimensions of all easements,tracts,building setbacks,tops of slopes,wetland boundaries, and floodplains. 2. Include pertinent restrictions as they apply to easements,tracts, and building setback lines. 3. State the maximum amount of added impervious surface and proposed clearing per lot as determined through engineering review. The maximum amount of impervious surface may be expressed in terms of percentage of lot coverage or square feet. 4. Include a recorded declaration of covenant and grant of casement for each lot on which on-site BMPs are installed or stipulated per Core Requirement 49, Section 1.2.9.4.1, and each lot for which on-site BMPs are installed in a separate dedicated tract per Section 1.2.9.4.1. 12/12/2016 2017 City of Renton Surface Water Design Manual 2-36 Section Page CHAPTER 3 3.1 Hydrologic Design Standards and Principles 3-3 HYDROLOGIC 3.1.1 Hydrologic Impacts and Mitigation 3-3 ANALYSIS & DESIGN 3.1.2 Flow Control Standards 3-5 3.1.3 Hydrologic Analysis Using Continuous Models 3-5 pew 3.2 Runoff Computation and Analysis rr, Methods 3-9 3.2.1 Rational Method 3-11 ✓ , n/ 3.2.2 Continuous Models and the Runoff Files Method 3-19 3.2.3 The Approved Model 3-30 3.2.4 The HSPF Model 3-30 3.3 Hydrologic Design Procedures and Considerations 3-33 3.3.1 General Hydrologic Design Process 3-33 3.3.2 Flow Control Design Using the Runoff CITY OF RENTON Files Method 3-34 3.3.3 Conveyance System Design with the SURFACE WATER Runoff Files Method 3-37 3.3.4 Safety Factors in Hydrologic Design 3-38 DESIGN MANUAL 3.3.5 Design Options for Addressing Downstream Drainage Problems 3-38 3.3.6 Point of Compliance Analysis 3-38 3.3.7 Onsite Closed Depressions and Ponding Areas 3-41 2017 City of Renton Surface Water Design Manual 12/12/2016 CITY OF RENTON SURFACE WATER DESIGN MANUAL (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL CHAPTER 3 HYDROLOGIC ANALYSIS & DESIGN This chapter presents the concepts and rationale for the surface water controls and designs required by this manual and the acceptable methods for estimating the quantity and characteristics of surface water runoff. These methods are used to analyze existing and to design proposed drainage systems and facilities. Hydrologic concepts,tools and methodologies, and an overview of the assumptions and data requirements of the methods, are described for the following tasks: • Calculating runoff time series and flow statistics • Designing detention and infiltration facilities Approved hydrologic modeling software are listed in Reference Section 6-D. Tools and methodologies specific to the software can be obtained from the software documentation and trainings provided by the software providers.At this writing,the approved models for stormwater runoff and water quality design include WWHM2012 and WWHM4, available from the Washington State Department of Ecology (Ecology),MGS Flood, available from MGS Engineering Consultants,Inc., and the Hydrologic Simulation Program(Fortran)(HSPF). The King County Reduced Time Series(KCRTS)software is no longer maintained by King County and is not an approved model for use with the Surface Water Design Manual(SWDM). Hydrologic tools and methodologies, and the assumptions and data requirements of the methods, are presented for the following tasks: • Sizing conveyance facilities • Analyzing conveyance capacities. Chapter Organization The information presented in this chapter is organized into three main sections: • Section 3.1, "Hydrologic Design Standards and Principles" • Section 3.2, "Runoff Computation and Analysis Methods" • Section 3.3, "Hydrologic Design Procedures and Considerations" These sections begin on odd pages so the user can insert tabs if desired for quicker reference. Other Supporting Information For specific guidance on the mechanics of using the approved modeling software for hydrologic analysis and design,refer to the associated approved model website and program documentation. See Reference Section 6-D for limited modeling guidance and requirements as applicable for specific tasks in this manual. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-1 CHAPTER 3 HYDROLOGIC ANALYSIS&DESIGN (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 3-2 CITY OF RENTON SURFACE WATER DESIGN MANUAL 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES This section presents the rationale for and approach to hydrologic analysis and design. Topics covered include the following: • "Hydrologic Impacts and Mitigation,"Section 3.1.1 • "Flow Control Standards,"Section 3.1.2 • "Hydrologic Analysis Using Continuous Models," Section 3.1.3 3.1.1 HYDROLOGIC IMPACTS AND MITIGATION Hydrologic Effects of Urbanization The hydrologic effects of development can cause a multitude of problems,including minor nuisance flooding,degradation of public resources,diminished fish production,and significant flooding endangering life and property. Increased stormwater flows expand floodplains,bringing flooding to locations where it did not occur before and worsening flood problems in areas already flood-prone. Increased stormwater flows also hasten channel erosion,alter channel structure, and degrade fish habitat. Human alteration of the landscape,including clearing,grading,paving,building construction, and landscaping,changes the physical and biological features that affect hydrologic processes. Soil compaction and paving reduce the infiltration and storage capacity of soils. This leads to a runoff process called Horton overland flow whereby the rainfall rate exceeds the infiltration rate,and the excess precipitation flows downhill over the soil surface. This type of flow rapidly transmits rainfall to the stream or conveyance system,causing much higher peak flow rates than would occur in the unaltered landscape. Horton overland flow is almost nonexistent in densely vegetated areas, such as forest or shrub land,where the vast majority of rainfall infiltrates into the soil. Some of this infiltrated water is used by plants, and depending on soil conditions, some of it percolates until it reaches the groundwater table. Sometimes the percolating soil water will encounter a low-permeability soil or rock layer. In this case,it flows laterally as interflow over the low-permeability layer until it reaches a stream channel. Generally, forested lands deliver water to streams by subsurface pathways,which are much slower than the runoff pathways from cleared and landscaped lands. Therefore,urbanization of forest and pasture land leads to increased stormwater flow volumes and higher peak flow rates. Land development increases not only peak flow rates but also changes annual and seasonal runoff volumes.In forested basins in King County, about 55%of the rain that falls each year eventually appears as streamflow. This percentage is called the yield of a basin. The remaining 45%of the rain evaporates and returns to the atmosphere.As trees are cleared and the soil is graded to make way for lawns and pastures, and as part of the land is covered with asphalt or concrete,the basin yield increases. More of the rain becomes streamflow, and less evaporates. In lowland King County,the yield of a basin covered with landscaped lawns would be about 65%,while the yield of an impervious basin would be about 85 to 90%. For these reasons, development without mitigation increases peak stormwater rates,stormwater volumes, and annual basin yields.Furthermore,the reduction of groundwater recharge decreases summer base flows. In summary,the following are the hydrologic impacts of unmitigated development: • Increased peak flows • Increased durations of high flows • Increased stormwater runoff volumes • Decreased groundwater recharge and base flows • Seasonal flow volume shifts • Altered wetland hydroperiods. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-3 SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES The resulting economic and ecological consequences of these hydrologic changes include the following: • Increased flooding • Increased stream erosion • Degraded aquatic habitat • Changes to wetland species composition. Mitigation of Hydrologic Effects of Urbanization Engineered facilities can mitigate many of the hydrologic changes associated with development. Detention facilities can maintain the rates and/or durations of high flows at predevelopment levels. Infiltration facilities can control flow volumes and increase groundwater recharge as well as control flow rates and durations. Conveyance problems can be avoided through analysis and appropriate sizing and design of conveyance facilities.Engineered mitigation of the hydrologic impacts of development include the following: • Managing peak flow rates with detention facilities • Managing high flow durations with detention facilities • Reducing flow volumes and maintaining or enhancing groundwater recharge with infiltration facilities • Avoiding flooding problems with appropriately sized and designed conveyance systems • Bypassing erosion problems with tightlines. Engineered facilities cannot mitigate all of the hydrologic impacts of development. Detention facilities do not mitigate seasonal volume shifts,wetland water level fluctuations,groundwater recharge reductions, or base flow changes. Such impacts can be further reduced through the use of low impact development (LID)techniques,beginning with careful site planning.For instance,clustering of units to reduce impervious cover while maintaining site density is an effective way to limit hydrologic change. Preserving native vegetation and minimizing soil disturbance or compaction in pervious areas also reduces hydrologic change. Such non-engineered mitigation measures are encouraged by the City and are discussed in Core Requirement 49 and Appendix C of this manual and are referred to as on-site BMPs. Other on-site BMPs, such as permeable pavements,bioretention,vegetated roofs, and rainwater harvesting can be effective in reducing increases in surface water volumes. The incorporation of these concepts in the design of the project is required,as detailed in Core Requirement 49 and Appendix C. Many of these approaches will result in a reduction in flow control facility size, so the on-site BMP requirements in Core Requirement 49 and Appendix C should be carefully considered and applied to maximize the benefits of this approach. Detention Facility Concepts The basic concept of a detention facility is simple:water is collected from developed areas and released at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days. The volume of storage needed is determined by(1)how much stormwater enters the facility(determined by the size and density of the contributing area), (2)how rapidly water is allowed to leave the facility, and(3)the level of hydrologic control the facility is designed to achieve. To prevent increases in the frequency of flooding due to new development, detention facilities are often designed to maintain peak flow rates at their predevelopment levels for recurrence intervals of concern (e.g.,2-and 10-year). Such mitigation can prevent increases in the frequency of downstream flooding. Facilities that control only peak flow rates,however,usually allow the duration of high flows to increase, which may cause increased erosion of the downstream system. For example,the magnitude of a 2-year flow may not increase,but the amount of time that flow rate occurs may double. Therefore, stream systems,including those with salmonid habitat,which require protection from erosion warrant detention systems that control the durations of geomorphically significant flows (flows capable of moving sediment). Such detention systems employ lower release rates and are therefore larger in volume. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-4 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS 3.1.2 FLOW CONTROL STANDARDS Refer to Chapter 1, Section 1.2.3, for flow control standards.1,2 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS The Need for Continuous Hydrologic Modeling This manual prescribes the use of a continuous hydrologic model for most hydrologic analyses rather than an event model. Event models such as the Santa Barbara Urban Hydrograph(SBUH),King County Runoff Series(KCRTS) and the Soil Conservation Service (SCS)3 method were used in previous versions of the King County Surface Water Design Manual.A continuous model was selected for the current version of the City of Renton SWDM because hydrologic problems in western Washington are associated with the high volumes of flow from sequential winter storms rather than high peak flows from short duration,high intensity rainfall events. The continuous hydrologic analysis tools prescribed in this manual are generically described as the "approved model'; a list of the approved models is found in Reference Section 6-D (as updated).At this writing,the approved continuous hydrologic models4 include the Western Washington Hydrologic Model(WWHM) and MGS Flood,both of which are variants of the Hydrologic Simulation Program- FORTRAN(HSPF)model. HSPF is also an approved model,but is more complex than other approved models and is typically used for basin planning and master drainage plan analyses. Continuous models are well suited to accounting for the climatological conditions in the lowland Puget Sound area. Continuous models include algorithms that maintain a continuous water balance for a catchment to account for soil moisture and hydraulic conditions antecedent to each storm event(Linsley, Kohler,Paulhus, 1982),whereas event models assume initial conditions and only address single hypothetical storm events.As a result,continuous hydrologic models are more appropriate for evaluating runoff during the extended wet winters typical of the Puget Sound area. The drawbacks of event models are summarized as follows: • Event methods inherently overestimate peak flows from undeveloped land cover conditions. The overestimation is due,in part,to the assumption that runoff from forest and pasture land covers flows across the ground surface. In actuality,the runoff from forests and pastures, on till soils,is dominated by shallow subsurface flows(interflow)which have hydrologic response times much longer than those used in event methods. This leads to an over estimation of predeveloped peak flows,which results in detention facility release rates being overestimated and storage requirements being underestimated. • A single event cannot represent the sequential storm characteristics of Puget Sound winters. • Event models assume detention facilities are empty at the start of a design event,whereas actual detention facilities may be partially full as a result of preceding storms. • Testing of event-designed detention facilities with calibrated,long-term continuous hydrologic simulations demonstrates that these facilities do not achieve desired performance goals. • Event methods do not allow analysis of flow durations or water level fluctuations. The benefits of continuous hydrologic modeling are summarized as follows: 1 Footnote 1 does not apply. 2 Footnote 2 does not apply. 3 The Soil Conservation Service(SCS)is now known as the National Resources Conservation Service(NRCS).The method described in Urban HydrologZ for Small Watersheds,Technical Release 55(TR-55},June 1986,published by the NRCS, is commonly referred to as the"SCS method." 4 KCRTS is no longer maintained by King County and is not an approved model for use with the SWDM. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-5 SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES • A continuous model accounts for the long duration and high precipitation volume of winter wet periods characterized by sequential,low-intensity rainfall events. Continuous simulation uses continuous long-term records of observed rainfall rather than short periods of data representing hypothetical storm events.As a result,continuous simulation explicitly accounts for the long duration rainfall events typically experienced in the Pacific Northwest as well as the effects of rainfall antecedent to major storm events. • HSPF has been shown to more accurately simulate runoff from basins with a wide range of sizes and land covers using the regional parameters developed by the United States Geologic Survey(USGS). • Continuous simulation allows direct examination of flow duration data for assessing the impacts of development on stream erosion and morphology.An event model,whether using a 1-day or a 7-day storm,cannot provide such information. • A continuous model allows water level analysis for wetlands,lakes, and closed depressions whose water level regime is often dependent on seasonal runoff rather than on 1-day or 7-day event runoff. • Continuous models produce flow control facilities that more accurately and effectively achieve desired performance goals. The importance of continuous modeling in the Puget Sound area is illustrated in Figure 3.1.3.A,which shows a small basin's runoff response to a series of winter storms and the outflow from a detention pond designed to control the peak annual flows from this basin.Note that the largest outflow from the detention pond corresponds not to the peak inflow on 11/6/86,but rather to the high volume of flow from the sequential storms beginning on 11/19/86.This demonstrates a key difference between continuous and event based models. With an event model, designers are accustomed to working with a single design storm event(e.g., 10-year),which by definition has the same return period once routed through a reservoir(10-year inflow will always generate 10-year outflow). With a continuous model, flow recurrence estimates are based on annual peak flow rates,with each time series being analyzed independently. Events that generate annual peak inflows to a reservoir may not generate annual peak discharges from the reservoir. In other words,the runoff event containing the 10-year inflow peak,when routed,may not create the 10-year outflow peak. This is due to natural variability of storm peaks and volumes(e.g.,high intensity/short duration thunderstorms as compared to moderate intensity/long duration winter storms)contained within a continuous record. Requirements of Continuous Hydrologic Modeling For the entire period of simulation, a continuous hydrologic model requires a continuous record of precipitation and evaporation at discrete time steps small enough to capture the temporal variability of hydrologic response, and it provides a continuous record of simulated flows at the same time step. The quicker a basin responds hydrologically(e.g., due to small size,land cover, or lack of detention),the smaller the time step should be. Time steps of 15 minutes are sufficient for most basins in the Puget Sound area. The continuous hydrologic model must include mathematical representations of hydrologic processes to determine the fate and movement of rainfall.For example, a good continuous hydrologic model must include representations of infiltration processes to determine how much water infiltrates the soil and how much runs off the surface. It must represent shallow and deep soil storage as well as the release of subsurface water to streams via interflow and groundwater flow,and it must also account for the loss of soil water to the atmosphere via evapotranspiration between rainfall events. The benefit of all this computation is a complete hydrologic assessment including information on peak flow rates, flow durations, storm volumes, seasonal volumes, annual volumes, and water levels of receiving bodies. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-6 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS FIGURE 3.1.3.A EFFECTS OF SEQUENTIAL STORMS ON DETENTION PERFORMANCE Small Basin Runoff Response: surface and interflows from 10-acre till site 2.5 I I I I Forest Condition Flows 2 I Detention Pond Outflows I - - - -Pond Inflows from Residential Development S S S S S 1.5 ti S S U LL ti U 5 O 5 LL S S 0.5 ti r y � ti r 0 11/2/86 11/9/86 11/16/86 11/23/86 11/30/86 Date 2017 City of Renton Surface Water Design Manual 12/12/2016 3-7 SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES (This page intentionally left blank) 12/12/2016 2017 City of Renton Surface Water Design Manual 3-8 CITY OF RENTON SURFACE WATER DESIGN MANUAL 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS This section presents the following four runoff computation methods accepted for hydrologic analysis and design: • The Rational Method described below and detailed in Section 3.2.1 • The TR-55 or SBUH methods described below. • The Runoff Files Method described below and detailed in Section 3.2.2 • The Hydrologic Simulation Program-FORTRAN(HSPF) model described below and detailed in Section 3.2.4. ❑ ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS Acceptable uses of the four runoff computation methods are summarized below and in Table 3.2: • Rational Method: This method is most appropriate for sizing new conveyance systems that drain smaller, quickly responding tributary areas(i.e.,less than 10 acres)where very short,intense storms tend to generate the highest peak flows. The Rational Method may also be used for conveyance sizing in any size basin if the attenuation effects of existing storage features within the basin are ignored. • TR-55/SBUH Methods: The Natural Resources Conservation Service(MRCS, formerly the Soil Conservation Service (SCS))TR-55 method or the SBUH method of the 1990 King County Surface Water Design Manual may be used for conveyance sizing where tributary areas are greater than or equal to 10 acres and if storage features are ignored.The peak flows from these single-event models are considered conservative for larger tributary areas if the flows are not routed through existing storage features. The TR-55 method is also used for water quality volume calculation in this manual. For more background information,refer to NRCS Publication 210-VI-TR-55, Second Edition(June 1986)or the 1990 SWDM. • The Runoff Files Method: This continuous modeling method using the approved model is the most versatile for quickly performing many of the computations summarized in Table 3.2.For conveyance sizing and analysis,the peak flows from the approved model are most accurate when the shortest possible time step is used.Unlike the Rational Method,the approved model may be used for tributary areas less than 10 acres where there is a significant storage feature(s). In previous editions of this manual, sizing and analysis of storage features and volume-based water quality facilities used hourly time steps for determination of predevelopment discharges and for routing purposes.As of this edition,the City requires 15-minute time steps for sizing of all flow control facilities,water quality facilities and conveyance to provide consistent management of surface water and protect against cumulative increases in peak flows on a basin-wide basis(see Sections 3.3.1 and 3.3.2). Methods for analysis and design of detention storage and water levels6 require the use of the approved model. See the user's documentation for background and guidance. • HSPF Model:For projects in Large Project Drainage Review(see Section 1.1.2.5),the City may require HSPF modeling for formulating a Master Drainage Plan(see Master Drainage Planning for Large Site Developments Process and Requirement Guidelines available from King County). The City also generally encourages use of HSPF for tributary areas larger than 200 acres. The HSPF model can be used wherever the approved model is allowed for sizing and analysis of conveyance systems, flow control facilities, and water quality facilities using a 15-minute time step.For such projects draining to a wetland or potentially impacting groundwater resources or stream base flows,the City 5 Footnote 5 does not apply. 6 One of the simplest and most commonly used level pool routing methods is described in the Handbook of Applied Hydrology (Chow,Ven Te, 1964)and elsewhere,and summarized in Reference Section 6-C, It is based on the continuity equation and can be completed with a spreadsheet.Although not approved for design with this manual, it provides a background for modeled routing techniques. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-9 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS may require the collection of actual rainfall and runoff data to be used in developing and calibrating the HSPF model. TABLE 3.2 ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS TYPE OF APPLIED TO RATIONAL TR 55/SBUH THE APPROVED HSPF COMPUTATION METHOD MODEL PEAK FLOW Tributary REQUIRED for OKAY if majority of OKAY if majority of CONVEYANCE Areas<10 ac undetained tributary area is tributary area is SIZING INC. (measured to areas,(2)and detained(4) detained(4) TESCM individual OKAY for (DESIGN FLOWS) conveyance detained areas if (See Chapter 4 for elements) no storage hydraulic analysis routing(3)is procedures) performed Tributary OKAY if no OKAY if no OKAY (storage OKAY (storage Areas Z 10 ac storage routing(3) storage routing is allowed) routing is allowed) is performed routing(3)is performed LEVEL-POOL Projects in OKAY OKAY ROUTING Full Drainage FLOW CONTROL Review (NEW/EXIST.)& Projects in MAY BE MAY BE WQ FACILITY Large Project ALLOWED(5) REQUIRED(5) SIZING AND Drainage ANALYSIS Review Projects in OKAY if no OKAY for OKAY OKAY Full or storage routing(3) tributary areas Targeted is performed >_ 10 ac. if no Drainage storage Review routing(3)is DOWNSTREAM performed ANALYSIS Projects in MAY BE MAY BE MAY BE Large Project ALLOWED(5) if ALLOWED(5)if ALLOWED(5)if Drainage used as as described in used as described Review described in the the box above in the box above box above PEAK FLOWS All Projects OKAY OKAY FOR APPLYING EXEMPTIONS& THRESHOLDS Notes: (1)Water quality design flow rates are determined as described in Section 6.2.1. (2) Undetained areas are those upstream of detention facilities or other storage features. (3) Storage routing uses the Level Pool Routing technique(described in Reference Section 6-C)or other similar method to account for the attenuation of peak flows passing through a detention facility or other storage feature. (4)The majority of the tributary area is considered detained if the runoff from more than 50%of the tributary area is detained by a detention facility or other storage facility. (5) For projects in Large Project Drainage Review,the selection of methodology for detention sizing and/or downstream analysis becomes a site-specific or basin-specific decision that is usually made by CED during the scoping process for master drainage plans. Guidelines for selecting the approved model, HSPF,or calibrated HSPF are found in the King County publication Master Drainage Planning for Large or Complex Site Developments,available from King County. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-10 3.2.1 RATIONAL METHOD 3.2.1 RATIONAL METHOD The Rational Method is a simple,conservative method for analyzing and sizing conveyance elements serving small drainage subbasins,subject to the following specific limitations: • Only for use in predicting peak flow rates for sizing conveyance elements • Drainage subbasin area A cannot exceed 10 acres for a single peak flow calculation • The time of concentration T,must be computed using the method described below and cannot exceed 100 minutes. It is also set equal to 6.3 minutes when computed to be less than 6.3 minutes. Note: Unlike other methods of computing times of concentration, the 6.3 minutes is not an initial collection time to be added to the total computed time of concentration. ❑ RATIONAL METHOD EQUATION The following is the traditional Rational Method equation: QR= CIRA (3-1) where QR= peak flow(cfs) for a storm of return frequency R C = estimated runoff coefficient(ratio of rainfall that becomes runoff) IR = peak rainfall intensity(inches/hour) for a storm of return frequency R A = drainage subbasin area(acres) "C" Values The allowable runoff coefficients to be used in this method are shown in Table 3.2.1.A by type of land cover. These values were selected following a review of the values previously accepted by King County for use in the Rational Method and as described in several engineering handbooks. The values for single family residential areas were computed as composite values(as illustrated in the following equation)based on the estimated percentage of coverage by roads,roofs,yards, and unimproved areas for each density. For drainage basins containing several land cover types,the following formula may be used to compute a composite runoff coefficient, C,: G=(CIAi+C2A2+... +GAn)lAt (3-2) where At = total area(acres) A1,2....n= areas of land cover types(acres) C1,2,...n= runoff coefficients for each area land cover type "IR" Peak Rainfall Intensity The peak rainfall intensity IR for the specified design storm of return frequency R is determined using a unit peak rainfall intensity factor iR in the following equation: IR=(PR)(iR) (3-3) where PR = the total precipitation at the project site for the 24-hour duration storm event for the given return frequency.Total precipitation is found on the Isopluvial Maps in Figure 3.2.1.A through Figure 3.2.1.D. iR = the unit peak rainfall intensity factor 2017 City of Renton Surface Water Design Manual 12/12/2016 3-11 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS The unit peak rainfall intensity factor iR is determined by the following equation: iR = (aR)(T,)( bR) (3-4) where T, = time of concentration(minutes),calculated using the method described below and subject to equation limitations(6.3 <_ T,<_ 100) aR, bR= coefficients from Table 3.2.1.13 used to adjust the equation for the design storm return frequency R This"iR"equation was developed by DNRP from equations originally created by Ron Mayo,P.E. It is based on the original Renton/Seattle Intensity/Duration/Frequency(LD.F.)curves. Rather than requiring a family of curves for various locations,this equation adjusts proportionally the Renton/Seattle I.D.F. curve data by using the 24-hour duration total precipitation isopluvial maps. This adjustment is based on the assumption that the localized geo-climatic conditions that control the total volume of precipitation at a specific location also control the peak intensities proportionally. Note:Due to the mathematical limits of the equation coefficients, values of T,less than 6.3 minutes or greater than 100 minutes cannot be used. Therefore, real values of T,less than 6.3 minutes must be assumed to be equal to 6.3 minutes, and values greater than 100 minutes must be assumed to be equal to 100 minutes. "TC" Time of Concentration The time of concentration is defined as the time it takes runoff to travel overland(from the onset of precipitation) from the most hydraulically distant location in the drainage basin to the point of discharge. Note: When G (see Equation 3-2)of a drainage basin exceeds 0.60, it may be important to compute T, and peak rate offlow from the impervious area separately. The computed peak rate of flow for the impervious surface alone may exceed that for the entire drainage basin using the value at T,for the total drainage basin. The higher of the two peak flow rates shall then be used to size the conveyance element. T,is computed by summation of the travel times Tt of overland flow across separate flowpath segments defined by the six categories of land cover listed in Table 3.2.1.C,which were derived from a chart published by the Soil Conservation Service in 1975. The equation for time of concentration is: T,=Ti+ T2+...+ Tn (3-5) where TI,z,...n= travel time for consecutive flowpath segments with different land cover categories or flowpath slope Travel time for each segment t is computed using the following equation: L (3-6) Tt 60V where Tt = travel time (minutes)Note: Tt through an open water body(such as a pond)shall be assumed to be zero with this method L = the distance of flow across a given segment(feet) V = average velocity(fps)across the land cover=kR J where kR = time of concentration velocity factor; see Table 3.2.1.0 s,, = slope of flowpath(feet/feet) 12/12/2016 2017 City of Renton Surface Water Design Manual 3-12 3.2.1 RATIONAL METHOD TABLE 3.2.1.A RUNOFF COEFFICIENTS-"C'VALUES FOR THE RATIONAL METHOD General Land Covers Single Family Residential Areas' Land Cover C Land Cover Density C Dense forest 0.10 0.20 DU/GA(1 unit per 5 ac.) 0.17 Light forest 0.15 0.40 DU/GA(1 unit per 2.5 ac.) 0.20 Pasture 0.20 0.80 DU/GA(1 unit per 1.25 ac.) 0.27 Lawns 0.25 1.00 DU/GA 0.30 Playgrounds 0.30 1.50 DU/GA 0.33 Gravel areas 0.80 2.00 DU/GA 0.36 Pavement and roofs 0.90 2.50 DU/GA 0.39 Open water(pond, lakes, 1.00 3.00 DU/GA 0.42 wetlands) 3.50 DU/GA 0.45 4.00 DU/GA 0.48 4.50 DU/GA 0.51 5.00 DU/GA 0.54 5.50 DU/GA 0.57 6.00 DU/GA 0.60 Based on average 2,500 square feet per lot of impervious coverage. For combinations of land covers listed above,an area-weighted"C,x Ai'sum should be computed based on the equation Cc x At=(C,x A,)+(C2 x A2)+ ...+(C„x A„),where Ae=(A,+A2+ ...+A„),the total drainage basin area. TABLE 3.2.1.B COEFFICIENTS FOR THE RATIONAL METHOD"IR"EQUATION Design Storm Return Frequency aR bR 2 years 1.58 0.58 5 years 2.33 0.63 10 years 2.44 0.64 25 years 2.66 0.65 50 years 2.75 0.65 100 years 1 2.61 1 0.63 TABLE 3.2.1.0 KR VALUES FOR TT USING THE RATIONAL METHOD Land Cover Category kR Forest with heavy ground litter and meadow 2.5 Fallow or minimum tillage cultivation 4.7 Short grass pasture and lawns 7.0 Nearly bare ground 10.1 Grassed waterway 15.0 Paved area (sheet flow) and shallow gutter flow 20.0 2017 City of Renton Surface Water Design Manual 12/12/2016 3-13 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS FIGURE 3.21,A 2-YEAR 24-HOUR ISOPLUVIALS G WE SNOHOMISH COUNTY K'HaC®uHTY j Ar,LA. i Ltapp N N N / 4 L 2. �i 0 PX f 18LAND�� - �,I j I STL ISSA4D f Q I r ENTND UIQ "JI I V 1 I ..A V� N� ES I_�I� ; p� Y V, LL.APL Jv 1 I ERAL D.i Ci - .. ey\ e ev / a �J/y1L*9NAUBs ,_KING COUNTY n0 PIERCE COUNTY WESTERN KING COUNTY tib. �Lwl N J , .. ...., 3.5 2-Year 24-Hour tib - o Precipitation e in Inches Miles 12/12/2016 2017 City of Renton Surface Water Design Manual 3-14 3.2.1 RATIONAL METHOD FIGURE 3.2.1.11 10-YEAR 24-HOUR ISOPLUVIALS j _ _ SNON RAISH COUNTY axo / — -- _—_— __— x*� KING COUNTY xvi66 I- 9 II 2s f rlJ1 4.0 I � i � xx 8 � � 9 " ( ............,,,,,,,,, I xx6 / .,.y I r.ax'un ,,,,, � c.. :: x ,ii P.xDI x' ';i Ic"iI / ptO ml ® KEr / w7N) p ox nny- � �/ Q I V 717y I I /60 ub � I p ,KING COUNTY PIERCE COUNTY WESTERN %- KING COUNTY - 4 r , 10-Year 24-Hour 4.0 Precipitation in Inches M`�S 1by �o 2017 City of Renton Surface Water Design Manual 12/12/2016 3-15 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS FIGURE 3,2.1,0 25-YEAR 24—HOUR ISOPLUVIALS' ---_ -- --9N NOICOUNTY n KING COUNTY ILL, / I AM- Isi 30 I I 16 „ ,�„ o' i I /iff ' ff o�ll RER.� , I Rex I .„„SRR F ERCV ai ND j Cb o Ye L ,KING COUNTY /L• PIERCE COUNTY WESTERN z KING COUNTY 5,5 50 „c6 I N `r 4.5 25-Year 24-Hour Precipitation Cb in Inches O�z�4Miles 12/12/2016 2017 City of Renton Surface Water Design Manual 3-16 3.2.1 RATIONAL METHOD FIGURE 3.2.1.1) 100—YEAR 24—HOUR ISOPLUVIALS a — O _ COUNTY �- l �— — — -- — N COUNTY RELI 9 OF l a f ti J A - J � L / I i f TY .l r V`AAL LEY r Ie 0 xt� r ,r ,r J IIII� _I��ono'v' ]K O COUNTY —ULLT — — PIE CE COU-NTY_ v� WESTERN KING COUNTY s.5 a � U. s.o N 0�• ey r 5.5 100-Year 24-Hour °`• �`� � ,�- '�� ' �r o� Precipitation °`• h -� in Inches Miles e• 2017 City of Renton Surface Water Design Manual 12/12/2016 3-17 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS ❑ RATIONAL METHOD EXAMPLE Compute the peak flow Q25 to size a new roadway cross culvert for a 9.8-acre drainage basin east of Kent, P25=3.42 inches. Given: AREAS Ai = 4.3 acres of single family residential area at 3.8 DU/GA A2 = 2.3 acres of light forest A3 = 3.2 acres of pasture At = 9.8 total acres DESCRIPTION OF FLOWPATH SEGMENTS FOR T, Li = 300 feet S1 = 0.08 forest land cover kR=2.5 L2 = 200 feet S2 = 0.03 meadow kR=2.5 L3 = 1000 feet S3 = 0.015 grassed waterway(ditch) kR= 15.0 Compute: COMPOSITE RUNOFF COEFFICIENT G Ai: Ci = From Table 3.2.1.A, C for 4.00 DU/GA=0.48, C for 3.50 DU/GA=0.45. Therefore, Ci for 3.80 DU/GA=0.47 by visual interpolation. A2: C2 = 0.15 A3: C3 = 0.20 G = [(Ci x A1)+(C2 x A2)+(C3 x A3)]lAt = [(0.47 x 4.3)+(0.15 x 2.3)+(0.20 x 3.2)]/9.8=0.31 PEAK RAINFALL INTENSITY IR First,compute T,: Tl Ll Ll 300 = 60Vi 60(kRsi) 60(2.5 0.08) = 7 minutes T2a = L2 _ L2 200 _ 60V2 60(kR s2) 60(2.5 0.03) = 8 minutes T3 _ L3 _ L3 _ 1000 60V3 60(kR S3) 60(15 0.015) = 9 minutes T, = Ti + T2+ T3 =7+ 8+9=24minutes Second,compute iR for R=25: its = (aR)(T,)(bR)=(2.66)(24) (0.65)=0.34 Third,compute IR for R=25: 125 = (P25)(i25)=(3.42)(0.34)= 1.16 PEAK RUNOFF RATE Q25 = C 125 A=G 125 A=(0.31)(1.16)(9.8)=3.5 cfs 12/12/2016 2017 City of Renton Surface Water Design Manual 3-18 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD The approved continuous model/runoff files implementations of HSPF were developed as tools that have the accuracy and versatility of HSPF but are much simpler to use and provide a framework for efficient design of onsite stormwater detention facilities. This section describes the Runoff Files Method. The term runofffiles refers to a database of continuous flows presimulated by HSPF. The KCRTS software package has formerly been a tool for using this flow database. Current approved continuous models are listed in Reference Section 6-D(as updated); as of this writing,they include the Western Washington Hydrology Model(WWHM)and MGSFlood7.Projects are required to use the same model throughout unless otherwise approved through the adjustment process described in Section 1.4. The Runoff Files method was developed as a hydrologic modeling tool for western King County to produce results(design flows, detention pond sizing, etc.)comparable to those obtained with the U.S. Environmental Protection Agency's HSPF model but with significantly less effort. This is achieved by providing the user with a set of time series files of unit area land surface runoff("runoff files") presimulated with HSPF for a range of land cover conditions and soil types within King County. The design flows are estimated and detention facilities are designed by directly accessing and manipulating the runoff file data by means of the continuous modeling software. Typical basic capabilities of the continuous modeling software include: • Estimating time series of flows for a specified land use and location within King County • Analyzing flow frequency and duration • Analyzing water surface frequency and duration • Plotting analysis results • Sizing detention facilities. ❑ DEVELOPMENT OF THE RUNOFF FILES To compile the runoff files,the land surface hydrologic response(represented by a time series of unit area land surface runoff)was generated by HSPF with regional parameters for a variety of land use classifications and for a long-term(over 50-year)rainfall station representing the western lowlands of King County(Sea-Tac Airport).A 158-year extended precipitation timeseries(Puget East)was also developed by MGS Consulting.The City allows the use of either the 50-year Sea-Tac Airport gage data or the 158-year simulated timeseries for sizing.The methods for developing the runoff files are specific to the individual approved models. Consult the program documentation and the software provider's website information for the particular model for background on the development of the runoff files for that model. Runoff time series were generated with data from these and other stations for the following eight soil/land cover types: • Impervious • Till forest • Till pasture • Till grass • Outwash forest • Outwash pasture • Outwash grass • Wetland. HSPF and the approved models simulate surface runoff,interflow, and groundwater flow. Groundwater flow,induced by surface runoff or occurring naturally,is usually lost from the system through the 7 King County no longer provides development,training and maintenance of the KCRTS model,and provides limited support dependent on staff availability. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-19 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS analysis,but may require consideration in the analysis if it expresses to the surface. Consult the user's guide for application of the interflow and groundwater components of runoff in the approved continuous model. 3.2.2.1 GENERATING TIME SERIES Most hydrologic analyses will require time series of flows for different land use conditions.For example, to size a flow control detention facility to meet the Peak Rate Flow Control Standard,the 2-, 10-, and 100-year peaks from the facility discharge time series must be compared with 2-, 10-, and 100-year peaks from the predevelopment time series. To generate a flow time series with the approved continuous model, depending on the model used,the program applies the following: 1. As determined by selecting the project's location on a map, • The rainfall region within which the project lies(i.e., Sea-Tac)and multiplier(a regional scale factor applied to the runoff files)to account for variations in rainfall volumes between the project site and the rainfall station,or • A calibrated area-specific rainfall map developed from the Sea-Tac rainfall data,or • A long-term (158-year) simulated precipitation timeseries(i.e.,Puget East),or • Site specific calibrated rainfall data. See the approved model's documentation for background on the development of the runoff files for the model. 2. The time step to be used in the analysis.As of this manual update, 15-minute time steps are required for all applications including detention sizing and volume analysis. 3. The complete historical runoff record used in the analysis: 4. The amount of land(acreage)of each soil/cover group for the subbasin under study, as calculated per model methodology and the methods described in this chapter. 5. If applicable,the percentage of impervious area that is effectively connected to the drainage system, typically accounted for by adjusting actual impervious area for the model inputs. See the user's documentation for the approved model for methodology and guidance for generating a new time series. See Reference Section 6-D for specific guidance to be used with this manual. ❑ SELECTION OF PRECIPITATION RECORD AND REGIONAL SCALE FACTOR As noted in the previous section,runoff files were developed using rainfall data from Sea-Tac Airport. The regional scale factor is a geographically variable multiplier applied to the flow time series to account for the variations in rainfall amounts, and hence runoff.Whereas previous models(e.g.,KCRTS)required determination by mapped values as data input,the scaling effects are determined in the currently approved continuous models(e.g.,WWHM and MGS Flood)by selecting the project location within the model. See the approved model user's documentation for background and guidance. Alternatively,the user can select the 158-year simulated precipitation timeseries(Puget East)for sizing. This precipitation timeseries can be found by selected"Use WS-DOT data"in WWHM or under "Extended Timeseries"in MGS Flood.A scaling factor does not need to be applied to the Puget East precipitation timeseries. ❑ CATEGORIZATION OF SOIL TYPES AND LAND COVER The Runoff Files method typically supports several land use classifications,including till forest,till pasture,till grass, outwash forest, outwash pasture, outwash grass,wetland, and impervious. These classifications incorporate both the effects of soil type and land cover. In the SCS method, four different hydrologic soil groups are defined(A,B, C, and D)based on soil type as mapped by the SCS. The SCS also defines hydrologic response for about a dozen different land use or cover types.The SCS method 12/12/2016 2017 City of Renton Surface Water Design Manual 3-20 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD therefore allows the user a considerably greater degree of flexibility in defining land cover and soil types than do continuous models. However,the flexibility and apparent detail available with the SCS method cannot be supported on the basis of the data used to develop that method. The Runoff Files method minimizes the number of land use classifications,thereby simplifying both the analysis and review of development proposals. Soil Groups for the Continuous Model The following soil characterization is generally true for continuous models;however,consult the model documentation for specific applicability. Till Soils Till soils are underlain at shallow depths by relatively impermeable glacial till. The principal SCS soil group within the City classified as a till soil is the Alderwood series(SCS hydrological soil group Q. The hydrologic response of till soils in an undeveloped, forested state is characterized by relatively slight surface runoff, substantial interflow occurring along the interface between the till soil and the underlying glacial till, and slight groundwater seepage into the glacial till. Bedrock soils,primarily Beausite and Ovall soils in King County, are underlain by either sandstone or andesite bedrock,and a large group of alluvial soils. Alluvial soils are found in valley bottoms. These are generally fine-grained and often have a high seasonal water table.There has been relatively little experience in calibrating the HSPF model to runoff from these soils, so in the absence of better information,these soils have been grouped as till soils.Most alluvial soils are classified by the SCS in hydrologic soil groups C and D. Outwash Soils Outwash soils are formed from highly permeable sands and gravels. The principal SCS soil group classified as an outwash soil is the Everett series. Where outwash soils are underlain at shallow depths (less than 5 feet)by glacial till or where outwash soils are saturated,they may need to be treated as till soils for the purpose of application in the model. Refer to the model documentation for specifics. Wetland Soils Wetland soils have a high water content, are poorly drained, and are seasonally saturated. For the purposes of applying continuous modeling in King County,wetland soils can be assumed to coincide with wetlands as defined in the critical areas code(RMC 4-3-050). The approximate correspondence between SCS soil types and the appropriate soil group for typical continuous modeling is given in Table 3.2.2.A(refer to the model documentation for specific soil group application for the model). If the soils underlying a proposed project have not been mapped, or if existing soils maps are in error or not of sufficient resolution,then a soils analysis and report shall be prepared and stamped by a civil engineer with expertise in soils to verify underlying soil conditions. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-21 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS TABLE 3.2.2.A EQUIVALENCE BETWEEN SCS SOIL TYPES AND TYPICAL CONTINUOUS MODELING SOIL TYPES SCS Soil Group for Hydrologic Continuous SCS Soil Type Soil Group Model Notes Alderwood (AgB, AgC, AgD) C Till Arents, Alderwood Material (AmB, AmC) C Till Arents, Everett Material (An) B Outwash 1 Beausite (BeC, BeD, BeF) C Till 2 Bellingham (Bh) D Till 3 Briscot(Br) D Till 3 Buckley (Bu) D Till 4 Earlmont(Ea) D Till 3 Edgewick (Ed) C Till 3 Everett(EvB, EvC, EvD, EwC) A/B Outwash 1 Indianola (InC, InA, InD) A Outwash 1 Kitsap (KpB, KpC, KpD) C Till Klaus (KsC) C Outwash 1 Neilton (NeC) A Outwash 1 Newberg (Ng) B Till 3 Nooksack (Nk) C Till 3 Norma (No) D Till 3 Orcas (Or) D Wetland Oridia (Os) D Till 3 Ovall (OvC, OvD, OvF) C Till 2 Pilchuck (Pc) C Till 3 Puget(Pu) D Till 3 Puyallup (Py) B Till 3 Ragnar(RaC, RaD, RaC, RaE) B Outwash 1 Renton (Re) D Till 3 Salal (Sa) C Till 3 Sammamish (Sh) D Till 3 Seattle (Sk) D Wetland Shalcar(Sm) D Till 3 Si (Sn) C Till 3 Snohomish (So, Sr) D Till 3 Sultan (Su) C Till 3 Tukwila (Tu) D Till 3 Woodinville (Wo) D Till 3 Notes: 1. Where outwash soils are saturated or underlain at shallow depth(<5 feet)by glacial till,they should be treated as till soils. 2. These are bedrock soils,but calibration of HSPF by King County shows bedrock soils to have similar hydrologic response to till soils. 3. These are alluvial soils,some of which are underlain by glacial till or have a seasonally high water table. In the absence of detailed study,these soils should be treated as till soils. 4. Buckley soils are formed on the low-permeability Osceola mudflow. Hydrologic response is assumed to be similar to that of till soils. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-22 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD Land Cover Types in Continuous Modeling Continuous models support land cover types including forest,pasture,grass, and impervious. These cover types shall be applied in accordance with Core Requirement 43 and as specified in Table 3.2.2.B. Predevelopment land cover types are determined by whether the project is in a Peak Rate Flow Control Standard Area or Flow Control Duration Standard Area and whether the area in question is a target surface,as defined in Section 1.2.3.1. Target surfaces within Peak Rate Flow Control Standard Areas and Flow Control Duration Standard Matching Existing Condition Areas and non-target surfaces are modeled as existing site conditions; for target surfaces in Flow Control Duration Standard Matching Forested Condition Areas,the predeveloped condition is assumed to be forested(historical)site conditions. TABLE 3.2.2.B CONTINUOUS MODEL COVER GROUPS AND AREAS OF APPLICATION Continuous Model Application Cover Group Predevelopment Post-Development Forest All forest/shrub cover, All permanent(e.g., protected by covenant or critical irrespective of age. area designation)onsite forest/shrub cover, irrespective of age, planted at densities sufficient to ensure 80%+canopy cover within 5 years. Pasture All grassland, pasture land, Unprotected forest in rural residential development lawns, and cultivated or cleared shall be considered half pasture, half grass. areas, except for lawns in Pasture areas to be retained on large rural residential redevelopment areas with lots (10 acres or greater)may be modeled as half predevelopment densities in pasture, half grass. excess of 4 DU/GA. Grass Lawns in redevelopment areas All post-development grassland and landscaping and with predevelopment densities in all onsite forested land not protected by covenant or excess of 4 DU/GA. designated as a protection area (wellhead,wetland, or buffer) in RMC 4-3-050 (except in rural areas as noted above). For purposes of runoff modeling, underdrained pervious areas may be modeled explicitly to account for attenuation and infiltration, or may be modeled as 50% impervious/50%grass where either: (a)there is no added liner, (b)where the added liner is a treatment liner, or(c)where the added liner is one that does not restrict infiltration rates below the in situ soil infiltration rate. Other lined underdrained systems must be modeled explicitly or as 100% impervious. Wetland All delineated wetland areas. All delineated wetland areas. Impervious(') All impervious surfaces, All impervious surfaces, including compacted gravel including heavily compacted and dirt roads, parking areas, etc., and open water gravel and dirt roads, parking bodies, including onsite detention and water quality areas, etc., and open water ponds.(2) bodies (ponds and lakes). (1) Impervious acreage used in computations should be the effective impervious area(EIA).This is the effective area as determined through layouts of the proposal and on-site BMP credit reductions from Table 1.2.9.A in Chapter 1. Non- effective impervious areas are considered the same as the surrounding pervious land cover. (2)To avoid iterations in the facility sizing process,the"assumed size"of the facility need only be within 80%of the final facility size when modeling its contribution of runoff from direct rainfall. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-23 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS The following factors are considered in specifying the above land cover types to be used in hydrologic analysis with continuous modeling: • Cover types are applied to anticipate ultimate land use conditions.For example,probable clearing of woodland after development is nominally complete suggests that the post-development land use be specified as grassland(either pasture or grass)unless the forest cover is protected by covenant. • In areas of redevelopment,there are often significant changes between the predevelopment and post- development efficiencies of the drainage system.For example,in conversion of low density residential areas to higher density land use,impervious areas prior to redevelopment may not be efficiently connected to a drainage system(e.g., downspouts draining to splash blocks,ditched instead of piped roadway systems). These problems are addressed by defining an"effective impervious fraction"for existing impervious areas and by generally requiring predevelopment grasslands to be modeled as pasture land. • All onsite,predevelopment forest/shrub cover and all offsite forest/shrub cover is defined as"forest," irrespective of age.Post-development onsite land use is defined as forested only if forested areas are in a critical area buffer or are otherwise protected and will have a minimum 80%canopy cover within 5 years. In urban areas,unprotected onsite forest cover should be treated as grass in the post- development analysis. In rural areas,unprotected forest cover should be assumed 50%grass, 50% pasture. • The HSPF grass parameters were developed by the USGS study of regional hydrology and have generally been interpreted as providing the hydrologic response for"urban"grasslands(lawns, etc.), which have relatively low infiltration rates and are drained effectively. The HSPF "pasture" parameters were developed to provide a hydrologic response intermediate to the USGS forest and grass parameters, as might be typified by ungrazed or lightly grazed pasture with good grass cover. Because it is impossible to adequately control grassland management after development, all post- development grassland should be modeled as"grass"(with the exception of unprotected forest, and pasture areas on large lots,in rural development as noted above).All predevelopment grassland should be modeled as"pasture"except for redevelopment of areas with predevelopment land use densities of 4 DU/GA or greater(which are modeled as grass). ❑ CALCULATION OF IMPERVIOUS AREA Total Impervious Coverage Table 3.2.2.0 lists percent impervious coverage for use in continuous runoff modeling analysis of existing residential,commercial, and industrial areas. The tabulated figures are useful in offsite analysis that includes large developed residential areas,making a detailed survey of impervious coverage impractical. Impervious coverage for proposed residential,commercial, and industrial development must be estimated for each specific proposal. Impervious coverage of streets, sidewalks,hard surface trails, etc., shall be taken from layouts of the proposal. House/driveway or building coverage shall be as follows: • For urban residential development,the assumed impervious coverage shall not be less than 4,000 square feet per lot or the maximum impervious coverage permitted by Table 3.2.2.C,whichever is less. • For commercial,multi-family, and industrial development,impervious coverage shall be estimated from layouts of the proposal. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-24 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD TABLE 3:2.....:2.0 MAXIMMIMPERVIOUS COVERAGE FOR RESIDENTIAL AREAS Zoning Dei nation Maximum Impervious Surface Area Resource Conservation (RC) Lots 5 acres or more: 20% Lots 10,000 sq ft: 55%. For each additional 10,000 sq ft increase in lot size, the impervious coverage shall be decreased by 1.75% to a minimum of 20% for a 5-acre lot Lots 10,000 sq ft or less: 55% Residential-1 (R-1) 30% Residential-4 (R-4) 55% Residential-8 (R-8) 75% Residential-10 (R-10) Detached units: 75% Attached units: 65% Residential-14 (R-14) 85% Effective Impervious Area The net hydrologic response of an impervious area depends on whether that area is effectively connected (usually by pipes or a channel)to a storm drainage system. The impervious area that the user inputs to the continuous model is the "Effective Impervious Area"(EIA). Non-effective impervious area(i.e.,total impervious area less EIA)is assumed to have the same hydrologic response as the immediately surrounding pervious area.For example, for existing residential areas with rooftops draining to splash pads on lawns or landscaping,the non-effective portion of the roof areas would be treated as pasture for predevelopment conditions(if DU/GA<4.0)and grass for post-development conditions.Note: Credits for infiltration/dispersion of downspouts on individual lots in proposed single family residential subdivisions are applied separately on a site-specific basis. Core Requirement 49 outlines where the use of on-site BMPs may be used to reduce the effective impervious area of the project.. The effective impervious area can be determined from detailed site surveys. 3.2.2.2 TIME SERIES STATISTICAL ANALYSIS When using a continuous runoff model to size flow control,water quality, and conveyance facilities, design flows and durations must be determined through statistical analysis of time series data generated by the software.Flow frequency analysis is used for determining design peak flows while flow duration analysis is used for determining durations of flow exceedance. ❑ FLOW FREQUENCY ANALYSIS Flow frequency is a commonly used but often misunderstood concept. The frequency of a given flow is the average return interval for flows equal to or greater than the given flow. The flow frequency is actually the inverse of the probability that the flow will be equaled or exceeded in any given year(the exceedance probability).For example,if the exceedance probability is 0.01, or 1 in 100,that flow is referred to as the 100-year flow.Assuming no underlying changes in local climate, one would expect to see about 10 peak annual flows equal to or greater than the 100-year flow in a 1,000-year period. Similarly,the 2-year flow is the flow with a probability of 0.5, or 1 in 2, of being equaled or exceeded in any given year. In a 100-year period, one would expect to observe 50 peak annual flows greater than or equal to the 2-year flow. The number of peak annual flows actually equal to the 2-year flow may be zero, since peak annual flows come from a continuous spectrum. There are many methods for estimating exceedance probabilities and therefore flow frequencies.The USGS Bulletin 17B methods are commonly used, as are graphical methods using the Gringorten, Cunane, 2017 City of Renton Surface Water Design Manual 12/12/2016 3-25 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS or Weibull plotting schemes(Maidment, 1993). Graphical methods for flow frequency estimation involve assigning exceedance probabilities, and therefore return intervals,to each annual peak in a series of annual peak observations, and then plotting the peak flows against their assigned return periods. This plot is known as a flow frequency curve, and it is a very useful tool for analyzing flood probabilities. Examples of flow-frequency curves for a small basin under various conditions are shown in Figure 3.2.2.A. Flow-frequency curves are used in continuous flow simulations to determine the effect of land use change and assess the effectiveness of detention facilities.Using continuous methodology to design detention facilities to control peak flows,the analyst must match(i.e.,not exceed)the post-development (detained)and predevelopment flow-frequency curves at the frequencies of interest, as shown in Figure 3.2.2.A,rather than match specific design events as when using an event model. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-26 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD FIGURE 3.2.2.A EXAMPLE FLOW FREQUENCY ANALYSIS Undetained Developed Return Period 2 5 16 2D 5o 166 X Post-developed o Pre-developed X X x x X O x x x o 0 D 9 0 D L.L L7 v Q ai T Q1 L 0 0 V d i� 1 2 5 10 20 30 40 50 60 70 80 40 R5 98 99 Cumulative Probabilrty Detained Developed — Return Period O 2 5 10 2D 50 106 X Detained Post-developed e7 Pre-developed X CD x C, X Ul a The 2-and 10-year annual peak flows are matched; however,the °} =y 100-year peak flow is only partially attenuated in this example, so the v detention volume would need to be increased to fully meet the Peak Rate Flow Control Standard 1 2 5 10 20 30 40 50 60 70 80 90 95 48 99 Cumulotive Probability 2017 City of Renton Surface Water Design Manual 12/12/2016 3-27 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS Flow frequency information is derived from the time series flow file by plotting the peak annual events in the runoff file and calculating runoff frequencies using a Log Pearson distribution or other statistical analysis. Typical return periods calculated in continuous models are the 100-year, 50-year,25-year, 10-year, 5-year,3-year,2-year, and lesser storms for low-flow regime,LID and water quality applications. ❑ FLOW DURATION ANALYSIS Flow duration analysis is important because it identifies the changes in durations of all high flows rather than simply the change in frequency of the peak annual flows. Channel scour and bank erosion rates rise proportionally with increases in flow durations. Flow duration analysis can only be conducted with continuous flow models or from gage records. A flow duration curve is a plot of flow rate against the percentage of time that the flow rate is exceeded. In a continuous flow model,the percent exceedance of a given flow is determined by counting the number of time steps during which that flow is equaled or exceeded and dividing that number by the total number of time steps in the simulation period.Flow duration curves are usually plotted with a linear flow scale versus a log scale of percent exceedance. The log scale for exceedance percentage is used because geomorphically significant flows(flows capable of moving sediment)and flows that exceed the 2-year flow typically occur less than one percent of the total time. ❑ DURATIONS AND PEAKS FOR FLOW CONTROL STANDARDS The Flow Control Duration Standard matching existing site conditions and Flow Control Duration Standard matching forested site conditions per Section 1.2.3.1 requires matching predevelopment and post-development flow duration curves for all flows from 50% of the 2-year flow up to the full 50-year flow. To simplify design,brief excursions$of post development durations above the target predevelopment durations are allowed for matching flows greater than 50%of the predevelopment 2-year peak flow. These excursions shall not increase the duration of discharge by more than 10% at any flow level and must be strictly below the target duration curve at the low end of the range of control from 50% of the 2-year peak flow to the 2-year peak flow. This allows efficient design using only two orifices for most applications, although two-orifice designs may not allow sizing with automatic pond sizing routines; see the software documentation for guidance.An example of a flow duration analysis is shown in Figure 3.2.2.13. The Flood Problem Flow Control Standard matches predevelopment and post-development flow durations over the same range of predevelopment flows as the Flow Control Duration Standard and requires matching the 100-year post-development peak flow. This standard provides additional storage volume over the Flow Control Duration Standard facility,which substantially mitigates the impacts of increased volumes of surface runoff on downstream,volume-sensitive flooding problems. The Peak Rate Flow Control Standard does not require flow duration analysis because it addresses peak flows only(the 2-year, 10-year, and 100-year peaks). The Low Impact Development(LID)performance standard requires that stormwater discharges shall match(i.e.,not exceed) developed discharge durations to pre-developed durations for the range of pre- developed discharge rates from 8%of the 2-year peak flow to 50%of the 2-year peak flow.No excursions above the pre-developed durations are allowed. 8 Brief excursions may not result in more than 50%of the target duration curve being exceeded. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-28 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD FIGURE 3.2.2.B EXAMPLE FLOW DURATION ANALYSIS Post-developed X Pre-developed 0 ty 0 a� 0 cn u K3P 4� (�Fo "m U p ifF d <:> O Probability Exceedence 0 10 ' 10� 10-' 10_2 10 ' 1 50-year ......................................................................................................................................................................................................Pre-developed FC Duration Existing Target X 10-year ...............................o ........................................................ L� 10%allowable horizontal 4N tolerance along portion of ¢ target curve above 2-year predevelopment peak flow •a 2-year ..................................................................................................................................................................................................'�........................... a � Strictly below target curve at low end of range of control (50%of 2-year peak w u, flow to 2-year peak flow). 50%2-year......................00................................................................................................................................................................................................� Predeveloped �T 0 Return Frequencies a 0 o Probability Exceedence 0- 10 10, 10_, 10z 10 1 2017 City of Renton Surface Water Design Manual 12/12/2016 3-29 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS When evaluating impacts to closed depressions,ponding areas and wetlands,or when evaluating for tightlined system requirements in critical areas per Core Requirement 41, frequencies of water levels or determination of average annual runoff volumes must be determined through statistical analysis of time series data generated using a continuous runoff model. ❑ ASSESSING WATER LEVEL STATISTICS Stage frequency analysis consists of estimating and plotting recurrence estimates for water levels within a storage feature in the same manner as flow frequency analysis is conducted for discharges. Stage frequency analysis is required for assessing runoff impacts to offsite closed depressions and ponding areas as required under Core Requirements 42 and 43, and as discussed Section 3.3.6, "Point of Compliance Analysis,"or as required for analyses of wetland impacts pursuant to Core Requirement 49. ❑ ASSESSING ANNUAL AVERAGE RUNOFF VOLUMES To compute the annual average runoff volume,the volume of runoff(surface+interflow) of a time series must be computed using the approved model. The analysis is performed using the entire period of record. The total volume is divided by the number of full water years being analyzed to determine the annual average runoff volume. 3.2.3 THE APPROVED MODEL The continuous hydrologic analysis tools prescribed in this manual are generically described as the "approved model"; a list of the approved models is found in Reference Section 6-D.At this writing,the approved continuous hydrologic models9 include the Western Washington Hydrologic Model (WWHM) and MGS Flood,both of which are variants of the Hydrologic Simulation Program- FORTRAN(HSPF)model. HSPF is also an approved model,but is more complex than other approved models and is typically used for basin planning and master drainage plan analyses. General instruction and guidance for use of the approved model is found in the user's documentation for the model. Guidance specific to the City for the continuous runoff models approved for use with this manual is contained in Reference Section 6-D.A brief overview of HSPF follows below. 3.2.4 THE HSPF MODEL HSPF is the parent model from which the other approved model methods are built. It is a very versatile continuous hydrologic/hydraulic model that allows for a complete range of hydrologic analysis. This model has been extensively used in King, Snohomish, and Thurston counties and found to be an accurate tool for representing hydrologic conditions in this area.The USGS has developed regional parameters to describe the common soil/cover combinations found in this area. In many cases,these regional parameters can be used to represent rainfall/runoff relationships in lieu of site-specific calibration parameters. Unfortunately,the HSPF model is very difficult to use. Design engineers using HSPF should study this model in detail and obtain training before using it on a project.For these reasons,the HSPF model is recommended only for large and complex projects where the capabilities of the approved model are too limited. The strengths of HSPF relative to the approved model are as follows: 1. HSPF can be calibrated to local conditions. 2. HSPF can model,link,and route many separate subbasins. 3. HSPF includes the groundwater component of streamflow. 9 KCRTS is no longer maintained by King County and is not an approved model for use with the SWDM 12/12/2016 2017 City of Renton Surface Water Design Manual 3-30 3.2.4 THE HSPF MODEL 4. HSPF can address groundwater connections and perform low-flow analysis. 5. HSPF can handle more complex hydrologic routing(e.g., evaporation, seasonal infiltration, etc.). The HSPF model is generally recommended for large sites where these additional features are required for comprehensive hydrologic and/or hydraulic analysis. Anyone planning a project that is large enough to require Large Project Drainage Review and submittal of a Master Drainage Plan(MDP)per Section 1.1.2.5 should meet with CED review staff regarding appropriate hydrologic analysis prior to initiating such analysis. If a project subject to Large Project Drainage Review drains to a wetland,a salmonid stream with low-flow sensitivities, or a ground water protection area,it is likely that the City will require a calibrated HSPF model. If such a project drains to erosion-sensitive streams or has features with complex hydraulics,the City may recommend or require an HSPF model using the USGS regional parameters. Smaller or less sensitive subbasins within a MDP area can be analyzed with the approved model. Additional data is required to develop an HSPF model.At a minimum, development of an HSPF model requires collection of onsite rainfall data for a period from seven to twelve months. This data is used to determine which regional long-term rainfall record is most appropriate for modeling the site and for determining transposition factors for the long-term records. If calibration is required,the onsite rainfall data is used. Calibration also requires the installation of flow gages and the collection of flow data against which simulated flows can be compared. HSPF analysis is based on simulations with long-term rainfall records(greater than 30 years). Long-term precipitation records in HSPF format can be obtained from King County for the Sea-Tac rain gage and the Puget East 158-year simulated precipitation timeseries. Land surface representation with HSPF follows the same procedures and classification as used with the approved model. Conceptually,the outputs required from an HSPF analysis are consistent with those required from an approved model analysis,including frequency and durational analysis. Flow and/or water level frequencies shall be estimated using the full set of annual peaks from the long-term simulations using the USGS Bulletin 17B methods as well as the Gringorten or Cunane graphical methods. Durational analyses can be produced from the HSPF model and the results presented graphically. If a wetland is modeled,water level analyses may be required.Monthly, seasonal, and annual water balance and flow information,if appropriate,can be calculated with the HSPF model. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-31 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 3-32 CITY OF RENTON SURFACE WATER DESIGN MANUAL 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS This section presents the design procedures and considerations for sizing flow control facilities to meet the required hydrologic performance specified in Core Requirement 43, Section 1.2.3. It includes the following procedures and special considerations for proper hydrologic design: • "General Hydrologic Design Process,"Section 3.3.1 • "Flow Control Design Using the Runoff Files Method,"Section 3.3.2 • "Conveyance System Design with the Runoff Files Method," Section 3.3.2 • "Safety Factors in Hydrologic Design,"Section 3.3.4 • "Design Options for Addressing Downstream Drainage Problems,"Section 3.3.5 • "Point of Compliance Analysis,"Section 3.3.6 • "Onsite Closed Depressions and Ponding Areas,"Section 3.3.7. 3.3.1 GENERAL HYDROLOGIC DESIGN PROCESS This section presents the general process involved in conducting a hydrologic analysis using the runoff computation and analysis tools described in Section 3.2 to design flow control facilities for a project. The process is described as follows: 1. Review the core and special requirements in Chapter 1 to determine all requirements that will apply to the proposed project. a) Determine the applicable flow control standard(outflow performance criteria and land cover assumptions). b) If downstream drainage problems are identified through offsite analysis per Core Requirement 42, determine if they will necessitate additional onsite flow control or other measures as described in Section 3.3.5. 2. Determine and demonstrate in the Technical Information Report(see Section 2.3)the predeveloped conditions per Core Requirement 43,Flow Control(see Section 1.2.3). 3. Identify and delineate the drainage basin for each natural discharge location from the project site. a) Identify existing drainage features such as streams,conveyance systems, detention facilities, ponding areas, depressions,wetlands, etc. b) Identify existing land uses. c) Identify soil types using SCS soil survey or onsite evaluation. d) Convert SCS soil types to soil classifications for the approved model. 4. Select and delineate appropriate subbasins,including subbasins tributary to major drainage features and important conveyance points, and subbasins for separate computation of onsite flows and offsite flows. 5. Determine hydrologic parameters for each subbasin under predeveloped conditions. a) Categorize soil types and land cover. b) Determine total impervious areas and effective impervious areas within each subbasin. c) Determine areas for each soil/cover type in each subbasin. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-33 SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6. Determine the runoff time series for predeveloped conditions at each natural discharge location. a) Compute the predeveloped condition runoff time series for each subbasin using 15-minute time steps. b) For subbasins that drain to a drainage feature with significant detention storage(e.g., existing detention facilities,ponding areas,closed depressions),route the runoff time series through the feature per the storage routing methods in the approved model. This will yield an attenuated flow series,which becomes the effective runoff time series for that subbasin. c) Sum the appropriate subbasin runoff time series to obtain the total runoff time series for each natural discharge location. d) Determine the 100-year peak flow for each natural discharge location. 7. Repeat Steps 4 through 6 for the proposed post-development condition. 8. Compare the 100-year peak flows for the appropriate predeveloped and post-development conditions at each natural discharge location. a) Check the"Discharge Requirements"criteria in Core Requirement 41 to determine the acceptable manner of discharge from the project site(using existing conditions). b) Check the flow control exemptions in Core Requirement 43 to determine if a flow control facility is required(using existing site or historical site conditions, as specified in Core Requirement 43). c) Check the requirement for bypass of runoff from non-target surfaces in Core Requirement 43 to determine if runoff from non-target surfaces must be conveyed around onsite flow control facilities(using existing conditions). 9. If flow control facilities are required, determine their location and make any necessary adjustments to the developed condition subbasins. 10. Design and size each flow control facility using the methods described in Section 3.2 and the Runoff Files Method design procedure in Section 3.3.2. a) Analyze the appropriate predeveloped condition runoff time series to determine target release rates for the proposed facility.Note:If the target release rates are zero, an infiltration facility will be required. b) Compute the post-development runoff time series for the proposed facility. c) Use the post-development runoff time series and an iterative process to size the facility to meet the required level of performance set forth in Core Requirement 43. See the approved model user's documentation for procedures in sizing flow control facilities using continuous flow time series. 11. Design required onsite conveyance systems using the appropriate runoff computation method(either the Rational method or the Runoff Files method with 15-minute time steps)as specified in Section 3.2. 3.3.2 FLOW CONTROL DESIGN USING THE RUNOFF FILES METHOD Flow control facility design using the approved modeling software involves four basic steps: 1. Determining the statistical characteristics(peaks or durations) of predevelopment flows(using 15-minute time steps)which set the targets for the facility release rates, 2. Developing preliminary facility volume and orifice configuration, 3. Routing post-development flow time series through the preliminary facility to check performance,and 4. Iteratively revising the facility and checking performance until the target flow conditions are achieved. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-34 3.3.2 FLOW CONTROL DESIGN USING THE RUNOFF FILES METHOD Instead of using individual design rainfall events as in an event model,the design of the facility is based on simulation of the facility's performance using the full historical(over 50-years)time series record of simulated post-development flows, and also on comparison of the outflow record to characteristics of the predevelopment flow record.Final design is achieved when the outflow time series meets the target flow specifications. Detention facility design with a continuous model is based on aggregate flow statistics,not upon individual storms. When designing detention facilities with a continuous model, the return period of the peak flow leaving the facility for a particular event may not have the same return period as the peak flow entering the facility during the same event. Unlike event models,continuous models have natural variability in the ratio of storm peak and volume. This lack of correspondence in the return periods of peak inflows and outflows in continuous models means that facility design using a continuous runoff model is more complicated than with an event method and in general has to be done on an iterative trial-and-error basis to obtain an optimal(i.e.,least volume)design. The effect of detention facilities in controlling peak flows is dependent on both the volume and peak of the inflowing hydrograph. Generally, it is high volume storms rather than high intensity storms that cause detention facilities to fill and overtop. The hydrographs produced by a continuous runoff model show considerable variability in the relationships between peak flows and storm volumes.For example, one event produced by high rainfall intensities in a relatively short duration storm may produce high peak flows with a relatively small hydrograph volume. By contrast, a second rainfall event may have relatively low intensities but long duration,producing a runoff hydrograph with large volumes and relatively small peak. Due to this natural variability,the peak annual outflows from a detention facility may not correspond in time to the annual peaks of the inflow record. Similarly, the predevelopment peak annual flows may not occur during the same storm as thepeak annual flows for the post-development flow series. This is because the types of storms that produce high flows from undeveloped land covers are different from those that produce high flows from impervious surfaces. Forests generate high streamflows in response to long-duration,high-volume rainfall events that soak the soil profile,whereas impervious surfaces produce the highest flow rates in response to high precipitation intensity. This is another reason why detention facility design with a continuous runoff model is based on aggregate flow statistics,not upon individual storm hydrographs. The following is a typical procedure for hydrologic design of detention/infiltration facilities using a continuous runoff model. Specific guidance for conducting hydrologic analysis and design with the approved model is provided in the approved model user's documentation. 1. Create time series of flows from the predevelopment area using graphic elements that detail the predevelopment land cover,the post-development area tributary to the facility, any onsite post- development bypass area, and any offsite flow-through areas. 2. Add any offsite flow-through time series to the predevelopment flow time series using similar graphic elements to produce a time series of total predevelopment outflows from the project site. Similarly, add the same offsite flow-through time series to the time series of post-development flows tributary to the facility to produce a time series of total post-development inflows to the facility. 3. Generate peak annual flow estimates,flow duration curves and flow frequency curves for pre- and post-development time series. 4. Enter the Facility element for the scenario and specify initial facility specifications for the type of facility proposed.Use of two orifices is usually sufficient for most designs. If designing an infiltration facility,the bottom orifice may be elevated or zero orifices may be specified. 5. Route the complete facility inflow time series through the facility. The outflow time series is automatically saved.Use the analysis tools to evaluate facility performance.When sizing the facility to account for credits from on-site BMPs per Core Requirement 49 and Appendix C, note that it is necessary to turn infiltration off for on-line on-site BMPs draining to the facility,to avoid 2017 City of Renton Surface Water Design Manual 12/12/2016 3-35 SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS counting the flow reduction effect twice.For facilities designed using this manual,explicit modeling of infiltrative BMPs for downstream flow control facility sizing is not allowed. 6. Adjust orifice configuration and facility size,iterate until desired performance is achieved.Use of the automatic facility sizing routine in the approved model is helpful. 7. Verify the facility performance by routing the complete time series of inflows and checking the post-development peak flows and/or durations at the project site boundary against the target flows and/or durations(see the criteria for"Evaluating Flow Control Performance"provided below). When explicitly modeling BMPs for compliance with the LID Performance standard,two separate routings are necessary to evaluate the flow control credit based facility performance and the explicitly modeled BMPs for the LID Performance standard. Evaluating Flow Control Performance Evaluating the performance of facility designs intended to provide flow frequency control is comparatively straightforward:the post-development facility annual peak flows should be strictly less than or equal to predevelopment annual peak flows at each of the specified return periods. Note:Peakflow matching is required per Core Requirement 43. The automatic sizing routines in the approved continuous runoff models are based on duration matching and do not evaluate for peak flow compliance. The user must complete this evaluation as an additional step to verb compliance. Evaluating the design performance of detention facilities providing flow duration control,however, generally requires several iterations. In fact,considerable time could be spent attempting to match predevelopment and post-development duration curves. Some flexibility in assessing the adequacy of fit is clearly needed to expedite both design and review. Therefore, flow duration designs will be accepted as meeting performance standards when the following conditions are met: 1. The post-development flow duration curve lies strictly on or below the predevelopment curve at the lower limit of the range of flow control(between 50%of the 2-year and the 2-year).10 2. At any flow value within the upper range of flow control(from the 2-year to the 50-year),the post- development duration of the flow is no more than 1.1 times the predevelopment flow duration. 3. The target duration curve may not be exceeded along more than 50%of the range of control. 4. Where a facility or BMP is used to meet the LID Performance Standard,the post-development flow duration curve lies strictly on or below the predevelopment curve for the range of pre-developed discharge rates for the LID Performance standard(from 8%of the 2-year peak flow to 50% of the 2-year peak flow)." 10 For small projects,the lower limit of the range of control for flow control duration standard matching existing site conditions is considered met with a minimum diameter(0.25 inches)lower orifice in a low head facility(maximum effective storage depth of 3 feet)where full duration control cannot be achieved at the lower limit. Predeveloped flow durations,within allowed tolerances, must be met for all flows above the best achievable lower limit.The LID Performance standard must also be met; performance results could be influenced by the minimum diameter. 11 See Core Requirement#9 and Appendix C for application of pre-sized on-site BMPs for mitigating the LID Performance standard in lieu of explicit modeling 12/12/2016 2017 City of Renton Surface Water Design Manual 3-36 3.3.3 CONVEYANCE SYSTEMS DESIGN WITH THE RUNOFF FILES METHOD 3.3.3 CONVEYANCE SYSTEM DESIGN WITH THE RUNOFF FILES METHOD This section provides guidance for use of the Runoff Files method in determining peak flows for the design and analysis of conveyance elements,overflow structures, and other peak flow sensitive drainage features. Rainfall events that create the highest rates of runoff from developed areas are typically shorter in duration and are characterized by brief periods of high intensity rainfall. To simulate the runoff from higher intensity, shorter duration rainfall events, a 15-minute time series is used. The following is the typical procedure for hydrologic design and analysis of conveyance facilities using the Runoff Files method: 1. Select and delineate appropriate subbasins. a) Select separate subbasins for major drainage features and important conveyance points. b) Identify existing land covers offsite and post-development land covers onsite. c) Identify soil types by using the SCS soil survey or by directly evaluating the site. d) Convert SCS soil types to the approved model soil classifications. 2. Determine hydrologic parameters for each subbasin. a) Within the approved model,locate the project to determine appropriate rainfall region and/or regional scale factor. b) Categorize soil types and land cover per Table 3.2.2.A and Table 3.2.2.13. c) Determine total impervious areas and effective impervious areas within each subbasin. d) Determine areas for each soil/cover type in each subbasin. 3. Determine peak flows for the conveyance element being analyzed. a) Following the approved model guidance, assemble the post-development scenario including an element for each subbasin and using 15-minute time steps. b) Set the point of compliance at the confluence of the post-developed subbasins being routed to the conveyance element. Run the scenario for the developed subbasins and conduct a flow frequency analysis on the results of the scenario run. From this analysis the 10-year,25-year, and 100-year peak flows can be determined. These design flows can then be used to size or assess the capacity of pipe systems,culverts,channels, spillways, and overflow structures. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-37 SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 3.3.4 SAFETY FACTORS IN HYDROLOGIC DESIGN It is often appropriate to apply safety factors to detention volumes or conveyance design flows. This manual does not require safety factors for detention or conveyance design,but it does recommend the use of safety factors when the designer believes the results of the approved model are not sufficiently conservative given local conditions. The approved model methodology does not include inherent safety factors as it is meant to account for "average"conditions. On a particular site, the approved model may overestimate or underestimate flow rates and detention volumes. Within any soil/cover group,there is a range of hydrologic response dependent on local soil and geologic conditions for which the approved model methodology does not account. The USGS regional parameters for HSPF that were used to create the runoff files produce"average"runoff time series that overestimate peak flows in some basins and underestimate them in others. Similarly,the detention volumes designed with the approved model for a given conversion type are in the middle of the range of volumes that would be created if exact local hydrologic conditions were known for every project of that type. Therefore, some of the detention facilities designed with the approved model are oversized and some are undersized, depending on variable site conditions. Because of the uncertainty in local hydrologic response, the City recommends, but does not require, that a volume safety factor of 10%be applied to all detention facilities. If downstream resources are especially sensitive, or if the designer believes that the approved model significantly overestimates predevelopment flows or underestimates post-development flows, a volume safety factor of up to 20%may be appropriate. If a volume safety factor is applied to a detention facility,the volume should be increased by the given percentage at each one-foot stage increment. Safety factors for conveyance systems should be evaluated with respect to the potential damages and costs of failures due to backwatering, overtopping, etc. Applications of safety factors fall strictly within a professional engineer's judgment and accountability for design. Section 4 of the Technical Information Report should state what safety factor was applied to the design of the flow control facility. 3.3.5 DESIGN OPTIONS FOR ADDRESSING DOWNSTREAM DRAINAGE PROBLEMS See Chapter 1,Table 1.2.3.A for options for addressing downstream drainage problems. 3.3.6 POINT OF COMPLIANCE ANALYSIS The point of compliance is the location where flow control performance standards are evaluated. In most cases,the point of compliance is the outlet of a proposed detention facility where, for example,2- and 10-year discharges must match predevelopment 2-and 10-year peak flow rates. The point of compliance for hydrologic control moves downstream of the detention facility outlet or the property boundary under the following circumstances: 1. The proposed project discharges to an offsite closed depression with a severe flooding problem per Section 1.2.2,and the project adds impervious surface greater than or equal to 10%of the 100-year water surface area of the closed depression(see Table 1.2.3.A). In these cases,the closed depression becomes the point of compliance,and the engineer must ensure that project site runoff does not aggravate the flooding problem(or create a new flooding problem). 2. The proposed project includes an onsite runoff bypass,a small developed area that bypasses the flow control facility(see Section 1.2.3.2). In such cases,runoff from the remainder of the project site is overdetained so that the sum of the detained and undetained flows meets the required flow control performance standard. The point of compliance for such projects is where the onsite bypass flows join the detained flows. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-38 3.3.6 POINT OF COMPLIANCE ANALYSIS 3. The proposed project bypasses offsite flows around an onsite closed depression,ponding area, or wetland(see Section 3.3.7).As with onsite bypasses,the point of compliance in this case is where detained flows converge with the bypassed flows. The approved model allows multiple points of compliance for evaluating runoff performance within a scenario. The automatic facility sizing routine in the approved model requires a point of compliance to size an individual facility; a separate point of compliance is required for downstream evaluation. See the approved model user's documentation for modeling application of points of compliance to meet the requirements of this manual. Note: When controlling flow durations at a downstream point of compliance to demonstrate no adverse impact, the 10%tolerance specified for Level 2 performance may not be used. Predevelopment condition flow durations should be matched to the extent feasible for all flows above the level of concern. The resultant facility should also be checked to verb that the minimum onsite performance standard(e.g., Level 1, Level 2, or Level 3 per Section 1.2.3.1)has also been met. ❑ OFFSITE CLOSED DEPRESSIONS If a project drains to an offsite closed depression with existing or potential flooding problems,then the water surface levels of the closed depression must not be allowed to increase for return frequencies at which flooding occurs,up to and including the 100-year frequency. This section describes the point of compliance analysis necessary to size detention facilities discharging to such a closed depression.If the closed depression is classified as a wetland, other requirements apply per Section 1.2.2, Core Requirement 42. The closed depression is first modeled(using the site's predevelopment condition)to determine the return frequency at which flooding currently occurs and the water levels associated with return frequencies in excess of this frequency. These flooding levels and their probabilities dictate the detention performance for the proposed development.The proposed detention facility is then iteratively sized such that discharge from the site's post-development condition does not increase water surface levels for the frequencies at which flooding occurs that is,after development,water level frequency curves must match for all frequencies equal to or greater than the frequency at which flooding occurs(up to the 100-year water level). The infiltration rate must be determined in order to accurately model the closed depression.In the case of a closed depression with an existing flooding problem, the infiltration rate is most realistically depicted by calibrating the model to known flooding events. This should be done using the full historical runoff files and setting the closed depression outflow(infiltration)such that recorded or anecdotal levels of flooding occur during the same storm events in the historical record. Where a flooding problem might be created by discharge of post-development flows to a closed depression, and in the absence of information on dates and water surface levels in the closed depression during past runoff events,infiltration rates must be determined through testing as follows: • For a closed depression without standing water,two or more test pits should be dug in the bottom of the closed depression to a depth of 10 feet or to the water table,whichever is reached first. The test pits shall be dug under the supervision of a geotechnical engineer, and a test pit log shall be kept. Evidence of high water table shall be noted. • If the test pit reveals deep homogeneous permeable material with no evidence of a high water table, then infiltration tests shall be performed in the bottom of the closed depression at locations of similar elevation and on opposite sides of the bottom area(as feasible). Surface infiltration rates shall be determined using the methods for assessing measured infiltration rates included in Section 5.2. The measured rates should be used directly,without applying correction factors. • If the closed depression has standing water or is a defined as a wetland according to RMC 4-3-050, or if test pits show evidence of a high water table or underlying impermeable material,then procedures for determining infiltration rates will be established on a case-by-case basis in coordination with CED. 2017 City of Renton Surface Water Design Manual 12/12/2016 3-39 SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS • In the event that a closed depression with a documented severe flooding problem is located on private property and all reasonable attempts to gain access to the closed depression have been denied,the Flood Problem Flow Control Standard shall be applied with a 20% factor of safety on the storage volume. ❑ ONSITE RUNOFF BYPASS It is sometimes impractical to collect and detain runoff from an entire project area, so provisions are made to allow undetained discharge from onsite bypass areas(see Section 1.2.3.2)while overdetaining the remainder of the runoff to compensate for unmitigated flows.A schematic of an onsite runoff bypass is shown in Figure 3.3.6.A. For projects employing onsite runoff bypass, flow control performance standards are evaluated at the point of compliance,the point where detained and undetained flows from the project site are combined. Point of Compliance Analysis for Onsite Bypass Areas 1. In the approved model,create a predeveloped condition element for the entire project area including the predevelopment detained area and the predevelopment bypass area. Route the scenario and apply the analysis tools to determine flow targets (either flow frequencies or durations, depending on the applicable design standard) from the predeveloped condition runoff time series. 2. Create and route separate developed condition elements for the detained area and the bypass area, producing a separate time series for each area. 3. Ensure that the flow characteristics of the developed runoff element for the bypass area do not exceed the targets determined in Step 1 or the 0.4 cfs threshold in Core Requirement 43. If the bypass area flows exceed the targets or threshold,then the bypass is not feasible. 4. Estimate allowable release rates from the detention facility for each return period of interest with the following equation: Allowable release=(Total Project Area F1oW)preaeveloped Iona.—(Bypass Area F1oW)aevelopea cond. Note: WWHM 2012 and later supports the direct sizing of onsite detention facilities based on the results at a downstream point-of-compliance. See the WWHM user's documentation for further details. 1. Develop a preliminary design of the flow control facility based on the estimated release rate(s). 2. Route post-development flows from the detained area through the detention facility to create a detention facility outflow time series.Provide a downstream point of compliance and route the bypass area and the facility outflow to the downstream POC. 3. The approved model determines the total project post-development outflow by adding the detention facility outflow runoff time series to the post-development runoff time series from the bypass area at the downstream point of compliance. Check characteristics of the total project post-development outflow against the targets determined in Step 1. 4. If compliance is not achieved(e.g.,2-and 10-year post-development flows exceed 2-and 10-year predevelopment flows),revise the facility design(or revise the project design to reduce the bypass area)and repeat Steps 6 through 8. For WWHM 2012 and later, Steps 6 through 8 have been automated for facility sizing by using the point of compliance option in the facility element of the model. See the WWHM user's documentation for guidance. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-40 3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS FIGURE 3.3.6.A SCHEMATIC OF AN ONSITE RUNOFF BYPASS 1 � YPASS � AREA I I , , � J DETAI ED � �•�• r' open i spme �.� detertim *JI 1 tract ,. i 1 3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS Onsite closed depressions,ponding areas, and wetlands require special consideration when determining detention performance targets;if altered,they can shift the point of compliance downstream. However,the critical areas code(RMC 4-3-050)regulates wetlands(note that most closed depressions and ponding areas are wetlands by definition)and generally does not permit alteration through either filling or gross hydrologic changes such as bypassing offsite flows.Note:Post-development discharges to offsite closed depressions,ponding areas, or wetlands (with the exception of those in Flood Problem Flow Control Areas per the Flow Control Applications Map or those discussed in Section 3.3.6)are normally not required to meet special performance standards unless there is a severe flooding problem as defined in Section 1.2.2. ❑ GENERAL REQUIREMENTS The following general requirements apply to onsite closed depressions,ponding areas,and wetlands (referred to below as"features"): 1. Flow attenuation provided by onsite wetlands and ponding areas, and storage provided by onsite closed depressions must be accounted for when computing both existing onsite and offsite flows. • Existing onsite flows must be routed through onsite wetlands and ponding areas to provide accurate target release rates for the developed site.Note: Closed depressions will have no ou flow for some portions of the site for some events, although overflow may occur during extreme events. • Existing offsite flows will increase at the project boundary if the feature is filled or if the offsite flows are bypassed around the feature. To compensate,post-development onsite flows must be 2017 City of Renton Surface Water Design Manual 12/12/2016 3-41 SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS overdetained, and the point of compliance will shift downstream to where the detained flows converge with the bypassed offsite flows. 2. If the onsite feature is used for detention,the 100-year floodplain must be delineated considering developed onsite and existing offsite flows to the feature.Note:Additional storage volume may be necessary within the feature, and the point of compliance is the discharge point from the feature. 3. If the detention facility for the proposed project discharges to an onsite wetland,ponding area, or closed depression that is not altered12 by the proposed project,AND Flow Control Duration or Flood Problem Flow Control is provided,the point of compliance is the discharge point of the detention facility,not the outlet of the onsite feature. If Peak Rate Flow Control is being provided, the point of compliance is the outlet of the onsite feature. ❑ FLOODPLAIN DELINEATION FOR LAKES,WETLANDS, CLOSED DEPRESSIONS,AND PONDING AREAS A minor floodplain analysis is required for onsite or adjacent lakes,wetlands, and closed depressions that do not have an approved floodplain or flood hazard study(see Section 4.4.2;note the exceptions). Minor floodplain studies establish an assumed base flood elevation below which development is not allowed. The following are guidelines for minor floodplain analysis of volume sensitive water bodies: 1. Create time series representing tributary flows to the feature from the entire tributary area. Where the feature is contained entirely onsite and where no offsite flows exist,use the tributary area for the proposed developed condition. 2. Where the feature is only partially onsite, or where there are offsite flows to the feature, assume the entire tributary area is fully built out under current zoning, accounting for required open space and protected critical areas in the basin as well as impervious surfaces and grass. 3. For potential future development,assume detention standards per Section 1.2.3.1.For simplicity the proposed detention may be simulated with a single assumed detention pond just upstream of the feature.This pond should be sized to the appropriate detention standard and predevelopment condition assumption as noted in Section 1.2.3.1 and will require generating a predevelopment time series for the basin. Large water bodies may provide significant floodwater storage and may also be included in the analysis. Most existing detention in the basin,with exception of that providing duration control,will have little effect on the analysis and should be discounted. 4. Sum all subbasin time series to create a single composite time series for the drainage feature. 5. Develop routing curves for the feature.As appropriate,consider infiltration as an outflow for closed depressions. 6. Route the time series through the storage feature,generate water surface frequency curves, and note the 100-year water surface elevation. 12 Not altered means existing on-and offsite flows to the feature will remain unchanged and the feature will not be excavated or filled. 12/12/2016 2017 City of Renton Surface Water Design Manual 3-42 Section Page CHAPTER 4 4.1 Route Design and Easement Requirements 4-3 CONVEYANCE SYSTEM 4.1.1 Route Design 4-3 ANALYSIS & DESIGN 4.1.2 Easement and Setback Requirements 4-3 4.2 Pipes, Outfalls,and Pumps 4-7 4.2.1 Pipe Systems 4-7 4.2.2 Outfall Systems 4-30 6 � 4.2.3 Pump Systems 4-36 4.3 Culverts and Bridges 4-37 4.3.1 Culverts 4-37 4.3.2 Culverts Providing for Fish Passage/Migration 4-50 Y G>, 4.3.3 Bridges 4-52 4.4 Open Channels, Floodplains, and Floodways 4-55 4.4.1 Open Channels 4-55 CITY OF RENTON 4.4.2 Flood plain/Floodway Analysis 4-71 SURFACE WATER DESIGN MANUAL 2017 City of Renton Surface Water Design Manual 12/12/2016 CITY OF RENTON SURFACE WATER DESIGN MANUAL (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual CITY OF RENTON SURFACE WATER DESIGN MANUAL CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & DESIGN This chapter presents the City of Renton's approved methods for the hydraulic analysis and design of conveyance systems.A conveyance system includes all portions of the surface water system, either natural or man-made,that transports surface and storm water runoff. This chapter contains the detailed design criteria,methods of analysis, and schematic representations for all components of the conveyance system. In some cases,reference is made to other adopted or accepted design standards and criteria such as the City of Renton Standard Details and the requirements of the City of Renton Transportation department and Surface Water Utility as applicable. The figures included in this chapter are provided as schematic representations and should not be used for design. Refer to the City of Renton Standard Details for specific design information. The figures provided in this chapter illustrate one example of how the conveyance system design criteria may be applied. Although the figures are meant to illustrate many of the most important design criteria,they may not show all criteria that apply. In general,the figures are not used to specify requirements unless they are indicated elsewhere in this manual. If this manual refers to a standard detail not included in the City of Renton Standard Details,the applicant shall use the figure provided in this manual. Chapter Organization The information presented in this chapter is organized into four main sections: • Section 4.1,"Route Design and Easement Requirements" • Section 4.2,"Pipes,Outfalls, and Pumps" • Section 4.3,"Culverts and Bridges" • Section 4.4,"Open Channels,Floodplains, and Floodways" These sections begin on odd pages so the user can insert tabs if desired for quicker reference. Required vs. Recommended Design Criteria Both required and recommended design criteria are presented in this chapter. Criteria stated using"shall" or"must"are mandatory,to be followed unless there is a good reason to deviate as allowed by the adjustment process(see Section 1.4). These criteria are required design criteria and generally affect facility performance or critical maintenance factors. Sometimes options are stated as part of the required design criteria using the language"should"or"may." These criteria are recommended design criteria,but are closely related to the required criteria, so they are placed in the same section. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-1 SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS (This page intentionally left blank) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-2 CITY OF RENTON SURFACE WATER DESIGN MANUAL 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS This section presents the general requirements for aligning conveyance systems and providing easements and setbacks to allow for proper maintenance and inspection of all conveyance system elements. 4.1.1 ROUTE DESIGN The most efficient route selected for new conveyance systems will result from careful consideration of the topography of the area to be traversed,the legal property boundaries, and access for inspection and maintenance.Additionally,topography and native soil characteristics beneficial to Low Impact Development(LID) applications may influence the route. The general requirements for route design are as follows: 1. Proposed new conveyance systems should be aligned to emulate the natural conveyance system to the extent feasible. Inflow to the system and discharge from the system should occur at the natural drainage points as determined by topography and existing drainage patterns. 2. New conveyance system alignments in residential subdivisions should be located adjacent and parallel to property lines so that required drainage easements can be situated along property lines. Drainage easements should be located entirely on one property and not split between adjacent properties. 3. Exception: Streams and natural drainage channels shall not be relocated to meet this requirement. 4. Aesthetic considerations,traffic routes and on-site BMP strategies may dictate the placement and alignment of open channels.Appropriate vehicular and pedestrian traffic crossings must be provided in the design. 5. For any reach or partial reach of new conveyance (ditch,channel or closed pipe system)proposed by a project, a geotechnical analysis and report is required if the conveyance is located within 200 feet of a steep slope hazard area or landslide hazard area,OR if the conveyance is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions.A low-permeability liner per Section 6.2.4 for the trench or channel may be required if warranted by soil stability conditions. 4.1.2 EASEMENT AND SETBACK REQUIREMENTS Proposed projects must comply with the following easement and setback requirements unless otherwise approved by the City: 1. Any onsite conveyance system element(including on-site BMPs used as conveyance)constructed as part of a subdivision project shall be located in a dedicated drainage easement,tract, or right-of-way that preserves the system's route and conveyance capacity as follows: • Onsite conveyance systems within the right-of-way will be inspected and maintained by the City. • Onsite conveyance systems within the drainage easements or tracts will be maintained by the property owners through the Homeowners Association created by the subdivision(with each property owner having equal responsibility for maintenance). These conveyance elements include those entering and exiting the tract from/to the public right-of-way. For conveyance pipes entering the tract from right-of-way,responsibility begins at the last structure prior to entering the tract. For conveyance pipes exiting the tract to right-of-way,responsibility ends at the next downstream structure. The easement shall grant the City rights for inspection. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-3 SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS Exception: Roof downspout,minor yard, and footing drains do not require easements,tracts, or right- of-way. If easements are provided for these minor drains (or for other utilities such as power,gas or telephone),they need not comply with the requirements of this section. Note: except for those facilities that have been formally accepted for maintenance by the City, maintenance and repair of drainage facilities and BMPs on private property is the responsibility of the property owner. Except for the inflow pipe and discharge pipe of a City-accepted f ow control or water quality facility, the City does not normally accept maintenance of conveyance systems constructed through private property. 2. Any onsite conveyance system element(including on-site BMPs used as conveyance)constructed under a commercial building or commercial development permit shall be covered by the drainage facility declaration of covenant and grant of easement in Reference Section 8-J(or equivalent)that provides the City right of access for inspection,maintenance, and repair. Note: except for those facilities that have been formally accepted for maintenance by the City, maintenance and repair of drainage facilities on private property is the responsibility of theproperty owner. 3. Any offsite conveyance system element(including on-site BMPs used as conveyance)constructed through private property as part of a proposed project that conveys runoff from public roads within the project site shall be located in a drainage easement dedicated to the City. If an offsite conveyance system through private property is proposed by a project to convey runoff diverted from the natural discharge location,the City may require a drainage release covenant per Reference Section 8-K as a condition of approval of the adjustment required in Section 1.2.1. 4. A river protection easement per Reference Section 8-P (or equivalent) shall be required for all properties adjoining or including major rivers' that may be dedicated to the City or County as applicable. The County shall review and approve river protection easements dedicated to the County. 5. Table 4.1 lists the required widths and building setback lines for drainage easements.For all pipes or any channels or constructed swales greater than 30 feet wide, facilities must be placed in the center of the easement.For channels or constructed swales less than or equal to 30 feet wide,the easement extends to only one side of the facility. 6. Any portion of a conveyance system drainage easement(shown in Table 4.1) shall not be located within an adjacent property or right-of-way.Building setback lines may cross into adjacent property. 7. The distance between the easement line and building or other structure footings shall be no less than the building setback line(BSBL) distance shown in Table 4.1. Exception: The BSBL distance indicated in Table 4.1 may be measured from the edge of a pipe in the easement plus 2 feet if all of the following conditions are met: a) As-builts showing the location of the pipe are submitted b) A geotechnical/structure analysis demonstrates stability of the proposed structure c) Access for maintenance/replacement remains unobstructed. Major rivers are defined in the King County Flood Hazard Management Plan. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-4 4.1.2 EASEMENT AND SETBACK REQUIREMENTS TABLE 4.1 EASEMENT WIDTHS AND BUILDING SETBACK LINES For Pipes:0) BSBL Inside Diameter(ID) Easement Width (From Easement) ID<_ 36" depth to invert< 8': 10 feet(2) 5 feet depth to invert> 8': 15 feet 36" < ID <_ 60" depth to invert< 8': 10 feet(2) 7.5 feet depth to invert>8': 15 feet ID > 60" ID plus 10 feet 10 feet For Channels and Swales: BSBL Top Width of Channel (W) Easement Width (From Easement) W< 10 feet W plus 10 feet on one side 5 feet W if no access required(3) 10 feet <W< 30 feet W plus 15 feet on one side 5 feet W>30 feet W plus 15 feet on both sides 5 feet BSBL For Major Rivers Easement Width (From Easement) See the King County Flood Varies per site conditions 5 feet Hazard Management Plan for Minimum 30 feet a list of the major rivers from stable top of bank(4) Notes: Pipes installed deeper than 10 feet require one of the following actions: • Increase the BSBL such that the distance from the BSBL to the centerline of the pipe is at least 1.5 times the depth to pipe invert,or • Place a restriction on adjacent lots that the footings be placed at a specific elevation,deep enough that the closest horizontal distance from the footing to the pipe centerline is 1.5 times the difference in elevation of the footing and pipe invert,or • Place a restriction on adjacent lots that the footings be designed by a geotechnical engineer or licensed engineering geologist,such that excavation of the pipe may be performed without necessitating shoring of adjacent structures. (2) Fifteen-foot easement width is required for maintenance access to all manholes, inlets,and culverts. (3)Access is not required for small channels if the channel gradient is greater than 5%(assumes steep channels will be self-cleaning). (4) Stable top of bank shall be as determined by King County. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-5 SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-6 CITY OF RENTON SURFACE WATER DESIGN MANUAL 4.2 PIPES, OUTFALLS, AND PUMPS This section presents the methods,criteria, and schematic representations for analysis and design of pipe systems, outfalls, and pump-dependent conveyance systems. The information presented is organized as follows: Section 4.2.1, "Pipe Systems" "Design Criteria," Section 4.2.1.1 "Methods of Analysis,"Section 4.2.1.2 Section 4.2.2, "Outfall Systems" "Design Criteria," Section 4.2.2.1 Section 4.2.3, "Pump Systems" "Design Criteria," Section 4.2.3.1 "Methods of Analysis,"Section 4.2.3.2 4.2.1 PIPE SYSTEMS Pipe systems are networks of storm drain pipes,catch basins,manholes,inlets, and outfalls designed and constructed to convey surface water. The hydraulic analysis of flow in storm drain pipes typically is limited to gravity flow;however,in analyzing existing systems it may be necessary to address pressurized conditions.A properly designed pipe system will maximize hydraulic efficiency by utilizing proper material, slope, and pipe size. 4.2.1.1 DESIGN CRITERIA General In addition to the design criteria described below,pipe systems shall be design to meet the hydraulic criteria as described in Section 1.2.4.1.These criteria supersede the methodology descriptions contained in Chapter 4. All pipe material,joints,protective treatment,construction workmanship, and inspection requirements shall be in accordance with the City of Renton Standard Details and the requirements of the City of Renton Transportation Department of Surface Water Utility as applicable. Note: The pipe materials and specifications included in this section are for conveyance systems installed according to engineering plans required for City permits/approvals. Other pipe materials and specifications may be used by private property owners for drainage systems they construct and maintain when such systems are not required by or granted to City. Acceptable Pipe Sizes and Length Between Structures Acceptable pipe sizes shall be per Table 4.2.LA1. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-7 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS TABLE 4.2.1.A1 PIPE SIZES AND LENGTHS BETWEEN STRUCTURES PIPE SIZES AND LENGTHS BETWEEN STRUCTURES UPSTREAM MINIMUM SLOPE(%)AND STRUCTURE TO PIPE MIN. FULL FLOW DOWNSTREAM DIAMETER VELOCITY(FPS)AT MAXIMUM WHERE STRUCTURE (IN) DESIGN FLOW LENGTH (FT) ALLOWED Inlet to CB/MH 12 0.5%, 3.0 fps 60 Public, See Note 1 Inlet to CB/MH 8 0.5%, 3.0 fps 40 Private CB to CB 8 0.5%, 3.0 fps 100 Private CB/MH to CB/MH 12 or 0.5%, 3.0 fps 300 Public greater Note 1. Minimum pipe size shall be 12-inch diameter.8-inch diameter may be permitted on cross street laterals less than 66 feet to avoid utility conflicts or to meet shallow grade. Note 2. Maximum spacing on surface drainage course between inlets and catch basins shall be 150 on grades less than 1% and 200 feet on grades from 1%to 3%. Otherwise, maximum spacing shall be 300 feet on grades over 3%,or as required by grate flow capacities. Maximum CB to inlet spacing may need to be reduced depending on street width and inlet capacity analysis in Section 4.2.1.2. Note 3. Minimum slope and full flow velocity is desirable unless it cannot be achieved due to outlet control,site topography, burial depth or other situations or conditions. Allowable Pipe Materials and Minimum Cover Requirements 1. The designer shall have the option of constructing storm sewers, drains and culverts of the pipe types listed below within the cover limits specified. In addition,concrete pipe shall be rubber gasketed and metal pipe shall be gasketed and securely banded. Leak testing shall be conducted if required by the City Engineer. 2. The pipe materials included in Table 4.2.LA2 are allowed for use in meeting the requirements of this manual. Refer to the current edition of WSDOT/APWA Standard Specifications 7-02, 7-03 and 7-04 for detailed specifications for acceptable pipe materials. Refer to the City of Renton Standard Details for pipe materials allowed in the City road right-of-way.2, 3, 4, 5 TABLE 4.2.1.A2 ALLOWABLE PIPE MATERIALS AND MINIMUM COVER Allowed Minimum in Zone 1 Cover of the Pipe Type (ft) Public Private APA Corrugated Steel Pie 2.0 Yes Yes Yes Spiral Rib Steel Pie 2.0 Yes Yes Yes Plain Concrete Pie PCP 2.0 No Yes No Reinforced Concrete Pie RCP 1.0 Yes Yes No Corrugated or Spiral Rib Aluminum Pie 2.0 Yes Yes No Ductile Iron 1.0 Yes Yes Yes Line Corrugated Polyethylene Pie LCPE 2.0 Yes Yes Yes Corrugated Polyethylene Pie CPE —Triple Wall 2.0 Yes Yes Yes Polyvinyl Chloride Pie PVC 3.0 Yes Yes Yes Solid Wall High Density Polyethylene Pie HDPE 2.0 Yes Yes Yes For pipe specifications including acceptable pipe joints see RMC 4-3-050S. 2 Footnote 2 is not used. 3 Footnote 3 is not used. 4 Footnote 4 is not used. 5 Footnote 5 is not used. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-8 4.2.1 PIPE SYSTEMS Allowable Pipe Joints 1. Concrete pipe shall be rubber gasketed. 2. CMP shall be rubber gasketed and securely banded. 3. Spiral rib pipe shall be "hat-banded"with neoprene gaskets. 4. Ductile pipe joints shall be flanged,bell and spigot,or restrained mechanical joints. 5. PP and CPE pipe joints (lined and single wall, fully corrugated)shall conform to the current WSDOTIAPWA Standard Specifications. 6. PVC pipe,CPE pipe and PP pipe shall be installed following procedures outlined in ASTM D2321.Solid wall HDPE pipe shall be jointed by butt fusion methods or flanged according to the City of Renton Standard Details. Pipe Alignment 1. Pipes must be laid true to line and grade with no curves,bends, or deflections in any direction. 2. Exception:Vertical deflections in solid wall HDPE and ductile iron pipe with flanged restrained mechanical joint bends(not greater than 30°)on steep slopes,provided the pipe drains. 3. A break in grade or alignment, or changes in pipe material shall occur only at catch basins or manholes. Maximum Pipe Slopes and Velocities Table 4.2.1.A3 presents maximum pipe slopes and velocities by pipe material. TABLE 4.2.1.A3 MAXIMUM PIPE SLOPES AND VELOCITIES Pipe Slope above which Pipe Maximum Anchors Required and Maximum Velocity at Pipe Material Minimum Anchor Spacing Slope Allowed Full Flow CMP, Spiral Rib, PVCM 20% 30%(3) 30 fps (1 anchor per 100 LF of pipe) Concrete, CPE, or PPM 10% 20%(3) 30 fps (1 anchor per 50 LF of pipe) Ductile Iron(2) 20% None None (1 anchor per pipe section) Solid wall HDPE(2) 20% None None (1 anchor per 100 LF of pipe, cross-slope installations only) Notes: (1)These materials are not allowed in landslide hazard areas. (2) Butt-fused or flanged pipe joints are required;above ground installation is recommended on slopes greater than 40%. (3)A maximum slope of 200%is allowed for these pipe materials with no joints(one section),with structures at each end, and with proper grouting. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-9 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS Changes in Pipe Size 1. Increase or decreases in pipe size are allowed only at structures. Exceptions may be allowed as follows: Connections to pipe systems may be made without placing a catch basin or manhole on the mainline by meeting all of the following conditions: a) The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe. b) Make connections in accordance with the manufacture's recommendations. Standard shop fabricated tees,wyes and saddles shall be used,except for concrete connections constructed in accordance with the City of Renton Standard Details. c) There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external wall of the main line. d) Offset angle of connecting pipe to mainline,horizontally and vertically shall be less than 45 degrees. e) Two-point survey control shall be used to set catch basin locations. 2. When connecting pipes at structures,match any of the following(in descending order of preference): crowns, 80%diameters,6 or inverts of pipes. Side lateral connectionS7, 12 inches and smaller, are exempt from this requirement. 3. Drop manholes may be used for energy dissipation when pipe velocities exceed 10 feet per second. External drop manholes are preferred where maintenance access to the upstream pipe is preserved by use of a tee section.Internal drop structures may be approved only if adequate scour protection is provided for the manhole walls. Drop structures must be individually engineered to account for design variations, such as flow rates,velocities, scour potential and tipping forces. 4. Downsizing pipes larger than 12 inches may be allowed provided pipe capacity is adequate for design flows. Note: The above criteria do not apply to detention tanks. Structures Table 4.2.1.13 lists typical drainage structures with corresponding maximum allowable pipe sizes. 1. Catch basin(or manhole)diameter shall be determined by pipe orientation at the junction structure.A plan view of the junction structure,drawn to scale,will be required when more than four pipes enter the structure on the same plane, or if angles of approach and clearance between pipes is of concern. The plan view(and sections if necessary)must ensure a minimum distance (of solid concrete wall) between pipe openings of 8 inches for 48-inch and 54-inch catch basins, and 12 inches for 72-inch and 96-inch catch basins. 2. Evaluation of the structural integrity for H-20 loading,or as required by the City of Renton Standard Details,may be required for multiple junction catch basins and other structures. 3. Catch basins shall be provided within 50 feet of the entrance to a pipe system to provide for silt and debris removal. 6 Match point is at 80%of the pipe diameter,measured from the invert of the respective pipes. 7 Side laterals include any 8-inch or smaller pipe connected to the main conveyance system at a catch basin,or manhole,as allowed under this manual and/or the City of Renton Standard Details. In addition, 12-inch and smaller pipes that serve a single inlet point(e.g., roadway simple inlets,footing drains,and lot stubouts including manifold systems serving multiple residential lots)are also included. Excluded from this definition are inlet pipes that contribute 30%or more of the total flow into a catch basin,or that collect or convey flows from a continuous source. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-10 4.2.1 PIPE SYSTEMS 4. All solid wall HDPE pipe systems(including buried solid wall HDPE pipe)must be secured at the upstream end. The downstream end shall be placed in a 4-foot section of the next larger pipe size. This sliding sleeve connection allows for the high thermal expansion/contraction coefficient of this pipe material. 5. The maximum slope of the ground surface for a radius of 5 feet around a catch basin grate or solid lid should be 5:1 (H:V)to facilitate maintenance access. Where not physically feasible, a maximum slope of 3:1 (H:V)shall be provided around at least 50%of the catch basin circumference. 6. Catch basins(see City ofRenton Standard Details)rather than inlets shall be used to collect storm water from road surfaces,unless approved by the City Engineer. 7. Type 2(see City of Renton Standard Details)catch basins shall be used where the depth to the invert of the pipe exceeds 5 feet. 8. Manholes(see City of Renton Standard Details)may be used in lieu of catch basins if they do not collect surface water. Manholes must be used if inverts are greater than 18 feet. 9. Roof and yard drains, or other concentrated flow from adjacent property shall not discharge over the surface of roadways, sidewalks,walkways, or shoulders. 10. Catch basins or manholes are required when joining differing types of pipes. 11. The location of at least two points of all catch basins shall be surveyed to ensure that the catch basin, frame and grate will properly align with finished curb,horizontally and vertically. 12. Metal castings for drainage structures shall not be dipped,painted,welded,plugged or repaired. 13. Porosity in metal castings for drainage structures shall be considered a workmanship defect subject to rejection by the inspector. 14. Grates and covers shall be seated properly to prevent rocking,including the replacement of existing covers with solid metal covers. 15. Unless otherwise specified,vaned grates(see City of Renton Standard Details), shall be used with standard frame in the traveled way,gutter, or shoulder.Vaned grates shall not be located within crosswalks. 16. At sag vertical curves, on the end of downgrade cul-de-sacs, or before intersections with a grade four percent or greater, an analysis shall be done to assure that typical catch basin grates will collect the surface runoff. To collect excessive volumes of runoff or protect against plugged grates and overflow situations,the City Road Engineer will require the use of through inlet frames on vertical curbs, (see City of Renton Standard Details). Where the through-curb inlets cannot be used,place a catch basin at the low point and two extra inlets located not greater than 0.1 foot above the low point grate within a spacing of 25 feet. 17. New catch basins and manholes that do not collect runoff shall use solid locking covers(per City of Renton Standard Details). Existing catch basins,which no longer collect runoff, shall have their frame and grates replaced with solid covers. 18. All storm drain covers and grates need to be locking regardless of their location. 19. Slot drains may be used when approved by the City Engineer.At a minimum, slot drains shall have catch basins at either end unless used as a driveway culvert. The maximum distance between catch basins along a slot drain shall be 50 feet. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-11 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS TABLE 4.2.1.B ALLOWABLE STRUCTURES AND PIPE SIZES Maximum Pipe Diameter CMP,Spiral Rib,Solid Wall HDPE, Concrete, Catch Basin Type0) PVC,and Ductile Iron(2) CPE, PP I n let(4) 12" 12" Type 1(3) 18"(2) 12" Type 1 L(3) 24" 18" Type 2—48-inch dia. 30" 24" Type 2—54-inch dia. 36" 30" Type 2—72-inch dia. 54" 42" Type 2—96-inch dia. 72" 60" Notes: (1) Catch basins(including manhole steps,ladder,and handholds)shall conform to the City of Renton Standard Details. (2) Generally these pipe materials will be one size larger than concrete, CPE or PP due to smaller wall thickness. However, for angled connections or those with several pipes on the same plane,this will not apply. (3)A maximum of 5 vertical feet is allowed between finished grade and invert elevation. (4) Inlets are normally allowed only for use in privately maintained drainage systems and must discharge to a catch basin immediately downstream. Pipe Design between Structures The following requirements are for privately maintained or City-maintained off-road right-of-way pipe systems. See the City of Renton Standard Details for pipe design between structures in City road right-of- way. 1. Minimum velocity at full flow should be 3.0 feet per second(fps). If site constraints result in velocities less than 3 fps at full flow,impacts from sedimentation in the pipe system shall be addressed with larger pipes,closer spacing of structures, sediment basins, or other similar measures. 2. Minimum slope for 8-inch pipes shall be 0.5%;minimum slope for 12-inch or larger pipes shall be 0.2%. 3. Maximum lengths between structures shall be 300 feet(for design flows greater than 3 fps). Solid wall HDPE tightlines down steep slopes are self-cleaning and do not require structures for maintenance. Pipe Cover 1. Pipe cover,measured from the finished grade elevation to the top of the outside surface of the pipe, shall be 2 feet minimum unless otherwise specified or allowed below or as allowed above in Table 4.2.1.A2.Under drainage easements, driveways,parking stalls, or other areas subject to light vehicular loading,pipe cover may be reduced to 1 foot minimum if the design considers expected vehicular loading and the cover is consistent with pipe manufacturer's recommendations.Pipe cover in areas not subject to vehicular loads, such as landscape planters and yards,may be reduced to 1 foot minimum. 2. All flexible storm sewer pipe and culvert material shall be covered by a minimum of 2 feet of cover unless the applicant submits detailed plans accompanied by manufacturer's recommendations specifying allowable cover less than 2 feet in depth.All non-flexible storm sewer pipe and culvert material shall be covered by a minimum of 1 foot of cover. Pipe cover over concrete pipe shall comply with Table 4.2.1.C.For other pipe types,the manufacturer's specifications or other documentation shall be provided for proposed cover in excess of 30 feet. Caution:Additional precautions to protect against crushing during construction may be needed under roadways if the road bed is included to meet minimum cover requirements. Damaged pipe shall be replaced. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-12 4.2.1 PIPE SYSTEMS 3. For proposed pipe arches,the manufacturer's specifications or other documentation shall be provided for proposed cover in excess of 8 feet. 4. Pipe cover over PVC SDR 35 shall be 3 feet minimum and 30 feet maximum. TABLE 4.2.1.0 MAXIMUM COVER(FEET)FOR CONCRETE PIPE Pipe Diameter (inches) Plain Class II Class III Class IV Class V 12 18 10 14 21 26 18 18 11 14 22 28 24 16 11 15 22 28 30 11 15 23 29 36 11 15 23 29 48 12 15 23 29 60 12 16 24 30 72 12 16 24 30 84 1 12 1 16 1 24 1 30 96 12 16 24 30 108 12 16 24 30 Note:See Figure 4.2.1.A for a schematic representation. Only Class IV and V are allowed in public right-of-way. Pipe Clearances A minimum 7-foot horizontal separation and 1-foot vertical separation(measured wall to wall)is required between storm pipe and other utilities with the exception of water lines where a minimum 10-foot horizontal separation is required. Pipe Bedding, Backfill and Compaction Pipe bedding and backfill shall be in accordance with the City of Renton Standard Details.Pipe compaction shall follow the current WSDOT Standard Specifications. Where pipes pass through flood containment structures,these standards shall be supplemented and modified as necessary in accordance with standards set forth in Corps of Engineers Manual for Design and Construction of Levees (EM 1110-2-1913). Pipe System Connections Connections to a pipe system shall be made only at catch basins or manholes.No wyes or tees are allowed except on roof/footing/yard drain systems on pipes 8 inches in diameter or less,with clean-outs upstream of each wye or tee. Additional exceptions may be made provided the following conditions are met: 1. The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe. 2. Make connections in accordance with the manufacturer's recommendations. Standard shop fabricated tees,wyes and saddles shall be used, except for concrete connections constructed in accordance with the City ofRenton Standard Details. 3. There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external wall of the main line. 4. Offset angle of connecting pipe to mainline,horizontally and vertically shall be less than 45 degrees. Storm drainage pipe systems shall not penetrate building foundations, except for sump pump discharge lines used to drain crawl spaces,provided the sump pump system includes a backflow prevention or a check valve. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-13 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS Pipe Anchors Table 4.2.1.A3 presents the requirements,by pipe material, for anchoring pipe systems.Figure 4.2.1.13 and Figure 4.2.1.0 show schematic representations of pipe anchors. Spill Control Where spill control is required as specified in Section 1.2.4.3.G, allowable options are as follows: a) A tee section (see Figure 5.1.4.A)in or subsequent to the last catch basin or manhole that collects runoff from non-roof-top pollution-generating impervious surface prior to discharge from the site or into an onsite natural drainage feature.$The tee section typically provided in a wetvault or detention facility may be used to meet the intent of this requirement.Unless otherwise specified, the riser top of the tee section shall be at or above the headwater elevation for the 10-year design flow and a minimum of 6 inches below the ceiling of the catch basin or manhole. The bottom end of the tee section shall be as illustrated in Figure 5.1.4.A. b) A wall section or other device as approved by the City that provides spill control equivalent to that of the tee section specified in a)above. c) A baffle or coalescing plate oil/water separator at or subsequent to the last catch basin or manhole that collects runoff from non-roof-top pollution generating impervious surface prior to discharge from the site or into an onsite natural drainage feature. d) An active spill control plan.To use this option,the spill control plan and summary of an existing or proposed training schedule must be submitted as part of the drainage review submittal.At a minimum, such plans must include the following: • Instructions for isolating the site to prevent spills from moving downstream(shutoff valves, blocking catch basins, etc.) • Onsite location of spill clean-up materials • Phone numbers to call for emergency response • Phone numbers of company officials to notify • Special safety precautions,if applicable. Debris Barriers Debris barriers (trash racks)are required on all pipes 18 to 36 inches in diameter entering a closed pipe system. Debris barriers for pipes smaller than 18 inches and larger than 36 inches in diameter may be required depending on conditions and safety concerns. Debris barriers shall have a bar spacing of 6 inches. See Figure 4.2.1.D for a schematic representation of debris barriers on pipe ends outside of roadways. See Figure 4.2.1.E and Section 4.3 for a schematic representation of debris barriers on pipe ends(culverts) projecting from driveway or roadway side slopes. Outfalls Outfalls shall be designed as detailed in Section 4.2.2. Other Details In addition to the schematic representations provided in Figure 4.2.1.A through Figure 4.2.1.E, standard construction details are available in the City of Renton Standard Details and APWA/WSDOT Standard Plans for Road,Bridge and Municipal Construction. Commonly used details include field tapping of concrete pipe,catch basins and catch basin details,manholes and manhole details,curb inlets, frames, grates, and covers. 8 Natural onsite drainage feature means a natural swale,channel,stream,closed depression,wetland,or lake. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-14 4.2.1 PIPE SYSTEMS FIGURE 4.2.1.A SCHEMATIC REPRESENTATION OF PIPE BEDDING AND BACKFILL DESIGNS TRENCH WIDTH (SEE NOTE 3) ro° CD PIPE ZONE BACKFILL o00�000 0 QDU' 0�� w z (SEE NOTE 1) �o°O� �OO��o°O� 0 O O O o GRAVEL BACKFILL FOR 00000�0�0 o00a�0ow00 L a PIPE ZONE BEDDING O o o o o00a (SEE NOTE 2) 15% FOUNDATION LEVEL RISE O O o �oUo Cho o �o o �o o �o o �� O O O PIPE ARCHES TRENCH WIDTH (SEE NOTE 3) CD 00 00 0 PIPE ZONE BACKFILL (SEE NOTE 1) o�oo�o oo�0 0 w o GRAVEL BACKFILL FOR 0 0085%O.D. N PIPE ZONE BEDDING-- Oo 0 0 LU (SEE NOTE 2) 0 15% a 0 �' o o d FOUNDATION LEVELS 0 C) 0 Cj- 0 O.D. 0 C00 UO Q0 i CONCRETE AND DUCTILE IRON PIPE TRENCH WIDTH (SEE NOTE 3) �C) �o°O 0C, NOTE. GRAVEL BACKFILL o000�0000�0�00 ALL DETAILS FOR PIPE ZONE O0OO O o w NOT TO SCALE BEDDING o�0 0�0 o p (SEE NOTE 2) N C oO UoO O OHO 000 a FOUNDATION LEVEL JoCOO�oC�00�oC�00 a NOTES: 1. SEE CURRENT WSDOT THERMOPLASTIC PIPE STANDARD SPECIFICATIONS SECTION 7-08.3(3)FOR PIPE ZONE BACKFILL. TRENCH WIDTH 2. SEE CURRENT WSDOT PIPE ZONE (SEE NOTE 3) STANDARD SPECIFICATIONS SECTION 9-03.12(3)FOR GRAVEL BACKFILL o 0 o BACKFILL FOR PIPE ZONE (SEE NOTE 1) 00� 00� 00� Q0 BEDDING. GRAVEL BACKFILL 00�00�0 0 C 0 w 3. SEE CURRENT WSDOT FOR PIPE ZONE �COo 000 0 0 0 STANDARD SPECIFICATIONS BEDDING LO N SECTION 2-09.4 FOR (SEE NOTE 2) U 0 O a WIDTH. 0 L MEASUREMENT OF TRENCH — o 00 o 00 o a FOUNDATION 4. SEE KCSWDM 4.2.1.1 FOR LEVE o a 0- a 0o� C� CLEARANCE BETWEEN PIPES AND OTHER UTILITIES. METAL PIPE 2017 City of Renton Surface Water Design Manual 12/12/2016 4-15 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE4.2.1.B SCHEMATIC REPRESENTATION OF A PIPE ANCHOR DETAIL A� 6" MIN, 2'Mi e 6" CONCRETE M//AV/. BLOCK CONCRETE BLOCK ANCHOR NTS e PIPE BEDDING -11 6"MIN. STRAP-FOOTING ANCHOR (TYPICAL) NTS _ _ ! 6"MIN. 6"MIN. SECTION A-A NOTE: FOR SOLID WALL HDPE,PIPE MUST BE CONCRETE FREE TO SLIDE INSIDE A 4'LONG SECTION FOOTING NTS OF PIPE ONE SIZE DIAMETER LARGER. KEYEDINTO UNDISTURBED SOIL AS SHOWN I,'I IIII 1"MIN. DIAMETER STEEL ROD(STRAP) CLAMPED SECURELY � ..- r TO PIPE. 6"MIN. 'I -� CONCRETE FOOTING AIN. KEYEDINTO UNDISTURBED SOIL AS SHOWN 2 x PIPE DIA. MIN. SECTION B-B NTS 12/12/2016 2017 City of Renton Surface Water Design Manual 4-16 4.2.1 PIPE SYSTEMS FIGURE 4.2.1.0 'SCHEMATIC REPRESENTATION OF CORRUGATED METAL PIPE COUPLING AND/OR GENERAL PIPE ANCHOR ASSEMBLY X12" � SMOOTH COUPLING BAND FOR SMOOTH PIPE NTS MATERIAL TO BE Y ASTM A36%4" PLATE GALVANIZED AFTER FABRICATION PER ASTM A123 �- ALL HOLES 3/4" DIAM. SLOTS TO BE m 119/32 X 3/4�� + + + + + PIPE STAKES 7"COUPLING BAND 4Y2" 4Y2" 12" OR 24"COUPLING BAND 12" PLATE DETAIL NTS COUPLING BAND V COLLAR(2" PIPE) WELD WELD PLATE SEE DETAIL MATERIAL TO BE ASTM A36 1 2"X 6' PIPE U GALVANIZED AFTER STAKES EACH FABRICATION PER ASTM Al 53 SIDE OF CULVERT NOTES: 1. THE SMOOTH COUPLING BAND SHALL BE USED IN FLATTEN TO POINT COMBINATION WITH CONCRETE PIPE. 2. CONCRETE PIPE WITHOUT BELL AND SPIGOT SHALL NOT BE INSTALLED ON GRADES IN EXCESS OF 20%. ANCHOR ASSEMBLY 3. THE FIRST ANCHOR SHALL BE INSTALLED ON THE CORRUGATED METAL PIPE FIRST SECTION OF THE LOWER END OF THE PIPE AND NTS REMAINING ANCHORS EVENLY SPACED THROUGHOUT THE INSTALLATION. 4. IF THE PIPE BEING INSTALLED HAS A MANHOLE OR CATCH BASIN ON THE LOWER END OF THE PIPE,THE FIRST PIPE ANCHOR MAY BE ELIMINATED. 5. WHEN CMP IS USED,THE ANCHORS MAY BE ATTACHED TO THE COUPLING BANDS USED TO JOIN THE PIPE AS LONG AS THE SPECIFIED SPACING IS NOT EXCEEDED. 6. ALL PIPE ANCHORS SHALL BE SECURELY INSTALLED BEFORE BACKFILLING AROUND THE PIPE. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-17 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.1.D SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER(OFF-ROAD RIGHT-OF- WAY) NOTES: 1. THIS DEBRIS BARRIER IS FOR USE OUTSIDE ROADWAYS ON PIPES 18" DIA.TO 36" DIA.. SEE FIGURE 4.2.1.E FOR DEBRIS BARRIERS ON PIPES PROJECTING FROM DRIVEWAY OR ROADWAY SIDE SLOPES. 2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). 3. LINED CPE PIPE REQUIRES BOLTS TO SECURE DEBRIS BARRIER TO PIPE. I PLAN NTS ISOMETRIC NTS 12" MIN. 6 �% 3/4" DIAMETER SMOOTH BARS Jill CMP OR LINED CPE 1 PIPE 45° 7— PIPE COUPLING6" MAX. (TYP.) SPOT WELD BARS TO AT SIDE VIEW LEAST 2 CORREGATIONS END VIEW OF METAL PIPE (TYPICAL) NTS NTS BOLT TO LINED CPE PIPE 12/12/2016 2017 City of Renton Surface Water Design Manual 4-18 4.2.1 PIPE SYSTEMS FIGURE 4.2.1.E SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER (IN ROAD RIGHT-OF-WAY) NOTES: 1. CMP OR LINED CPE PIPE END-SECTION SHOWN; FOR CONCRETE PIPE BEVELED END SECTION, SEE KCRDCS DRAWING NO. 7-001. 2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). i MAY BE REMOVED I i i _L_ 3/4" DIA. SMOOTH BARS WITH ENDS WELDED TO BAR-FRAME 1' MIN. 6" O.C. MAX. BAR SPACING 3/4" DIAMETER J 3 BAR F RAM E I 1 I c i BEVELED PIPE END SECTION 3"-5" FOR 18" DIA. PIPE- 5"-8" FOR 24" DIA. COUPLING 7"-9" FOR 30" DIA. &GREATER 2" X 5"ANCHOR STRIPS WELDED TO 3/4" DIA. BAR-FRAME 4 PLACES SPACED UNIFORMLY. FASTEN W/ 1/2" GALV. OR NON-CORROSIVE BOLTS &NUTS. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-19 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS 4.2.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing pipe systems for compliance with the conveyance capacity requirements set forth in Section 1.2.4, "Core Requirement 44: Conveyance System." ❑ DESIGN FLOWS Design flows for sizing or assessing the capacity of pipe systems shall be determined using the hydrologic analysis methods described in Chapter 3. ❑ INLET GRATE CAPACITY The methods described in Chapter 5, Sections 4 and 5, of the Washington State Department of Transportation (WSDOT)Hydraulics Manual may be used in determining the capacity of inlet grates when capacity is of concern,with the following exceptions: 1. Use design flows as required in Section 1.2.4 of this manual. 2. Assume grate areas on slopes are 80% free of debris; "vaned"grates,95% free. 3. Assume grate areas in sags or low spots are 50% free of debris; "vaned"grates, 75% free. ❑ CONVEYANCE CAPACITY Two methods of hydraulic analysis using Manning's equation are used sequentially for the design and analysis of pipe systems. First,the Uniform Flow Analysis method is used for the preliminary design of new pipe systems. Second,the Backwater Analysis method is used to analyze both proposed and existing pipe systems to verify adequate capacity. See Core Requirement 44, Section 1.2.4, for sizing requirements of pipe systems. Note: Use of the Uniform Flow Analysis method to determine preliminary pipe sizes is only suggested as a first step in the design process and is not required. Results of the Backwater Analysis method determine final pipe sizes in all cases. Uniform Flow Analysis Method In addition to the design criteria described below,new pipe systems shall be design to meet the hydraulic criteria as described in Section 1.2.4.1. This method is used for preliminary sizing of new pipe systems to convey the design flow (i.e.,the 10-year or 25-year peak flow rate as specified in Core Requirement 44, Section 1.2.4). Assumptions: • Flow is uniform in each pipe(i.e., depth and velocity remain constant throughout the pipe for a given flow). • Friction head loss in the pipe barrel alone controls capacity. Other head losses (e.g., entrance, exit, junction, etc.)and any backwater effects or inlet control conditions are not specifically addressed. Each pipe within the system is sized and sloped such that its barrel capacity at normal full flow (computed by Manning's equation)is equal to or greater than the design flow. The nomograph in Figure 4.2.1.F may be used for an approximate solution of Manning's equation.For more precise results, or for partial pipe full conditions, solve Manning's equation directly: 12/12/2016 2017 City of Renton Surface Water Design Manual 4-20 4.2.1 PIPE SYSTEMS V = 1.49 R2/3 S1/2 (4-1) n or use the continuity equation,Q=AV,such that: 1.49 A R2/3 Sv2 (4-2) Q n where Q = discharge (cfs) V = velocity(fps) A = area(sf) n = Manning's roughness coefficient; see Table 4.2.1.1)below R = hydraulic radius=area/wetted perimeter(ft) S = slope of the energy grade line (ft/ft) For pipes flowing partially full,the actual velocity may be estimated from the hydraulic properties shown in Figure 4.2.1.G by calculating Qfiai and Vf,,ii and using the ratio Qaesig lQf,,ii to fmd V and d(depth of flow). Table 4.2.1.1)provides the recommended Manning's"n"values for preliminary design using the Uniform Flow Analysis method for pipe systems.Note: The "n"values for this method are 15%higher in order to account for entrance, exit,junction, and bend head losses. TABLE 4.2.1.1) MANNING'S "n"VALUES FOR PIPES Type of Pipe Material Analysis Method Uniform Flow Backwater Flow (preliminary (capacity design) verification) A. Concrete pipe, lined CPE pipe and lined PP pipe 0.014 0.012 B. Annular Corrugated Metal Pipe or Pipe Arch: 1. 2-2/3"x '/2' corrugation (riveted): a. plain or fully coated 0.028 0.024 b. paved invert(40% of circumference paved): 1) flow at full depth 0.021 0.018 2) flow at 80%full depth 0.018 0.016 3) flow at 60%full depth 0.015 0.013 c. treatment 5 0.015 0.013 2. 3"x 1" corrugation 0.031 0.027 3. 6"x 2" corrugation (field bolted) 0.035 0.030 C. Helical 2-2/3"x '/2' corrugation and unlined CPE pipe 0.028 0.024 D. Spiral rib metal pipe and PVC pipe 0.013 0.011 E. Ductile iron pipe cement lined 0.014 0.012 F. Solid wall HDPE pipe (butt fused only) 1 0.009 1 0.009 2017 City of Renton Surface Water Design Manual 12/12/2016 4-21 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS Backwater Analysis Method This method is used to analyze the capacity of both new and existing pipe systems to convey the required design flow(i.e., either the 10-year or 25-year peak flow,whichever is specified in Core Requirement 44, Section 1.2.4). In either case,pipe system structures must be demonstrated to contain the headwater surface(hydraulic grade line)for the specified peak flow rate. Structures may overtop for the 100-year peak flow as allowed by Core Requirement 44. When this occurs,the additional flow over the ground surface is analyzed using the methods for open channels described in Section 0 and added to the flow capacity of the pipe system. This method is used to compute a simple backwater profile(hydraulic grade line)through a proposed or existing pipe system for the purposes of verifying adequate capacity. It incorporates a re-arranged form of Manning's equation expressed in terms offriction slope(slope of the energy grade line in ft/ft). The friction slope is used to determine the head loss in each pipe segment due to barrel friction,which can then be combined with other head losses to obtain water surface elevations at all structures along the pipe system. The backwater analysis begins at the downstream end of the pipe system and is computed back through each pipe segment and structure upstream. The friction, entrance, and exit head losses computed for each pipe segment are added to that segment's tailwater elevation(the water surface elevation at the pipe's outlet)to obtain its outlet control headwater elevation. This elevation is then compared with the inlet control headwater elevation,computed assuming the pipe's inlet alone is controlling capacity using the methods for inlet control presented in Section 4.3.1.2. The condition that creates the highest headwater elevation determines the pipe's capacity. The approach velocity head is then subtracted from the controlling headwater elevation, and the junction and bend head losses are added to compute the total headwater elevation,which is then used as the tailwater elevation for the upstream pipe segment. The Backwater Calculation Sheet in Figure 4.2.1.H may be used to compile the head losses and headwater elevations for each pipe segment. The numbered columns on this sheet are described in Figure 4.2.LL An example calculation is performed in Figure 4.2.1.J. Note: This method should not be used to compute stage/discharge curves for level pool routing purposes. Instead, a more sophisticated backwater analysis using the computer software provided with this manual is recommended as described below. Computer Applications The King County Backwater(KCBW)computer program includes a subroutine BWPIPE,which may be used to quickly compute a family of backwater profiles for a given range of flows through a proposed or existing pipe system.A schematic description of the nomenclature used in this program is provided in Figure 4.3.1.G.Program documentation providing instructions on the use of this and the other KCBW subroutines is available from King County Department of Natural Resources and Parks(DNRP). 12/12/2016 2017 City of Renton Surface Water Design Manual 4-22 4.2.1 PIPE SYSTEMS FIGURE 4.2,1,F NOMOGRAPH FOR SIZING CIRCULAR DRAINS FLOWING FULL 1,000 900 800 .0001 700 2.0 600 .0002 500 .0003 Minimum 400 .0004 .0001 Allowable .0005 Velocity .0006 (Flow!)g 3.0 300 .0008 .0002 Full 120 N .001 108 .0003 200 gg 0ii .0004 9804 .002 .0005 4.0 .0006 72 O 003 .0008 004 100 60 p .005 001 ❑ 5.0 90 54 U) .008 .002 o O 80 U) 48 .01 0 / w 6.0 70 w 003 i� cn 60 v 42 .004- w 7.0 .02 .005 O 36 LL U 50 z 33 .03 006 w 8.0 008 o w ? 40 w 30 .04 .010 0 Lu Lu a 27 :Q5 co Z 9.0 30 � 24 .08 .020 10.0 = O 21 .10 0 Q� .030 UO Zp 18 .040 w Q 15 .060 o .080 12 .100 10 10 9 8 SAMPLE USE 7 8 246 dia.CMP @ 2%slope yields 20.0 6 17cfs @ 5.4 fps velocity 5 6 (n=0.024) 4 Values per Manning6s equation Q=( 1.49 ) AR2/3 S0/2 3 n 30.0 This table can be converted to other dn6 values by applying 2 formula: 40.0 Q1 n2 Q2 n1 1 2017 City of Renton Surface Water Design Manual 12/12/2016 4-23 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.1.G CIRCULAR CHANNEL RATIOS ' 1.4 PR PORTIO AL HYDRAULIC RA DIUS —uuiui�uuuui 1.2 u u� uuiu�uuuu U zROPORT ONAL >: X \F, I\ DISCHARGE U OJ 0.8 LU PR PORTIO AL A > \ w C7 a U N 0.6 a LU a J Z O O 0.4 a O a \ 0.2 0 \ 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 RATIO OF FLOW DEPTH TO DIAMETER(d/D) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-24 4.2.1 PIPE SYSTEMS FIGURE 4.2.1.11 BACKWATER CALCULATION SHEET o C14 = W cc g -a — °r° m ami o �-- mcc = J „ L -u Q 2 U O U W 'X R co T Wco = J r W J `i L J � W = W v C> r U- D w J T W to w ca Q CO m N c0 C Q Cr m — W O W _ aD -O CIO e- N a- W N c� J � � U C m 0 o U E o m m U 2017 City of Renton Surface Water Design Manual 12/12/2016 4-25 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.1.I BACKWATER CALCULATION SHEET NOTES' Column(1) Design flow to be conveyed by pipe segment. Column(2) Length of pipe segment. Column(3) Pipe Size;indicate pipe diameter or span x rise. Column(4) Manning's"n"value. Column(5) Outlet Elevation of pipe segment. Column(6) Inlet Elevation of pipe segment. Column(7) Barrel Area,this is the full cross-sectional area of the pipe. Column(8) Barrel Velocity;this is the full velocity in the pipe as determined by: V= QiA or Col.(8) = Col.(1)/Col.(7) Column(9) Barrel Velocity Head = T'�/2g or (Col.(8))%2g where g=32.2 ft/sec (acceleration due to gravity) Column(10) Tailwater(TW)Elevation;this is the water surface elevation at the outlet of the pipe segment.If the pipe's outlet is not submerged by the TW and the TW depth is less than(D+d)l2,set TW equal to(D+d)12 to keep the analysis simple and still obtain reasonable results (D=pipe barrel height and d=critical depth,both in feet.See Figure 4.3.1.E for determination of d). Column(11) Friction Loss = SfxL [or Sfx Col.(2)] where Sfis the friction slope or head loss per linear foot of pipe as determined by Manning's equation expressed in the form: Sf= (nP)/2.22 R'33 Column(12) Hydraulic Grade Line(HGL)Elevation just inside the entrance of the pipe barrel;this is determined by adding the friction loss to the TW elevation: Col.(12) =Col.(11)+Col.(10) If this elevation falls below the pipe's inlet crown,it no longer represents the true HGL when computed in this manner.The true HGL will fall somewhere between the pipe's crown and either normal flow depth or critical flow depth,whichever is greater.To keep the analysis simple and still obtain reasonable results(i.e.,erring on the conservative side),set the HGL elevation equal to the crown elevation. Column(13) Entrance Head Loss = Ke x V12g [or Ke x Col.(9)] where Ke = Entrance Loss Coefficient(from Table 4.3.LB).This is the head lost due to flow contractions at the pipe entrance. Column(14) Exit Head Loss = 1.0 x V12g or 1.0 x Col.(9) This is the velocity head lost or transferred downstream. Column(15) Outlet Control Elevation = Col.(12)+Col.(13)+Col.(14) This is the maximum headwater elevation assuming the pipe's barrel and inlet/outlet characteristics are controlling capacity.It does not include structure losses or approach velocity considerations. Column(16) Inlet Control Elevation(see Section 4.3.1.2,for computation of inlet control on culverts);this is the maximum headwater elevation assuming the pipe's inlet is controlling capacity.It does not include structure losses or approach velocity considerations. Column(17) Approach Velocity Head;this is the amount ofhead/energy being supplied by the discharge from an upstream pipe or channel section, which serves to reduce the headwater elevation.If the discharge is from a pipe,the approach velocity head is equal to the barrel velocity head computed for the upstream pipe.If the upstream pipe outlet is significantly higher in elevation(as in a drop manhole)or lower in elevation such that its discharge energy would be dissipated,an approach velocity head of zero should be assumed. Column(18) Bend Head Loss = Kb x V12g [or Kb x Col.(17)] where Kb = Bend Loss Coefficient(from Figure 4.2.LK).This is the loss of head/energy required to change direction of flow in an access structure. Column(19) Junction Head Loss.This is the loss in head/energy that results from the turbulence created when two or more streams are merged into one within the access structure.Figure 4.2.LL may be used to determine this loss,or it may be computed using the following equations derived from Figure 4.2.1.L: Junction Head Loss = K x V%2g [or Kj x Col.(17)] where Kf is the Junction Loss Coefficient determined by: K, — (Q31Qi)/(1.1$+0.63(Q31Qi)) Column(20) Headwater(HW)Elevation;this is determined by combining the energy heads in Columns 17,18,and 19 with the highest control elevation in either Column 15 or 16,as follows: Col.(20) = Col.(15 or 16)-Col.(17)+Col.(18)+Col.(19) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-26 4.2.1 PIPE SYSTEMS FIGURE 4.2.1J BACKWATER PIPE CALCULATION EXAMPLE co N rn ami'. �. =W c o 0 e 9t =J o OW C> fA III a Z W m=� o No 0 o L I vQ>=v o o c 1 rn aoo. o 0 0 Q o �r.OUW M , o c 0`� r r r _ t ~ vxMCO 'on aT aro I V p0- LQ wr W =� o o C; o_ W 1 vaNO N O r W = Ill 0 O O O 900 BEND cv 0 R t=� ,` M III =W=Ei - o Mo 00 U o r r Z 0 2 4 N C ~ o w o _o a' 00 to J� t7 j =1itOO V a�n c"o, M can' J m> Ln ai m �n Q r V m Q rn r I I W V $ o ©' cW� o c r N FLOW JUNCTION m Wch r>I�� O W a� >„ U o Q,=6 ds ~ 0=W- Co 0 Q o 0 0 Q3=4 ds �O w N N N Y > c ' o 0 U V ain 10 0- 00 V Q r r o ,0 o L II d cfl m _ oo m' '00 �\ Jv � V 0 cLEo _ III in0 O r N H 2017 City of Renton Surface Water Design Manual 12/12/2016 4-27 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.1.x BEND HEAD LOSSES IN STRUCTURES 1.2 r D 1.0 Y � Y 0.8 c Bend at Manhole, 29 no Special Shaping o JDeflector 0.6 Curved ABendat Manhole, Curved or Deflector Y 0.4 Curved Sewer r/DI=2 0.2 . .... Sewer r/D>6 0.0 0. 20' 40' 60' 80' 90' 100' Deflection Angle Y,Degrees 12/12/2016 2017 City of Renton Surface Water Design Manual 4-28 4.2.1 PIPE SYSTEMS FIGURE 4.2.1.L JUNCTION HEAD LOSS IN STRUCTURES 3.4 3.2 Q1 Qz Q3 = 100% 3.0 Q1 2.8 Q3 2.6 Typical junction chamber Q3= 130,Q2=195, Q3 65 V1 =13.5, V2 =12.3 2.4 Q3TQ7- 0.50(50%) Head Loss=0.940 2.2 Q3 = 50% 2.0 Q1 t � 1.8 R m = 1.6 1.4 Q = 30% 1.2 1.0 Graphic Example .8 .6 Q = 10% 0or ;F0 .4 .2 0 2 4 6 8 10 12 14 16 18 20 Velocity in upstream pipe, V (fps) 2017 City of Renton Surface Water Design Manual 12/12/2016 4-29 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS 4.2.2 OUTFALL SYSTEMS Properly designed outfalls are critical to ensuring no adverse impacts occur as the result of concentrated discharges from pipe systems and culverts,both onsite and downstream. Outfall systems include rock splash pads, flow dispersal trenches,gabion or other energy dissipaters, and tightline systems.A tightline system is typically a continuous length of pipe used to convey flows down a steep or sensitive slope with appropriate energy dissipation at the discharge end. In general,it is recommended that conveyance systems be designed to reduce velocity above outfalls to the extent feasible. 4.2.2.1 DESIGN CRITERIA General At a minimum, all outfalls shall be provided with a rock splash pad(see Figure 4.2.2.A)except as specified below and in Table 4.2.2.A: 1. The flow dispersal trench shown in Figure 4.2.2.13 shall only be used as an outfall as described in Core Requirement 41, Section 1.2.1. 2. For outfalls with a velocity at design flow greater than 10 fps, a gabion dissipater or engineered energy dissipater shall be required.Note the gabion outfall detail shown in Figure 4.2.2.D is illustrative only; a design engineered to specific site conditions is required. Gabions shall conform to WDSOT/APWA specifications. 3. Engineered energy dissipaters,including stilling basins, drop pools,hydraulic jump basins,baffled aprons, and bucket aprons, are required for outfalls with velocity at design flow greater than 20 fps. These should be designed using published or commonly known techniques found in such references as Hydraulic Design of Energy Dissipaters for Culverts and Channels,published by the Federal Highway Administration of the United States Department of Transportation; Open Channel Flow,by V.T. Chow;Hydraulic Design of Stilling Basins and Energy Dissipaters,EM 25,Bureau of Reclamation(1978); and other publications, such as those prepared by the Soil Conservation Service (now Natural Resource Conservation Service). Alternate mechanisms, such as bubble-up structures (which will eventually drain) and structures fitted with reinforced concrete posts,may require an approved adjustment and must be designed using sound hydraulic principles and considering constructability and ease of maintenance. 4. Tightline systems shall be used when required by the discharge requirements of Core Requirement 41 or the outfall requirements of Core Requirement 44. Tightline systems may also be used to prevent aggravation or creation of a downstream erosion problem. 5. Flood closure devices shall be provided on new outfalls passing through existing levees or other features that contain floodwaters. Such structures shall be designed to the Corps of Engineers Manual for Design and Construction ofLevees (EM 1110-2-1913). 6. Backup (secondary gate)closure devices shall be required for new outfalls through flood containment levees unless this requirement is specifically waived by the City. 7. New outfalls through levees along the Green River between River Mile 6 and State Route 18 shall comply with the terms of the adopted Lower Green River Pump Operation Procedures Plan. Tightline Systems 1. Outfall tightlines may be installed in trenches with standard bedding on slopes up to 40%. In order to minimize disturbance to slopes greater than 40%,it is recommended that tightlines be placed at grade with proper pipe anchorage and support. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-30 4.2.2 OUTFALL SYSTEMS 2. Solid wall HDPE tightlines must be designed to address the material limitations,particularly thermal expansion and contraction and pressure design, as specified by the manufacturer. The coefficient of thermal expansion and contraction for solid wall HDPE is on the order of 0.001 inch per foot per Fahrenheit degree. Sliding sleeve connections shall be used to address this thermal expansion and contraction. These sleeve connections consist of a section of the appropriate length of the next larger size diameter of pipe into which the outfall pipe is fitted. These sleeve connections must be located as close to the discharge end of the outfall system as is practical. 3. Solid wall HDPE tightlines shall be designed and sized using the applicable design criteria and methods of analysis specified for pipe systems in Section 4.2.1. 4. Due to the ability of solid wall HDPE tightlines to transmit flows of very high energy, special consideration for energy dissipation must be made.A schematic representation of a"gabion mattress energy dissipater"has been provided as Figure 4.2.2.D .Flows of very high energy will require a specifically engineered energy dissipater structure, as described above in General Criterion 43. Caution, the in-stream sample gabion mattress energy dissipater may not be acceptable within the ordinary high water mark offish-bearing waters or where gabion will be subject to abrasion from upstream channel sediments.A four-sided gabion basket located outside the ordinary high water mark should be considered for these applications. TABLE 4.2.2.A, ROCK PROTECTION AT OUTFALLS Discharge Velocity at Design Flow(fps) REQUIRED PROTECTION Greater Less than Minimum DimensionsM than or equal to Type Thickness Width Length Height 0 5 Rock 1 foot Diameter 8 feet Crown lining(2) + 6 feet or + 1 foot 4 x diameter, whichever is greater 5 10 Riprap(3) 2 feet Diameter 12 feet Crown + 6 feet or + 1 foot or 4 x diameter, 3 x diameter, whichever is whichever is greater greater 10 20 Gabion As required As required As required Crown outfall + 1 foot 20 N/A Engineered energy dissipater required (1)These sizes assume that erosion is dominated by outfall energy. In many cases sizing will be governed by conditions in the receiving waters. (2) Rock lining shall be quarry spalls with gradation as follows: Passing 8-inch square sieve: 100% Passing 3-inch square sieve: 40 to 60%maximum Passing 3/4-inch square sieve: 0 to 10%maximum (3) Riprap shall be reasonably well graded with gradation as follows: Maximum stone size: 24 inches(nominal diameter) Median stone size: 16 inches Minimum stone size: 4 inches Note:Riprap sizing governed by side slopes on outlet channel is assumed to be approximately 3:1. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-31 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.2.A SCHEMATIC REPRESENTATION OF PIPE/CULVERT DISCHARGE PROTECTION+ q CHANNEL q O REQUIRED DIMENSIONS LL8'FOR ROCK LINING AYB 12' FOR RIP RAP (4'MIN.) SEE TABLE 4.2.2.A. Q m m LL ILLO d d O O I 2' 30° ppr MIN. MIN. .. .. ................j7 m m �/ ILL ILL O O LU w O O PLAN NTS PLACE ROCK 1'ABOVE CROWN BOTH SIDES OF CHANNEL FOR"A"<8' ONE SIDE OF CHANNEL FOR"A">8' - CMIN. DISCHARGE PIPE , FILTER FABRIC LINER UNDER 1'OR 2' ROCK ROCK THICKNESS SEE TABLE 4.4.1.A SECTION A-A NTS 12/12/2016 2017 City of Renton Surface Water Design Manual 4-32 4.2.2 OUTFALL SYSTEMS FIGURE 4.2.2,B SCHEMATIC REPRESENTATION OF A FLOW DISPERSAL TRENCH PIPE O.D. 11-0 11-0 MIN. MIN. END CAP OR PLUG FLOW TO SECOND DISPERSAL TRENCH CLEAN OUT WYE FROM PIPE IF NECESSARY a a MIN 6"PERFORATED PIPE LAID FLAT/LEVEL TYPE I CB TYPE I CB W/SOLID W/SOLID COVER(LOCKING) COVER A ° A 50'MAX. INFLUENT PIPE(MAX DESIGN a FLOW<0.5 CFS PER TRENCH) 71 CLEAN OUT WYE FROM PIPE a � FLOW TO OTHER BRANCHING CB'S AS NECESSARY NOTCHED PLAN 18"O.C.- GRADE BOARD NTS 2"X 2" NOTCHES 2" 18"O.C.' GALVANIZED PIPE O.D. BOLTS, SEE 1'-0 1'-0 20%MAX' 2"GRADE BOARD NOTE 4 MIN. MIN. SES 6 NOTCHES* 'FOR WATER QUALITY FACILITIES, \_xZ �X'A ��AA�� SEE SECTION 6.2.6.1, OPTION A FOR A � x\� NOTCH DIMENSIONS AND SPACING 2 Xx12A /�� FILTER FABRIC TRENCH LINER g BOARD%V�%V� ° °a '�/j CLEAN(<5% FINES)q"- 1 "'WASHED ROCK MIN 6"PERFORATED PIPE LAID FLAT 4"X 4" V� a a NOTES: SUPPORT �� a ° 1. THIS TRENCH SHALL BE CONSTRUCTED TO POST PREVENT POINT DISCHARGE AND/OR EROSION. 2. TRENCHES MAY BE PLACED NO CLOSER THAN 50 FEET TO ONE ANOTHER(100 FEET ALONG a FLOWLINE). 3 TRENCH IGN TO FOLLOW CONTOURS LEVEL. OURS OF SITE. ALFOLLOW 4. VERTICALLY SLOTTED BOLT HOLES,2"SLOT LENGTH,ALLOWED FOR GRADE BOARD LEVEL 6„ FASTEN ADJUSTMENT. PROVIDE BACKING WASHERS AND MIN. SECTION A-A 5. SUPPORT POST SPACING AS REQUIRED BY SOIL NTS CONDITIONS TO ENSURE GRADE BOARD REMAINS LEVEL. 6. 15% MAX UNLESS OTHERWISE EVALUATED AND APPROVED, SEE SECTION C.2.1.1 2017 City of Renton Surface Water Design Manual 12/12/2016 4-33 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS FIGURE 4.2.2.0 SCHEMATIC REPRESENTATION OF AN ALTERNATIVE FLOW DISPERSAL TRENCH NOTES: 1. THIS TRENCH SHALL BE CONSTRUCTED TO PREVENT POINT DISCHARGE AND/OR EROSION. GALVANIZED00" 2. TRENCHES MAY BE PLACED NO BOLTS, SEE MIN•20� �0NO'C-6 r CLOSER THAN 50 FEET TO ONE NOTE 4 S� /moi ANOTHER(100 FEET ALONG FLOWLINE). yx�j�j�A/� 3. TRENCH AND GRADE BOARD MUST 2"X 12" j o°�O��o°� �A/��AjA�� BE LEVEL. ALIGN TO FOLLOW GRADE i // // / X00000 ��A��A��`� CONTOURS OF SITE. BOARD ?3010oo 4. VERTICALLY SLOTTED BOLT HOLES, / 0 000 o � 4"X 4"SUPPORT 2"SLOT LENGTH,ALLOWED FOR "o C) 0o' 2"X 2" z_ ��j/ °off oo POST GRADE BOARD LEVEL ADJUSTMENT. NOTCHES § \/ �0�0�0� PROVIDE BACKING WASHERS AND M ° Vo° Op� o FASTEN SECURELY. 18"O.C. N � o�Q�O� CLEAN (<5/o FINES) c 5. PROVIDE SUPPORT POST SPACING 0-0 0 R -K WASHED AS REQUIRED BY SOIL CONDITIONS ROCK TO ENSURE GRADE BOARD REMAINS LEVEL. FILTER FABRIC 6. 15% MAX UNLESS OTHERWISE EVALUATED AND APPROVED,SEE SECTION A-A SECTION C.2.1.1 NTS 2"GRADE BOARD NOTCHES` `FOR WATER QUALITY FACILITIES, SEE SECTION 6.2.6.1, OPTION A FOR 2 NOTCH DIMENSIONS AND SPACING NOTCH DETAIL 18..O.C.- NTS 12/12/2016 2017 City of Renton Surface Water Design Manual 4-34 4.2.2 OUTFALL SYSTEMS FIGURE 4.2.2.D SCHEMATIC REPRESENTATION OF A GABION MATTRESS ENERGY DISSIPATOR PLAN VIEW 0 NTS OC�O OC�O oo�Ogoor,000o�00 c°nU��nU��C�U�c�Qn GABION MATTRESS NON°9-(p, \oU ° �00�000o Oo B O�o�� 0�0J00 0 000 0 0 0 000 0000,o oogo� o �a�o�u o 00 GABIONS(TYPICAL) �OUOo00 4S°PREFERRED (SIZE AS REQUIRED) NOTE: �OJoO 00 0' 0 0 0 0 O�OOo��o ,_ 00 >,0 �OpO Ooo IF PIPE DISCHARGES Oo0 0�' 'o�r,�o�' ��O000r,0 Oo Ooo Ooq VPERPENDICULAR TO STREAM o°O Oo°�O Oo°OUCPo°O O O�OU O �O�O�O� P� G OR GABIONS ARE LOCATED WITHIN THE ORDINARY HIGH O) DSO WATER MARK(OHWM)OR A o�0�0o WILL BE SUBJECT TO RIP RAP ABRASION FROM UPSTREAM SOLID WALL SEDIMENTS,A FOUR-SIDED HDPE PIPE GABION BASKET LOCATED OUTSIDE THE OHWM SHOULD BE CONSIDERED. PIPE ANCHOR 1, � GABION MATTRESS \�A l00�Q00 iVo 0 0 o o00000 00 \ Onr(O��O��O��O� o o 0000 00�o SECTION A-A SECTION B-B r EXISTING SLEEVE OF NEXT LARGER GROUND LINEV� SIZE DIAMETER PIPE FOR \j�Ajam j� THERMAL EXPANSION D AND CONTRACTION 3 x D 2 x D �0 0 0 0000 �0 000 0 0��0 AAS 0 00 Oo �Oo Foo�a o ao�o ao�o ao�o ao�o vv��` L>Iv\I A /i.<w,</\.✓ /yV/jV/jVA/j�AA/��i.��y�/ice/��A//�/ PLACE FILTER FABRIC BETWEEN SECTION C-C GABIONS AT SOIL BEDDING NTS 2017 City of Renton Surface Water Design Manual 12/12/2016 4-35 SECTION 4.2 PIPES,OUTFALLS,AND PUMPS 4.2.3 PUMP SYSTEMS As allowed in Core Requirement 44, Section 1.2.4.3,pump systems may be used for conveyance of flows internal to a site if located on private property and privately maintained. Pump systems discharging to the Green River between River Mile 6 and State Route 18 (within the Green River Flood Control Zone District)shall comply with the standards of the adopted Green River Pump Operation Procedures Plan. 4.2.3.1 DESIGN CRITERIA Proposed pump systems must meet the following minimum requirements: 1. The pump system must be privately owned and maintained. 2. The pump system shall be used to convey water from one location or elevation to another within the site. 3. The pump system must have a dual pump (alternating)equipped with an external alarm system. 4. The pump system shall not be used to circumvent any other City drainage requirements, and construction and operation of the pump system shall not violate any other City requirements. 5. The gravity-flow components of the drainage system to and from the pump system must be designed so that pump failure does not result in flooding of a building or emergency access, or overflow to a location other than the natural discharge point for the site. 6. The pump system shall have either installed emergency backup power or the ability for portable backup power generator in the event of a loss of primary power. If portable backup emergency power is provided,the appliance must include a description of how the backup power will be brought to the site during an emergency within an emergency response plan(discussed below). 7. The applicant must provide an emergency response plan that details how backup power will be activated during an emergency and include method for delivering to the site and energizing portable backup power. The emergency response plan must also describe response for pump failures including repair and replacement of damaged pumps/motors and generators. 4.2.3.2 METHODS OF ANALYSIS Pump systems must be sized in accordance with the conveyance capacity requirements for pipe systems set forth in Section 1.2.4, "Core Requirement 44: Conveyance System." 12/12/2016 2017 City of Renton Surface Water Design Manual 4-36 CITY OF RENTON SURFACE WATER DESIGN MANUAL 4.3 CULVERTS AND BRIDGES This section presents the methods,criteria, and details for hydraulic analysis and design of culverts and bridges. The information presented is organized as follows: Section 4.3.1, "Culverts" "Design Criteria," Section 4.3.1.1 "Methods of Analysis,"Section 4.3.1.2 Section 4.3.2, "Culverts Providing for Fish Passage/Migration" "Design Criteria," Section 4.3.2.1 "Methods of Analysis,"Section 4.3.2.2 Section 4.3.3, "Bridges" "Design Criteria,"Section 4.3.3.1 "Methods of Analysis," Section 4.3.3.2. 4.3.1 CULVERTS Culverts are relatively short segments of pipe of circular, elliptical,rectangular, or arch cross section. They are usually placed under road embankments or driveways to convey surface water flow safely under the embankment. They may be used to convey flow from constructed or natural channels including streams. The Critical Areas Code (RMC 4-3-050)contains definitions of streams(termed "aquatic areas")and requirements for crossing of streams. In addition to those requirements and the design criteria described below, other agencies such as the Washington State Department of Fish and Wildlife (WDFW)may have additional requirements affecting the design of proposed culverts. 4.3.1.1 DESIGN CRITERIA General In addition to the design criteria described below,culverts shall be designed to meet the hydraulic criteria as described in Section 1.2.4.1. 1. All circular pipe culverts shall conform to any applicable design criteria specified for pipe systems in Section 4.2.1. 2. All other types of culverts shall conform to manufacturer's specifications. See the City of Renton Standard Details for types of culverts allowed in City right-of-way. Headwater 1. For culverts 18-inch diameter or less,the maximum allowable headwater elevation(measured from the inlet invert)shall not exceed 2 times the pipe diameter or arch-culvert-rise at design flow(i.e.,the 10-year or 25-year peak flow rate as specified in Core Requirement 44, Section 1.2.4). 2. For culverts larger than 18-inch diameter,the maximum allowable design flow headwater elevation (measured from the inlet invert)shall not exceed 1.5 times the pipe diameter or arch-culvert-rise at design flow. 3. The maximum headwater elevation at design flow shall be below any road or parking lot subgrade. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-37 SECTION 4.3 CULVERTS AND BRIDGES Inlets and Outlets 1. All inlets and outlets in or near roadway embankments must be flush with and conforming to the slope of the embankment. 2. For culverts 18-inch diameter and larger,the embankment around the culvert inlet shall be protected from erosion by rock lining or riprap as specified in Table 4.2.2.A,except the length shall extend at least 5 feet upstream of the culvert, and the height shall be at or above the design headwater elevation. 3. Inlet structures, such as concrete headwalls,may provide a more economical design by allowing the use of smaller entrance coefficients and,hence, smaller diameter culverts. When properly designed, they will also protect the embankment from erosion and eliminate the need for rock lining. 4. In order to maintain the stability of roadway embankments,concrete headwalls,wingwalls, or tapered inlets and outlets may be required if right-of-way or casement constraints prohibit the culvert from extending to the toe of the embankment slopes.All inlet structures or headwalls installed in or near roadway embankments must be flush with and conforming to the slope of the embankment. 5. Debris barriers(trash racks) are required on the inlets of all culverts that are over 60 feet in length and are 18 to 36 inches in diameter. Debris barriers for pipes smaller than 18 inches and larger than 36 inches in diameter may be required depending on conditions and safety concerns. Debris barriers shall have a bar spacing of 6 inches. This requirement also applies to the inlets of pipe systems. See Figure 4.2.1.D and Figure 4.2.1.E for schematic representations of debris barriers. 6. For culverts 18-inch diameter and larger,the receiving channel of the outlet shall be protected from erosion by rock lining specified in Table 4.2.2.A, except the height shall be one foot above maximum tailwater elevation or one foot above the crown,whichever is higher(See Figure 4.2.2.A). 4.3.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing culverts for compliance with the conveyance capacity requirements set forth in Section 1.2.4, "Core Requirement 44: Conveyance System." ❑ DESIGN FLOWS Design flows for sizing or assessing the capacity of culverts shall be determined using the hydrologic analysis methods described in Chapter 3. ❑ CONVEYANCE CAPACITY The theoretical analysis of culvert capacity can be extremely complex because of the wide range of possible flow conditions that can occur due to various combinations of inlet and outlet submergence and flow regime within the culvert barrel.An exact analysis usually involves detailed backwater calculations, energy and momentum balance, and application of the results of hydraulic model studies. However, simple procedures have been developed where the various flow conditions are classified and analyzed on the basis of a control section.A control section is a location where there is a unique relationship between the flow rate and the upstream water surface elevation. Many different flow conditions exist over time,but at any given time the flow is either governed by the culvert's inlet geometry (inlet control) or by a combination of inlet geometry,barrel characteristics, and tailwater elevation(outlet control). Figure 4.3.1.A illustrates typical conditions of inlet and outlet control. The procedures presented in this section provide for the analysis of both inlet and outlet control conditions to determine which governs. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-38 4.3.1 CULVERTS Inlet Control Analysis Nomographs such as those provided in Figure 4.3.1.B and Figure 4.3.1.0 may be used to determine the inlet control headwater depth at design flow for various types of culverts and inlet configurations. These nomographs were originally developed by the Bureau of Public Roads—now the Federal Highway Administration(FHWA)—based on their studies of culvert hydraulics. These and other nomographs can be found in the FHWA publication Hydraulic Design of Highway Culverts,HDS No. 45(Report No. FHWA-IP-85-I S)(September 1985), or the WSDOT Hydraulic Manual. Also available in the FHWA publication, are the design equations used to develop the inlet control nomographs. These equations are presented below. For unsubmerged inlet conditions(defined by Q/AD0.5<3.5); Form 1*:HW/D = HdD+K(Q/AD0.5)M-0.5S** (4-3) Form 2*:HW/D = K(Q/ADo-s)M (4-4) For submerged inlet conditions(defined by Q/AD0.5>4.0); HW/D = c(Q/AD0.5)2+Y-0.55** (4-5) where HW = headwater depth above inlet invert(ft) D = interior height of culvert barrel(ft) H, = specific head(ft)at critical depth(dc+ W12g) Q = flow(cfs) A = full cross-sectional area of culvert barrel(sf) S = culvert barrel slope (ft/ft) K,M,c,Y = constants from Table 4.3.1.A. The specified head H,is determined by the following equation: Hc = dc+Vc'/2g (4-6) where dc = critical depth(ft); see Figure 4.3.1.F Vc = flow velocity at critical depth(fps) g = acceleration due to gravity(32.2 ft/sec'). * The appropriate equation form for various inlet types is specified in Table 4.3.1.A below. **For mitered inlets,use+0.75 instead of-0.55. Note:Between the unsubmerged and submerged conditions, there is a transition zone (3.5 < Q/AD0.5< 4.0)for which there is only limited hydraulic study information. The transition zone is defined empirically by drawing a curve between and tangent to the curves defined by the unsubmerged and submerged equations.In most cases, the transition zone is short and the curve is easily constructed. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-39 SECTION 4.3 CULVERTS AND BRIDGES TABLE 4.3.1.A CONSTANTS FOR INLET CONTROL EQUATIONS* Unsubmerged Submerged Equation Shape and Material Inlet Edge Description Form K M c Y Circular Concrete Square edge with headwall 1 0.0098 2.0 0.0398 0.67 Groove end with headwall 0.0078 2.0 0.0292 0.74 Groove end projecting 0.0045 2.0 0.0317 0.69 Circular CMP Headwall 1 0.0078 2.0 0.0379 0.69 Mitered to slope 0.0210 1.33 0.0463 0.75 Projecting 0.0340 1.50 0.0553 0.54 Rectangular Box 30°to 75°wingwall flares 1 0.026 1.0 0.0385 0.81 90°and 15°wingwall flares 0.061 0.75 0.0400 0.80 0°wingwall flares 0.061 0.75 0.0423 0.82 CM Boxes 90° headwall 1 0.0083 2.0 0.0379 0.69 Thick wall projecting 0.0145 1.75 0.0419 0.64 Thin wall projecting 0.0340 1.5 0.0496 0.57 Arch CMP 90° headwall 1 0.0083 2.0 0.0496 0.57 Mitered to slope 0.0300 1.0 0.0463 0.75 Projecting 0.0340 1.5 0.0496 0.53 Bottomless Arch 90° headwall 1 0.0083 2.0 0.0379 0.69 CMP Mitered to slope 0.0300 2.0 0.0463 0.75 Thin wall projecting 0.0340 1.5 0.0496 0.57 Circular with Smooth tapered inlet throat 2 0.534 0.333 0.0196 0.89 Tapered Inlet Rough tapered inlet throat 0.519 0.64 0.0289 0.90 E Source: FHWA HDS No.5 Outlet Control Analysis Nomographs such as those provided in Figure 4.3.1.1) and Figure 4.3.1.E may be used to determine the outlet control headwater depth at design flow for various types of culverts and inlets. Outlet control nomographs other than those provided can be found in FHWA HDS No.5 or the WSDOT Hydraulic Manual. The outlet control headwater depth may also be determined using the simple Backwater Analysis method presented in Section 4.2.1.2 for analyzing pipe system capacity. This procedure is summarized as follows for culverts: HW = H+ TW-LS (4-7) where H = Hf+ He + Hex Hf = friction loss(ft) = (k n2L)l(2.22R1 ') Note:If(Hf+TW-LS) <D, adjust Hf such that(Hf+TW-LS) D. This will keep the analysis simple and still yield reasonable results (erring on the conservative side). He = entrance head loss(ft) = Ke(V212g) H, = exit head loss(ft) = C12g TW= tailwater depth above invert of culvert outlet(ft) Note:If TW< (D+de)/2, set TW (D+de)/2. This will keep the analysis simple and still yield reasonable results. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-40 4.3.1 CULVERTS L = length of culvert(ft) S = slope of culvert barrel(ft/ft) D = interior height of culvert barrel(ft) V = barrel velocity(fps) n = Manning's roughness coefficient from Table 4.2.1.1) R = hydraulic radius(ft) Ke = entrance loss coefficient(from Table 4.3.13) g = acceleration due to gravity(32.2 ft/sect) d, = critical depth(ft); see Figure 4.3.11 Note: The above procedure should not be used to develop stage/discharge curves for level pool routing purposes because its results are not precise for flow conditions where the hydraulic grade line falls significantly below the culvert crown (i.e., less than full flow conditions). 2017 City of Renton Surface Water Design Manual 12/12/2016 4-41 SECTION 4.3 CULVERTS AND BRIDGES TABLE 4.3.1.B ENTRANCE LOSS COEFFICIENTS Type of Structure and Design Entrance Coefficient, Ke Pipe, Concrete, PVC, Spiral Rib, DI, and Lined CPE Projecting from fill,socket(bell)end 0.2 Projecting from fill, square cut end 0.5 Headwall, or headwall and wingwalls Socket end of pipe (groove-end) 0.2 Square-edge 0.5 Rounded (radius= '/12D) 0.2 Mitered to conform to fill slope 0.7 End section conforming to fill slope* 0.5 Beveled edges, 33.7°or 45° bevels 0.2 Side-or slope-tapered inlet 0.2 Pipe, or Pipe-Arch, Corrugated Metal and Other Non-Concrete or D.I. Projecting from fill (no headwall) 0.9 Headwall, or headwall and wingwalls (square-edge) 0.5 Mitered to conform to fill slope (paved or unpaved slope) 0.7 End section conforming to fill slope* 0.5 Beveled edges, 33.7°or 45° bevels 0.2 Side-or slope-tapered inlet 0.2 Box, Reinforced Concrete Headwall parallel to embankment(no wingwalls) Square-edged on 3 edges 0.5 Rounded on 3 edges to radius Of barrel dimension or beveled edges on 0.2 3 sides Wingwalls at 30°to 75°to barrel Square-edged at crown 0.4 Crown edge rounded to radius Of barrel dimension or beveled top edge 0.2 Wingwall at 10°to 25°to barrel Square-edged at crown 0.5 Wingwalls parallel (extension of sides) Square-edged at crown 0.7 Side-or slope-tapered inlet 0.2 * Note: "End section conforming to fill slope"are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections incorporating a closed taper in their design have a superior hydraulic performance. Computer Applications The"King County Backwater"(KCBW)computer program available with this manual contains two subroutines(BWPIPE and BWCULV)that may be used to analyze culvert capacity and develop stage/discharge curves for level pool routing purposes.A schematic description of the nomenclature used in these subroutines is provided in Figure 4.3.1.G. The KCBW program documentation available from King County Department of Natural Resources and Parks(DNRP)includes more detailed descriptions of program features. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-42 4.3.1 CULVERTS FIGURE 4.3.1.A INLET/OUTLET CONTROL CONDITIONS ee= \ 11 HW �n9Y rade� D hydraulic grade line Inlet Control - Submerged Inlet / \ HW -needo �rade line \ � hydraulic grade liney Inlet Control - Unsubmerged Inlet K v e 2g ---� �— e�9 rade line (VZn2L � h draulic 2.22 R s rade line\ 2 HW 2g Hy S L JTW Outlet Control - Submerged Inlet and Outlet NOTE: See FHWA no. 5 for other possible conditions 2017 City of Renton Surface Water Design Manual 12/12/2016 4-43 SECTION 4.3 CULVERTS AND BRIDGES FIGURE 4.3.1.B HEADWATER DEPTH FOR SMOOTH INTERIOR PIPE CULVERTS WITH INLET CONTROL CHART I �B 090 of$ EXAMIPLE (3) pp-r4 wm C 1.6 440 pas "000 s.oq 144 1*0 4. 120 M 9.1 F,4 �2,000 W27- kj �300 too 0.5 z 2 45 Z., 100, 40 0 ................. 42 130 H* SCAL E C' 4,0 Itypt Vo 10 S10 "do 01,0 110 49p W04 vill* 30 4*4,0401 X — .0 10 —.24 42) 0$1 pf'o-WO Nof 1woffilthIl'o*cap$pi�,q*o,* 41 us,&SIMMIN't ImeNlIma ghm#I'Vough 0,and Q wea[m,of UtAff%M,44 Mulfrafffit it HEADWATER DEPTH FOR AEAOWA%R SC-A�ES 263 CONCRETE PIPE CULVERTS REVIUD MAY P"4 WtTH INLET CONTROL 12/12/2016 2017 City of Renton Surface Water Design Manual 4-44 4.3.1 CULVERTS FIGURE 4.3.1.0 HEADWATER DEPTH FOR CORRUGATED PIPE CULVERTS WITH INLET 'CONTROL ' 180 10,000 168 8,000 EXAMPLE 156 6,000 Do 36 Inches (3.0 feet). 6. (2) 5,000 Q=66 cfs 144 4,000 �3� HW" HW 5' 6. ENTRANCE TYPE 132 3,000 D (feet) U (1) 1.8 5.4 5' 6. HEADWALL PLAN 120 w 2,000 (2) 2.1 6.3 5. ; 108 g (3) 2.2 6.6 4• *D in feet 3. 4. I 96 1,000 3 I I 800 3. r 84 y 600 2. ) 500 72 400 -- 2-~ MITERED TO 2. CONFORM 300 0 1.5 TO SLOPE V y SECTIO Z60 Z 200 �� N 1.5 /� w 1.5 I1C 6 - 54 a � w 100 / w 48 Q 80, a �2) 2 Z PROJECTING V 42 N 50 = 1.0 1.0 SECTION LL O40 w 1.0 LLI30 HW .9 I j SCAL 36 E ENTRANCE w D TYPE a .9 Q 33 20 G (1) Headwall G .$ .8 30 (2) Mitered to conform W V to slope $ (3) C 27 10 (3) Projecting odc 8 .7 .7 z 24 .7 6 w S 21 To use scale(2)or(3)project 4 horizontally to scale(1),then use straight inclined line through 6 .6 3 D and Q scales,or reverse as .6 18 illustrated 2 15 .5 1.0 .5 12 2017 City of Renton Surface Water Design Manual 12/12/2016 4-45 SECTION 4.3 CULVERTS AND BRIDGES FIGURE 4.3.1.1) HEAD FOR CULVERTS(PIPE W/"n"=0.012) FLOWING FULL WITH OUTLET CONTROL 2000 H HW ho cs .4 Z W Slope Sp� 000 ? SUBMERGED OUTLET CULVERT FLOWING FULL .5 H J HW=H+ho-LSO 800 120 For outlet crown not submerged,compute HW by .6 methods described in the design procedure 600 108 .8 500 96 400 84 / 300 72 66 w 2 200 60 / ADO tiC� LL CO LL 54 0 y� Z Z N D=4 / L=110 LU 48-7-71 ke=o.s ^p� ti� 2 3 x F LU 00 '1 Z 2 ry00 X00 w 4 o/— _ $0% ? 00 = 0 36 �p0 b 5 U 60 33 00 y00 6 p w pc 50 w 30 —40 500 8 0 D 27 0 0 O e� 0 30 24 20 21 18 20 p 15 8 6 12 5 4 12/12/2016 2017 City of Renton Surface Water Design Manual 4-46 4.3.1 CULVERTS FIGURE 4.31.E HEAD FOR CULVERTS(PIPE W/"n"=0.024) FLOWING FULL WITH OUTLET CONTROL 2000 2000 — — H .4 1000 2 z Lu HW ho .5 800 ~ J Slope SO� .6 SUBMERGED OUTLET CULVERT FLOWING FULL 600 120 HW=H+ho-LSO 500 08 For outlet crown not submerged,compute HW by •8 methods described in the design procedure 400 96 1.0 300 84 p 'mss yo 200 72 66 09 010 yc� w 2 LU V) 60 1.1. LL V 54 2 Z .. 80 048 T� 2 3 LU Cl �N, 0 O .. ? 42 / 9 10 2 4 U, 60 Z = 50 - 5 5 U 40 le 33 010 6 _ 0=35 W — 30 LU _30 1— — —_— EXAMPLE —_ 00_ H=7.5 3 8 Q 27 0 00 20 10 24 400 21 600 6010 10 18 120 8 6 15 5 4 L12 3 2 2017 City of Renton Surface Water Design Manual 12/12/2016 447 SECTION 4.3 CULVERTS AND BRIDGES FIGURE 4.3.1.F CRITICAL DEPTH OF FLOW FOR CIRCULAR CULVERTS' 120 3,000 0.99 114 108 2,000 102 96 1,000 90 84 0.90 500 78 400 72 300 0.80 6 200 0.70 60 54 100 0.60 � w 48 ¢ 50 = 40 0 0.50 U � t o 30 Z. 42 20 0.40 36 0 10 g 33y Q T =1.0 30 5 gAs D 0.30 4 v 27 _ 3 A d 24 2 T 21 1 Note: For all cross-sectional shapes, do can be calculated by 0.20 18 trial and error knowing that the quantity(Q2T/gA3)=1.0 at critical depth. 15 EXAMPLE D =66 inches, Q = 100cfs dc/D - Ratio= 0.50 12 do= (0.50)(66 inches)= 33 inches�(12 inches/ft) dC 2.75 feet 12/12/2016 2017 City of Renton Surface Water Design Manual 4-48 4.3.1 CULVERTS FIGURE 4.3.1.G COMPUTERSUBROUTINES BWPIPE AND BWCULV:VARIABLE DEFINITIONS - ----------- FLOW DATA EHU 0 KE"VEH --------- VEH VNH H.G.L. VOH=VCH i HWO DE ------ — — ---- O DN D DC DO TW (DXN*X) X FLOW DATA COEFFICIENTS/INLET DATA DC- Critical Depth (ft) KE- Entrance Coefficient under Outlet Control DN- Normal Depth (ft) KB- Bend Loss Coefficient TW- Tailwater Depth (ft) KJ - Junction Loss Coefficient DO- Outlet Depth (ft) K- Inlet Control Equation parameter (See Table 4.3.1.A) DE- Entrance Depth (ft) M- Inlet Control Equation parameter (See Table 4.3.1.A) HWO- Headwater(ft)assuming Outlet Control C- Inlet Control Equation parameter (See Table 4.3.1.A) HWI - Headwater(ft)assuming Inlet Control Y- Inlet Control Equation parameter (See Table 4.3.1.A) DXN - Distance (expressed as a fraction of the pipe Q-Ratio- Ratio of tributary flow to main upstream length)from the outlet to where the flow profile flow(Q3/Q1) intersects with normal depth. DXN will equal one under full-flow conditions and will equal zero when a hydraulic jump occurs at the outlet or when normal depth equals zero(normal depth will equal zero when the pipe grade is flat or reversed). VBH- Barrel Velocity Head (ft)based on the average velocity determined by V=Q/Afull VUH - Upstream Velocity Head (ft)based on an inputted velocity. EHU - Upstream Energy Head (ft)available after bend losses and junction losses have been subtracted from VUH. VCH - Critical Depth Velocity Head (ft) VNH - Normal Depth Velocity Head (ft) VEH- Entrance Depth Velocity Head (ft) VOH - Outlet Depth Velocity Head ft 2017 City of Renton Surface Water Design Manual 12/12/2016 4-49 SECTION 4.3 CULVERTS AND BRIDGES 4.3.2 CULVERTS PROVIDING FOR FISH PASSAGE/MIGRATION In fish-bearing waters,water-crossing structures must usually provide for fish passage as required for Washington State Department of Fish and Wildlife(WDFW)Hydraulic Project Approval or as a condition of permitting under the critical areas code (RMC 4-3-050). Culverts designed for fish passage must also meet the requirements of Section 1.2.4, "Core Requirement 44: Conveyance System." Fish passage can generally be ensured by providing structures that do not confine the streambed—that is, a structure wide enough so that the stream can maintain its natural channel within the culvert. Bridges, bottomless arch culverts, arch culverts, and rectangular box culverts("utility vaults")can often be used to accommodate stream channels. Where it is unfeasible to construct these types of structures,round pipe culverts may be used if high flow velocities are minimized and low flow depths are maximized. The Hydraulic Code Rules (Title 220 WAC) detail requirements for WDFW Hydraulic Project Approval. See the WDFW manual"Design of Road Culverts for Fish Passage"for detailed design methodologies. Materials Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations, sometimes in the range that can be toxic to aquatic life,have been observed in the region. Therefore,use of galvanized materials in stormwater facilities is not allowed, and their use in conveyance systems is discouraged. Where other metals, such as aluminum or stainless steel, or plastics are available, they should be used. 4.3.2.1 DESIGN CRITERIA Table 4.3.2.A lists allowable velocities, flow depths, and hydraulic drops for culverts in fish-bearing streams.Velocities are for the high flow design discharge;water depths are for the low flow design discharge.The hydraulic drop(a vertical drop in the water surface profile at any point within culvert influence)is for all flows between the high and low flow design discharges. TABLE 4.3.2.A FISH PASSAGE DESIGN CRITERIA Adult Pink, Chum Adult Chinook, Coho, Adult Trout Salmon Sockeye,Steelhead 1. Max Velocity(fps) Culvert Length: 10-60 ft 4.0 5.0 6.0 60-100 ft 4.0 4.0 5.0 100-200 ft 3.0 3.0 4.0 2. Min Flow Depth (ft) 0.8 0.8 1.0 3. Max Hydraulic Drop(ft) 0.8 0.8 1.0 Source: WDFW manual `Design of Road Culverts for Fish Passage" (2003), Chapter 5,p. 21, Table 5-1. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-50 4.3.2 CULVERTS PROVIDING FOR FISH PASSAGE/MIGRATION 4.3.2.2 METHODS OF ANALYSIS High Flow Design Discharge For gaged streams,the high flow design discharge shall be estimated by the 10%exceedance flow for October through April inclusive,proportioned by tributary area to the culvert using the technique described in Section 4.4.2.4 under"Flood Flows from Stream Gage Data." For ungaged streams,the high flow design discharge shall be estimated by one of the following: • The 10%exceedance flow for October through April inclusive for the nearest hydrologically similar gaged stream,proportioned by tributary area • The 5%exceedance flow determined through duration analysis with the approved model • The 10%exceedance flow for October through April inclusive determined with the HSPF model or the approved model using the full historical record. Low Flow Design Discharge For gaged streams,the low flow design discharge shall be estimated by the 95%exceedance flow for October through April inclusive,proportioned by tributary area. For ungaged streams,the low flow design discharge shall be estimated by one of the following: • The 95%exceedance flow for October through April inclusive for the nearest hydrologically similar gaged stream,proportioned by tributary area • The 95%exceedance flow for October through April inclusive, determined by the HSPF model or the approved model using the full historical record • The following equation,using input data from the approved model(Note: Equation 4-9 is not used): For the Sea-Tac rainfall region: Q1 = f(0.46A�f+0.56At,+0.46Atg+0.72A,,f+0.96A,,p+ 1.10Aog)/ 1000 (4-7) where Q1 = low flow design discharge(cfs) f = regional rainfall scale factor from the WWHM2012 Site Information map screen A�f= area of till forest(acres) At,= area of till pasture (acres) Atg= area of till grass(acres) A,,f= area of outwash forest(acres) A,,p= area of outwash pasture(acres) A,,g= area of outwash grass(acres) Note:Minimum depths may also be met by providing an "installed no flow depth,"per Title 220 WAC, where the static water surface level meets minimum flow depth criteria. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-51 SECTION 4.3 CULVERTS AND BRIDGES 4.3.3 BRIDGES Bridges over waterways are considered conveyance structures and are generally constructed to allow the continuation of a thoroughfare (such as a road). They generally consist of foundation abutments and/or piers that support a deck spanning the waterway. In addition to the design criteria for conveyance described below,bridge designs must meet the City of Renton Transportation requirements,AASHTO Standard Specifications for Highway Bridges or AASHTO LRFD Bridge Design Specifications and the most current WSDOT/APWA Standard Specifications as well as the requirements of other agencies such as the Washington State Department of Fish and Wildlife (WDFW). 4.3.3.1 DESIGN CRITERIA Bridges shall be designed to convey flows and pass sediments and debris for runoff events up to and including the 100-year event in a manner that does not increase the potential for flooding or erosion to properties and structures near or adjacent to the bridge, or cause bridge failure. Inadequate conveyance capacity may cause flooding to increase by restricting flow through the hydraulic openings,by placing approach fill or abutments in floodplains,by causing changes in channel gradient and alignment or by trapping debris.A common mode of bridge failure involving debris is the resultant scour and undermining of piers or abutments where debris accumulates. Openings between the structural elements of the bridge and the bottom of the channel or floodplain ground surface must be large enough to allow for passage of water, sediment, and debris. The horizontal openings are defined by the bridge span,the horizontal distances between piers or abutments. Bridge clearance is the vertical distance between the 100-year water surface and the low chord of the bridge. For stream crossing locations where the 100-year peak flow exceeds 100 cfs,the height of a bridge clearance above rivers and streams shall be a minimum three feet above the 100-year water surface elevation unless otherwise required by the City based on evaluation of the design criteria in this section. For stream crossing locations where the 100-year peak flow is 100 cfs or less,there is no specific clearance requirement. Hydraulic Capacity Bridge and approach roads must pass the 100-year flow without creating hydraulic restrictions that cause or increase flooding. Design of bridge and approach roads shall demonstrate compliance with the compensatory storage provisions of RMC 4-3-050. Of necessity,bridge and approach roads are sometimes constructed within 100-year floodplains. In some cases, approach roads will be inundated and the bridge will not be accessible during extreme events. In other cases,both the bridge and approach roads will be inundated by the 100-year flood. In these cases,the bridge shall be designed to withstand the expected condition while inundated.The design shall employ means to facilitate flow over the bridge and to minimize the potential for erosion of the roadway fill in the approach roads. Bed Aggradation Where bed aggradation is probable,the analysis of hydraulic capacity shall assume the bed raised by an amount expected during a suitable design life(40 years minimum)of the bridge.Aggradation estimates shall be based on a sediment transport analysis that,where possible,is calibrated to direct cross-section comparisons over time. This analysis shall extend upstream and downstream a sufficient distance to adequately characterize bed aggradation that may affect the hydraulic capacity at the bridge location. Bed aggradation is frequently associated with channel migration. The location and design of bridges and approach roads shall consider channel migration hazards. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-52 4.3.3 BRIDGES Debris Passage Since debris can pass through an opening either partly or totally submerged,the total vertical clearance from the bottom of the structure to the streambed needs to be considered. Required clearance for debris shall include an assessment of the maximum material size available,the ability of the stream to transport it, and the proximity of debris sources. The following factors also must be considered:history of debris problems in the river reaches upstream and downstream of the proposed bridge location,history of debris accumulations on an existing bridge structure or nearby structures upstream and downstream from the proposed bridge location,mapped channel migration hazard and channel migration history of the reach of stream, and skew of the bridge alignment such that piers in floodplain may be in the path of the debris. For a detailed qualitative analysis of debris accumulation on bridges, see the U.S. Department of Transportation,Federal Highway Administration Publication FHWA-RD-97-028, Potential Drift Accumulation at Bridges,by Timothy H. Diehl(1997). Safety Margin When designing bridges to convey flows and pass sediments and debris, a safety margin shall be considered by the design engineer to account for uncertainties in flow rates, debris hazards,water surface elevations, aggradation, and channel migration over time. The safety margin should be increased when the surrounding community is especially susceptible to flood damages that could be exacerbated by a debris jam at the bridge. Section 5 of the Technical Information Report submitted with the project's engineering plans shall include a discussion of the need for a safety margin and the rationale for its selection. Bridges and Levees Where bridge structures and approach roads intersect flood containment levees,the bridge structure and approach roads shall be designed and constructed to preserve existing levels of flood containment provided by the existing levee. Where the existing levee currently provides containment of the 100-year flood,the bridge structure and approach roads shall be designed and constructed to meet FEMA levee and structural performance standards,including sufficient freeboard on the levee in the bridge vicinity, as provided for in 44 CFR (also see Section 1.3.3, Special Requirement 43,Flood Protection Facilities). Bridge Piers and Abutments Bridge pier and abutment locations are governed by provisions of the City's critical areas code, RMC 4-3-050. 4.3.3.2 METHODS OF ANALYSIS The following methods are acceptable for hydraulic analysis of bridges and approach roads: 1. The Direct Step backwater method described in Section 4.4.1.2 shall be used to analyze the hydraulic impacts of bridge piers, abutments, and approach roads to the water surface profile. 2. The Army Corps of Engineers Hydraulic Engineering Center publishes technical papers on methods used to address the hydraulic effects of bridge piers, abutments, and approach roads. The book Open Channel Hydraulics by V.T. Chow also contains techniques for analyzing hydraulic effects. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-53 SECTION 4.3 CULVERTS AND BRIDGES (This page intentionally left blank.) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-54 CITY OF RENTON SURFACE WATER DESIGN MANUAL 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS This section presents the methods,criteria, and details for hydraulic analysis and design of open channels, and the determination and analysis of floodplains and floodways. The information presented is organized as follows: Section 4.4.1, "Open Channels" "Design Criteria,"Section 4.4.1.1 "Methods of Analysis," Section 4.4.1.2 Section 4.4.2, "Floodplain/Floodway Analysis" "No Floodplain Study Required," Section 4.4.2.1 "Approximate Floodplain Study,"Section 4.4.2.2 "Minor Floodplain Study,"Section 4.4.2.3 "Major Floodplain/Floodway Study," Section 4.4.2.4. 4.4.1 OPEN CHANNELS Open channels may be classified as either natural or constructed.Natural channels are generally referred to as rivers, streams,creeks, or swales,while constructed channels are most often called ditches, or simply channels. The Critical Areas, Shorelines, and Clearing and Grading Codes as well as Chapter 1 of this manual should be reviewed for requirements related to streams. Natural Channels Natural channels are defined as those that have occurred naturally due to the flow of surface waters, or those that, although originally constructed by human activity,have taken on the appearance of a natural channel including a stable route and biological community. They may vary hydraulically along each channel reach and should be left in their natural condition,wherever feasible or required,in order to maintain natural hydrologic functions and wildlife habitat benefits from established vegetation. Constructed Channels Constructed channels are those constructed or maintained by human activity and include bank stabilization of natural channels. Constructed channels shall be either vegetation-lined,rock-lined, or lined with appropriately bioengineered vegetation9. • Vegetation-lined channels are the most desirable of the constructed channels when properly designed and constructed. The vegetation stabilizes the slopes of the channel,controls erosion of the channel surface, and removes pollutants. The channel storage,low velocities,water quality benefits, and greenbelt multiple-use benefits create significant advantages over other constructed channels. The presence of vegetation in channels creates turbulence that results in loss of energy and increased flow retardation;therefore,the design engineer must consider sediment deposition and scour, as well as flow capacity,when designing the channel. • Rock-lined channels are necessary where a vegetative lining will not provide adequate protection from erosive velocities. They may be constructed with riprap,gabions, or slope mattress linings. The rock lining increases the turbulence,resulting in a loss of energy and increased flow retardation. Rock 9 Bioengineered vegetation lining as referenced here applies to channel stabilization methods.See Appendix C, Simplified Drainage Requirements for bioswale design criteria. Note,for bioswales and other infiltrative BMPs that may be placed in-line with conveyance,any infiltration option in the modeling shall be turned off when evaluating conveyance capacity. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-55 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS lining also permits a higher design velocity and therefore a steeper design slope than in grass-lined channels. Rock linings are also used for erosion control at culvert and storm drain outlets, sharp channel bends,channel confluences, and locally steepened channel sections. • Bioengineered vegetation lining is a desirable alternative to the conventional methods of rock armoring. Soil bioengineering is a highly specialized science that uses living plants and plant parts to stabilize eroded or damaged land.Properly bioengineered systems are capable of providing a measure of immediate soil protection and mechanical reinforcement.As the plants grow they produce a vegetative protective cover and a root reinforcing matrix in the soil mantle. This root reinforcement serves several purposes: a) The developed anchor roots provide both shear and tensile strength to the soil,thereby providing protection from the frictional shear and tensile velocity components to the soil mantle during the time when flows are receding and pore pressure is high in the saturated bank. b) The root mat provides a living filter in the soil mantle that allows for the natural release of water after the high flows have receded. c) The combined root system exhibits active friction transfer along the length of the living roots. This consolidates soil particles in the bank and serves to protect the soil structure from collapsing and the stabilization measures from failing. The vegetative cover of bioengineered systems provides immediate protection during high flows by laying flat against the bank and covering the soil like a blanket. It also reduces pore pressure in saturated banks through transpiration by acting as a natural"pump"to "pull"the water out of the banks after flows have receded. The King County publication Guidelines for Bank Stabilization Projects primarily focuses on projects on larger rivers and streams,but the concepts it contains may be used in conjunction with other natural resource information for stabilization projects on smaller systems. The WDFWIntegrated Streambank Protection Guidelines is another useful reference. 4.4.1.1 DESIGN CRITERIA General 1. Open channels shall be designed to provide required conveyance capacity and bank stability while allowing for aesthetics,habitat preservation, and enhancement. Open channels shall be consistent with the WDFWIntegrated StreambankProtection Guidelines. 2. An access casement for maintenance is required along all constructed channels located on private property. Required easement widths and building setback lines vary with channel top width as shown in Table 4.1. 3. Channel cross-section geometry shall be trapezoidal,triangular,parabolic, or segmental as shown in Figure 4.4.1.0 through Figure 4.4.I.E. Side slopes shall be no steeper than 3:1 for vegetation-lined channels and 2:1 for rock-lined channels.Note:Roadside ditches shall comply with the City ofRenton Standard Details. 4. To reduce the likelihood that pollutants will be discharged to groundwater when untreated runoff is conveyed in ditches or channels constructed in soils with high infiltration rates, a low permeability liner or a treatment liner shall be provided for any reach of new ditch or channel proposed by a project in which the untreated runoff from 5,000 square feet or more ofpollution-generating impervious surface comes into direct contact with an outwash soil, except where it can be demonstrated that the soil meets the soil suitability criteria listed in Section 5.2.1. The low permeability liner or treatment liner shall be consistent with the specifications for such liners in Section 6.2.4. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-56 4.4.1 OPEN CHANNELS 5. Vegetation-lined channels shall have bottom slope gradients of 6%or less and a maximum velocity at design flow of 5 fps (see Table 4.4.1.A). 6. Rock-lined channels or bank stabilization of natural channels shall be used when design flow velocities exceed 5 feet per second. Rock stabilization shall be in accordance with Table 4.4.1.A or stabilized with bioengineering methods as described above in"Constructed Channels." TABLE 44,1.A CHANNEL PROTECTION Velocity at Design Flow(fps) Required Protection Minimum Height Greater Less than Above Design than or Equal to Type of Protection Thickness Water Surface 0 5 Grass lining N/A N/A or Bioengineered lining 5 8 Rock lining(') 1 foot 1 foot or Bioengineered lining 8 12 Riprap(2) 2 feet 2 feet 12 20 Slope mattress gabion, Varies 2 feet etc. Rock Lining shall be reasonably well graded as follows: Maximum stone size: 12 inches Median stone size: 8 inches Minimum stone size: 2 inches (2) Riprap shall be reasonably well graded as follows: Maximum stone size: 24 inches Median stone size: 16 inches Minimum stone size: 4 inches Note:Riprap sizing is governed by side slopes on channel, assumed to be approximately 3:1. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-57 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS Riprap Designio When riprap is set, stones are placed on the channel sides and bottom to protect the underlying material from being eroded.Proper riprap design requires the determination of the median size of stone,the thickness of the riprap layer,the gradation of stone sizes, and the selection of angular stones that will interlock when placed.Research by the U.S.Army Corps of Engineers has provided criteria for selecting the median stone weight, W5o(Figure 4.4.1.A). If the riprap is to be used in a highly turbulent zone (such as at a culvert outfall, downstream of a stilling basin, at sharp changes in channel geometry, etc.),the median stone W5o should be increased from 200%to 600%depending on the severity of the locally high turbulence. The thickness of the riprap layer should generally be twice the median stone diameter(D50) or at least that of the maximum stone. The riprap should have a reasonably well graded assortment of stone sizes within the following gradation: 1.25 :—D,n,,,lD50 :— 1.50 D151D50 =0.50 D.idD50=0.25 Detailed design methodology may be found in the Corps publication EM 1110-02-1601,Engineering and Design—Hydraulic Design of Flood Control Channels. For a more detailed analysis and design procedure for riprap requiring water surface profiles and estimates of tractive force,refer to the paper by Maynord et al. in Journal of Hydraulic Engineering(A.S.C.E.), July 1989. Riprap Filter Design Riprap should be underlain by a sand and gravel filter(or filter fabric)to keep the fine materials in the underlying channel bed from being washed through the voids in the riprap. Likewise,the filter material must be selected so that it is not washed through the voids in the riprap.Adequate filters can usually be provided by a reasonably well graded sand and gravel material where: D15 < 5ds5 The variable d85 refers to the sieve opening through which 85%of the material being protected will pass, and D15 has the same interpretation for the filter material. A filter material with a D50of 0.5 mm will protect any finer material including clay. Where very large riprap is used,it is sometimes necessary to use two filter layers between the material being protected and the riprap. Example: What embedded riprap design should be used to protect a streambank at a level culvert outfall where the outfall velocities in the vicinity of the downstream toe are expected to be about 8 fps? From Figure 4.4.1.A, W5o=6.5 lbs,but since the downstream area below the outfall will be subjected to severe turbulence,increase W5o by 400% so that: W5o=26 lbs,D50= 8.0 inches The gradation of the riprap is shown in Figure 4.4.1.13,and the minimum thickness would be 1 foot(from Table 4.4.1.A);however, 16 inches to 24 inches of riprap thickness would provide some additional insurance that the riprap will function properly in this highly turbulent area. Figure 4.4.1.13 shows that the gradation curve for ASTM C33, size number 57 coarse aggregate (used in concrete mixes),would meet the filter criteria.Applying the filter criteria to the coarse aggregate demonstrates that any underlying material whose gradation was coarser than that of a concrete sand would be protected. 10 From a paper prepared by M. Schaefer, Dam Safety Section,Washington State Department of Ecology. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-58 4.4.1 OPEN CHANNELS FIGURE 4.4.1.A MEAN CHANNEL VELOCITY VS.MEDIUM STONE WEIGHT(W5o)AND EQUIVALENT STONE DIAMETER sqj(M)j4Bi9M9uojS OR C) Oil 0 04 w — 0 00 0 co 0 U) -a ID E OC W r- 44 E 3 w z 0, 00 ;2 co 0 .2 C,4 to E 26 LU 6 CL CL E co x w -j > T- 10 0 C cc > C; w Z 0 CL a bi (0) O C � 0 to Ed 0 00o 10 44 II II> 0 0 00 — 6 o (puooes jed jeej)f4laojeA 2017 City of Renton Surface Water Design Manual 12/12/2016 4-59 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS FIGURE 4.4.1.B RIPRAP/FILTER EXAMPLE GRADATION CURVE 20 RIP-RAP N 10 V IN 1100,01, i Coarse Aggregate size number 57 ASTM 14 C-33 1 0.1 0 10 20 30 40 50 60 70 80 90 100 % Finer by Weight 12/12/2016 2017 City of Renton Surface Water Design Manual 4-60 4.4.1 OPEN CHANNELS 4.4.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing open channels for compliance with the conveyance capacity requirements set forth in Section 1.2.4, "Core Requirement 44: Conveyance System." ❑ DESIGN FLOWS Design flows for sizing and assessing the capacity of open channels shall be determined using the hydrologic analysis methods described in Chapter 3. ❑ CONVEYANCE CAPACITY There are three acceptable methods of analysis for sizing and analyzing the capacity of open channels: 1. Manning's equation for preliminary sizing 2. Direct Step backwater method 3. Standard Step backwater method. Manning's Equation for Preliminary Sizing Manning's equation is used for preliminary sizing of open channel reaches of uniform cross section and slope (i.e.,prismatic channels)and uniform roughness.This method assumes the flow depth(or normal depth) and flow velocity remain constant throughout the channel reach for a given flow. The charts in Figure 4.4.1.0 and Figure 4.4.1.D may be used to obtain graphic solutions of Manning's equation for common ditch sections.For conditions outside the range of these charts or for more precise results,Manning's equation can be solved directly from its classic forms shown in Equations(4-1)and(4- 2). Table 4.4.1.13 provides a reference for selecting the appropriate"n"values for open channels.A number of engineering reference books,such as Open-Channel Hydraulics by V.T. Chow,may also be used as guides to select"n"values.Figure 4.4.1.E contains the geometric elements of common channel sections useful in determining area A,wetted perimeter WP, and hydraulic radius(R=AW). If flow restrictions occur that raise the water level above normal depth within a given channel reach, a backwater condition(or subcritical flow)is said to exist. This condition can result from flow restrictions created by a downstream culvert,bridge, dam,pond,lake, etc., and even a downstream channel reach having a higher flow depth. If backwater conditions are found to exist for the design flow, a backwater profile must be computed to verify that the channel's capacity is still adequate as designed. The Direct Step or Standard Step backwater methods presented in this section may be used for this purpose. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-61 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS TABLE 4.4.I.B VALUES OF ROUGHNESS COEFFICIENT"n"FOR OPEN CHANNELS Manning's Manning's Type of Channel "n,,* Type of Channel "n,,* and Description (normal) and Description (normal) A. Constructed Channels 6. Sluggish reaches,weedy 0.070 a. Earth, straight and uniform deep pools 1. Clean, recently completed 0.018 7. Very weedy reaches, 0.100 2. Gravel, uniform section, 0.025 deep pools, or floodways clean with heavy stand of 3. With short grass,few 0.027 timber and underbrush weeds b. Mountain streams, no b. Earth,winding and sluggish vegetation in channel, banks 0.025 usually steep,trees and 1. No vegetation 0.030 brush along banks 2. Grass, some weeds 0.035 submerged at high stages 3. Dense weeds or aquatic 1. Bottom: gravel, cobbles, plants in deep channels 0.030 and few boulders 0.040 4. Earth bottom and rubble 2. Bottom: cobbles with sides large boulders 5. Stony bottom and weedy 0.035 B-2 Floodplains 0.050 banks a. Pasture, no brush 6. Cobble bottom and clean 0.040 1. Short grass sides 2. High grass c. Rock lined b. Cultivated areas 0.030 1. Smooth and uniform 0.035 1. No crop 0.035 2. Jagged and irregular 0.040 2. Mature row crops d. Channels not maintained, 3. Mature field crops 0.030 weeds and brush uncut 0.035 1. Dense weeds, high as flow C. Brush 9 0.080 1. Scattered brush, heavy 0.040 depth weeds 2. Clean bottom, brush on sides 0.050 2. Light brush and trees 0.050 3. Same as#2, highest stage 0070 3. Medium to dense brush 0.060 of flow . 4. Heavy, dense brush 4. Dense brush, high stage 0.100 d. Trees 0.070 B. Natural Streams 1. Dense willows, straight 0.100 B-1 Minor streams (top width at 2. Cleared land with tree flood stage < 100 ft.) stumps, no sprouts 0.150 a. Streams on plain 3. Same as#2, but with 0.040 1. Clean,straight, full stage 0.030 heavy growth of sprouts no rifts or deep pools 4. Heavy stand of timber, a 0.060 2. Same as#1, but more 0.035 few down trees, little stones and weeds undergrowth, flood stage 0.100 below branches 3. Clean,winding,some pools 0.040 5. Same as#4, but with and shoals flood stage reaching 4. Same as#3, but some 0.040 branches weeds 0.120 5. Same as#4, but more 0.050 stones Note:These n"values are `normal"values for use in analysis of channels.For conservative design of channel capacity, the maximum values listed in other references should be considered. For channel bank stability,the minimum values should be considered. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-62 4.4.1 OPEN CHANNELS Direct Step Backwater Method The Direct Step backwater method may be used to compute backwater profiles on prismatic channel reaches(i.e.,reaches having uniform cross section and slope)where a backwater condition or restriction to normal flow is known to exist. The method may be applied to a series of prismatic channel reaches in secession beginning at the downstream end of the channel and computing the profile upstream. Calculating the coordinates of the water surface profile using this method is an iterative process achieved by choosing a range of flow depths,beginning at the downstream end, and proceeding incrementally up to the point of interest or to the point of normal flow depth. This is best accomplished by the use of a table (see Figure 4.4.1.G) or computer programs(as discussed in"Computer Applications"in Section 4.4.1.2). To illustrate analysis of a single reach,consider the following diagram: 1 2 I energ line S I � vlZ I f I hf=Sfox 1 2g I I � 0 water surtace, Sw I C>(:2Vzz Y' 2g I So Asx a � channel bottom, So Yz I ox Equating the total head at cross sections 1 and 2,the following equation may be written: z z 4-8 S,Ax+yi+ a V, = yz+ a Vz +SfAx 2g z 2g where, Ax = distance between cross sections(ft) yi,yz = depth of flow(ft)at cross sections 1 and 2 Vi, V = velocity(fps)at cross sections 1 and 2 al, a2= energy coefficient at cross sections 1 and 2 S,, = bottom slope(ft/ft) Sf = friction slope=(n2r)/(2.21R133) g = acceleration due to gravity, (32.2 ft/sect) If the specific energy E at any one cross-section is defined as follows: E + a V 2 (4-9) Y+ 2g and assuming a = ai= az where a is the energy coefficient that corrects for the non-uniform distribution of velocity over the channel cross section,Equations 4-10 and 4-11 can be combined and rearranged to solve for Ax as follows: 2017 City of Renton Surface Water Design Manual 12/12/2016 4-63 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS Ax= (E2-E,)I(S,,-Sf) = AE/(S,,-Sf) (4-10) Typical values of the energy coefficient a are as follows: Channels,regular section 1.15 Natural streams 1.3 Shallow vegetated flood fringes(includes channel) 1.75 For a given flow,channel slope,Manning's"n,"and energy coefficient a,together with a beginning water surface elevation y2,the values of Ax may be calculated for arbitrarily chosen values ofyi. The coordinates defining the water surface profile are obtained from the cumulative sum of Ax and corresponding values ofy. The normal flow depth,y,,,should first be calculated from Manning's equation to establish the upper limit of the backwater effect. Standard Step Backwater Method The Standard Step Backwater Method is a variation of the Direct Step Backwater Method and may be used to compute backwater profiles on both prismatic and non-prismatic channels. In this method, stations are established along the channel where cross section data is known or has been determined through field survey. The computation is carried out in steps from station to station rather than throughout a given channel reach as is done in the Direct Step method.As a result,the analysis involves significantly more trial-and-error calculation in order to determine the flow depth at each station. Computer Applications Because of the iterative calculations involved,use of a computer to perform the analysis is recommended. The King County Backwater(KCBW) computer program included in the software package available with this manual includes a subroutine,BWCHAN,based on the Standard Step backwater method,which may be used for all channel capacity analysis. It can also be combined with the BWPIPE and BWCULV subroutines to analyze an entire drainage conveyance system.A schematic description of the nomenclature used in the BWCHAN subroutine is provided in Figure 4.4.1.H. See the KCBW program documentation for further information. There are a number of commercial software programs for use on personal computers that use variations of the Standard Step backwater method for determining water surface profiles. The most common and widely accepted program is called HEC-RAS,published and supported by the United States Army Corps of Engineers Hydraulic Engineering Center. It is one of the models accepted by FEMA for use in performing flood hazard studies for preparing flood insurance maps. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-64 4.4.1 OPEN CHANNELS FIGURE 4.4.1.0 DITCHES-COMMON SECTIONS PROPERTIES OF DITCHES DIMENSIONS HYDRAULICS NO. Side Slopes B H W A WP R R(2/3) D-1 - - 6.5" 5'-0" 1.84 5.16 0.356 0.502 D-1 C - - 6" 25'-0" 6.25 25.50 0.245 0.392 D-2A 1.5:1 2'-0" 1'-0" 5'-0" 3.50 5.61 0.624 0.731 B 2:1 2'-0" 1'-0" 6'-0" 4.00 6.47 0.618 0.726 C 3:1 2'-0" 1'-0" 8'-0" 5.00 8.32 0.601 0.712 D-3A 1.5:1 3'-0" 1'-6" 7'-6" 7.88 8.41 0.937 0.957 B 2:1 3'-0" 1'-6" 9'-0" 9.00 9.71 0.927 0.951 C 3:1 3'-0" 1'-6" 12'-0" 11.25 12.49 0.901 0.933 D-4A 1.5:1 3'-0" 2'-0" 9'-0" 12.00 10.21 1.175 1.114 B 2:1 3'-0" 2'-0" 11'-0" 14.00 11.94 1.172 1.112 C 3:1 3'-0" 2'-0" 15'-0" 18.00 15.65 1.150 1.098 D-5A 1.5:1 4'-0" 3'-0" 13'-0" 25.50 13.82 1.846 1.505 B 2:1 4'-0" 3'-0" 16'-0" 30.00 16.42 1.827 1.495 C 3:1 4'-0" 3'-0" 22'-0" 39.00 21.97 1.775 1.466 D-6A 2:1 - 1'-0" 4'-0" 2.00 4.47 0.447 0.585 B 3:1 - 1'-0" 6'-0" 3.00 6.32 0.474 0.608 D-7A 2:1 - 2'-0" 8'-0" 8.00 8.94 0.894 0.928 B 3:1 - 2'-0" 12'-0" 12.00 12.65 0.949 0.965 D-8A 2:1 - 3'-0" 12'-0" 18.00 13.42 1.342 1.216 B 3:1 - 3'-0" 18'-0" 27.00 18.97 1.423 1.265 D-9 7:1 - 1'-0" 14'-0" 7.00 14.14 0.495 0.626 D-10 7:1 - 2'-0" 28'-0" 28.00 28.28 0.990 0.993 D-11 7:1 - 3'-0" 42'-0" 63.00 42.43 1.485 1.302 R=66 W 21 2% W H D-1 Segmental (parabolic) D-1 c Curbed crowned street W W lsee400) e �5O Vab�el 7 7 � V carie H S � �� S ��A B D-2, D-3, D-4, D-5 Trapezoidal Isosceles triangular D-6 through D-11 2017 City of Renton Surface Water Design Manual 12/12/2016 4-65 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS FIGURE 4.4,1,1) DRAINAGE DITCHES-COMMON SECTIONS NOTE: A)Chart based on Manning formula Q=1.491n*A*R2f3*S1 f2 with n=0.030,except D-1 C which is based on n=0.015. For other values of n,multiply discharge by 0.0301n B) 1 indicates a velocity of 1 ft.per sec. EAF iple: Given-Slope=3.3'per 1000',discharge=6.3 c.f.s.,n=0.025. D-1 Required-Size of ditch and velocity. Solution-To use chart,multiply discharge,6.3 by(.03!.025)=7.56 c.f.s. Point satisfying given conditions D-6A , lies between lines for D-2A and D-2B. Select larger of the two ditches,in ' this case D-2B. Velocity approx.2.1 ft.per sec. U-6B U- 0.07 2 d-213 D-2 3 D- 1 D1 fL d D-3 d-313 2 -0 7 LU -713 jE1 LU D-4A 4LU S ¢ 7 13-413 ". W � § LL d_ 6 LJ 10 2 D-10 I 4 S 7 LV 13-813 4 4 Q D-5 13-513 _� S 7 8 10 §§ 10 4I4I 7 8 8 1 12 ,..... .........� 4 I 1 10 13 d-'�'� 11 11 1 ^II 10 12 12 � I ❑ 7 ...,.�,... 7..§ I v I 11 12 ❑ 1 d 1 . 110 10 13 .,. IT— 11 14 7 8 12 6 * E „13 3 7 J 110 10 11 § 14 hll ld § S 12 12 11 qh qh 6 .......�,,... 11 12 13`.14 10 13 14 1 12 13 .. 10 ...11 .. 12 13 0.001 0.01 0.1 SLOPE IN FEET PER FOOT 12/12/2016 2017 City of Renton Surface Water Design Manual 4-66 4.4.1 OPEN CHANNELS FIGURE 4.4.I.E GEOMETRIC ELEMENTS OF COMMON SECTIONS N <� i N r1 0 + N � O N I.- (D(D \� J .. •V � + o Q rn N >~ CU to N O L O + + O + + 41 X •� + M N L N X d NN N N L E ILL C L y-+ N a N p + N O O E X O O T i � T 7- CIS Ta I, TII-. j;l 1 ;I CC co w 1 1 1 2017 City of Renton Surface Water Design Manual 12/12/2016 4-67 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS FIGURE 4.4.I.FOPEN CHANNEL FLOW PROFILE COMPUTATION Q= n= So= a= Y= y A R R4/3 V a V2/2g E AE Sr _ _ Ax x Sr So-Sr (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) 12/12/2016 2017 City of Renton Surface Water Design Manual 4-68 4.4.1 OPEN CHANNELS FIGURE 4.4.1.G DIRECT STEP BACKWATER METHOD-EXAMPLE Y A R R4/3 V aVl2g E AE Sf _ _ Ax x Sf So- Sf (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) 6.0 72.0 2.68 3.72 0.42 0.0031 6.0031 - 0.00002 - - - - 5.5 60.5 2.46 3.31 0.50 0.0040 5.5040 0.4990 0.00003 0.000025 0.00698 71.50 71.5 5.0 50.0 2.24 2.92 0.60 0.0064 5.0064 0.4976 0.00005 0.000040 0.00696 71.49 142.99 4.5 40.5 2.01 2.54 0.74 0.0098 4.5098 0.4966 0.00009 0.000070 0.00693 71.64 214.63 4.0 1 32.0 1.79 1 2.17 0.94 1 0.0157 4.0157 0.4941 1 0.00016 0.000127 0.00687 1 71.89 286.52 3.5 24.5 1.57 1.82 1.22 0.0268 3.5268 0.4889 0.00033 0.000246 0.00675 72.38 358.90 3.0 18.0 1.34 1.48 1.67 0.0496 3.0496 0.4772 0.00076 0.000547 0.00645 73.95 432.85 2.5 12.5 1.12 1.16 2.40 0.1029 2.6029 0.4467 0.00201 0.001387 0.00561 79.58 512.43 2.0 8.0 0.89 0.86 3.75 0.2511 12.2511 0.3518 0.00663 1 0.004320 0.00268 131.27 1 643.70 The step computations are carried out as shown in the above table. The values in each column of the table are explained as follows: Col. 1. Depth of flow(ft)assigned from 6 to 2 feet Col.2. Water area(ft)corresponding to depth in Col. 1 Col. 3 Hydraulic radius(ft)corresponding to y in Col. 1 Col. 4. Four-thirds power of the hydraulic radius Col. 5. Mean velocity(fps)obtained by dividing Q(30 cfs)by the water area in Col. 2 Col. 6. Velocity head(ft) Col. 7. Specific energy(ft)obtained by adding the velocity head in Col. 6 to depth of flow in Col. 1 Col. 8. Change of specific energy(ft)equal to the difference between the E value in Col. 7 and that of the previous step. Col.9. Friction slope Sf,computed from V as given in Col. 5 and R"in Col.4 Col. 10.Average friction slope between the steps, equal to the arithmetic mean of the friction slope just computed in Col.9 and that of the previous step Col. 11.Difference between the bottom slope,So, and the average friction slope,Sf Col. 12.Length of the reach(ft)between the consecutive steps; Computed by Ax=AEl(S,,-Sf)or by dividing the value in Col. 8 by the value in Col. 11 Col. 13.Distance from the beginning point to the section under consideration. This is equal to the cumulative sum of the values in Col. 12 computed for previous steps. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-69 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS FIGURE 4.4.1.11 BWCRAN COMPUTER SUBROUTINE—VARIABLE DEFINITIONS SF = SF1 + SF2 2 EC*2.0 222 1 g V 2 E.G.L I _ ... 0 E1 I EC* 2g _ ...... 0 YN-IN I YC-IN I Y2 E2 I Y1 YN-OT YC-OT I_ � X .I I— ox �- current previous cross-section cross-section BWCRAN-VARIABLE DEFINITIONS YC-IN Critical Depth (ft) at current section based on incoming flow rate. YC-OT Critical Depth (ft) at current section based on outgoing flow rate. YN-IN Normal Depth (ft) at current section based on incoming flow rate/channel grade. YN-OT Normal Depth (ft) at current section based on outgoing flow rate/channel grade. Y1 Final Water Depth (ft) at current cross section N-Y1 Composite n-factor of current section for final depth, Y1. A-Y1 Cross-sectional Area of current section for final depth, Y1. WP-Y1 Wetted Perimeter(ft)of current section for final depth, Y1. V-Y1 Average Velocity(fps)of current section for final depth, Y1. E1 Total Energy Head (ft)at current section Y1+EC*V 2 E2 Total Energy Head (ft)at pervious or downstream section. SF1 Friction Slope of current section. SF2 Friction Slope of previous or downstream section. DXY Distance (expressed as a fraction of the current reach length)from the previous or downstream section to where the flow profile would intersect the final water depth, Y1, assuming Y1 were to remain constant EC Energy Coefficient"a," Q-TW The flow rate used to determine Tailwater Height from an inputted HW/TW Data File. TW-HT Tailwater Height. Q-Y1 Flow rate (cfs) in channel at current section, for depth, Y1 VU-Y1 Upstream Velocity(fps)at current section for depth, Y1 ("Adjust"option). V1-HD Channel Velocity Head (ft) at current section. VU-HD Upstream Velocity Head (ft)at current section. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-70 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS This section describes the floodplain/floodway studies required by Special Requirement 42,Flood Hazard Area Delineation,in Section 1.3.2.Floodplain/floodway studies, as required by this manual, establish base flood elevations and delineate floodplains and/or floodways when the City determines that a proposed project contains or is adjacent to a flood hazard area for a river, stream,lake,wetland,closed depression, marine shoreline, or other water feature. Furthermore,when development is proposed within the floodplain,the floodplain/floodway study is used to show compliance with the critical areas code (RMC 4-3-050)flood hazard area regulations. There are four conditions affecting the requirements for floodplain/floodway studies. Each condition is considered a threshold for determining the type of studies required and the documentation needed to meet the study requirements. Each study threshold and related study requirements are shown in the table below, and described further in this section. Note that any projects or related flood studies that are expected to result in a change to Base Flood Elevations published in FEMA Flood Insurance Studies and Rate Maps,must also comply with 44 CFR Part 65. TABLE 4.4.2.A FLOODPLAIN/FLOODWAY STUDY THRESHOLDS AND REQUIREMENTS Threshold Study Requirements The project site is on land that is No floodplain study • Show delineation of floodplain on outside of an already delineated required the site improvement plan and floodplain and above the floodplain's indicate base flood elevation base flood elevation based on best . Record a notice on title available floodplain data. See Section 4.4.2.1 for more details The project site is on land that is at Approximate Floodplain • Submit an engineering plan with least 10 feet above the ordinary high Study per Section 4.4.2.2 approximate base flood elevation water mark or 2 feet above the . Record a notice on title downstream overflow elevation of a See Section 4.4.2.2 for more details water feature for which a floodplain has not been determined. The project site does not meet the Minor Floodplain Study per • Backwater model above thresholds and is either on land Section 4.4.2.3 . Submit an engineering plan with that is outside of an already determined base flood elevations delineated Zone A floodplain (i.e., . Record a notice on title without base flood elevations See Section 4.4.2.3 for more details determined), or is adjacent to a water feature for which a floodplain has not been determined. The project site is on land that is Major Floodplain/Floodway • Show mapped floodplain/floodway partially or fully within an already Study per Section 4.4.2.4 on the site improvement plan and delineated floodplain of a river or indicate base flood elevation stream, or is determined by a Minor . Record a notice on title Floodplain Study to be partially or fully . See further requirements in within the floodplain of a river or Section 4.4.2.4 stream. For any project site or study that is intended to result in a change to FEMA Flood Insurance Study or Rate Maps, including changing published based flood elevations,the applicant must comply with documentation and approval requirements of FEMA regulations 44 CFR Part 65. 1 For marine shorelines, refer to the FEMA Guidelines and Specifications for Flood Hazard Mapping Partners. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-71 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS 4.4.2.1 NO FLOODPLAIN STUDY REQUIRED IF the proposed project site is on land that is outside of an already delineated floodplain and is above the already determined base flood elevation for that floodplain,based on best available floodplain data,THEN no floodplain study is required. In this situation,if the already determined floodplain covers any portion of the site,the boundary of that floodplain and its base flood elevation must be shown on the project's site improvement plan. In addition, a notice on title must be recorded for the site, alerting future property owners of the presence of a flood hazard area on the site and its base flood elevation. The notice on title requirement may be waived if the floodplain is not on any portion of the site. 4.4.2.2 APPROXIMATE FLOODPLAIN STUDY If the proposed project site is on land that is at least 10 feet above the ordinary high water mark or 2 feet above the downstream overflow elevation of a water feature for which the floodplain has not been delineated,then an Approximate Floodplain Study may be used to determine an approximate floodplain and base flood elevation. The intent of the Approximate Floodplain Study is to reduce required analysis in those situations where the project site is adjacent to a flood hazard area,but by virtue of significant topographical relief,is clearly in no danger of flooding. The minimum 10 feet of separation from ordinary high water reduces the level of required analysis for those projects adjacent to streams confined to deep channels or ravines, or near lakes or wetlands. The minimum 2 feet clearance above the downstream overflow elevation is intended to avoid flood hazard areas created by a downstream impoundment of water behind a road fill or in a lake,wetland, or closed depression. Use of the Approximate Floodplain Study requires submittal of an engineering plan" showing the proposed project site is at least 10 feet above the ordinary high water elevation of the water feature in question, or at least 2 feet above the downstream overflow elevation of the water feature,whichever is less, subject to the following conditions: 1. The design engineer preparing the engineering plan shall determine an approximate base flood elevation and include a narrative describing his/her level of confidence in the approximate base flood elevation. The narrative must include,but is not limited to, an assessment of potential backwater effects(such as might result from nearby river flooding, for example); observations and/or anecdotal information on water surface elevations during previous flood events; and an assessment of potential for significantly higher future flows at basin build out.Note:Many of these issues will have been addressed in a Level I downstream analysis, if required. Acceptance of the approximate base flood elevation shall be at the sole discretion of the City. If the approximate base flood elevation is not acceptable, a Minor Floodplain Study or Major Floodplain/Floodway Study may be required. 2. That portion of the site that is at or below the assumed base flood elevation must be delineated and designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site, notifying future property owners of the approximate floodplain and base flood elevation. 4.4.2.3 MINOR FLOODPLAIN STUDY IF the proposed project site does not meet the conditions for"no floodplain study required"per Section 4.4.2.1 or for use of the Approximate Floodplain Study per Section 4.4.2.2,AND the project site is either on land that is outside of an already delineated Zone A floodplain(i.e.,without base flood Engineering plan means a site improvement plan, including supporting documentation,stamped by a licensed civil engineer. In some instances,CED review staff may determine that the proposed project is sufficiently above the clearances specified in this exception and may not require an engineering plan.Typically,this is done for projects in Simplified Drainage Review that clearly exceed minimum clearances and otherwise would not require engineering design. 12/12/2016 2017 City of Renton Surface Water Design Manual 4-72 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS elevations determined)or is adjacent to a water feature for which a floodplain has not been determined, THEN a Minor Floodplain Study may be used to determine the floodplain. However,if the Minor Floodplain Study determines that all or a portion of the project site is at or below the base flood elevation of a river or stream and thus within the floodplain,then the applicant must either redesign the project site to be out of the floodplain or complete a Major Floodplain/Floodway Study per Section 4.4.2.4. Use of the Minor Floodplain Study requires submittal of an engineering plan and supporting calculations. That portion of the site that is at or below the determined base flood elevation must be delineated and designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site, notifying future property owners of the floodplain and base flood elevation. Methods of Analysis For streams without a floodplain or flood hazard study, or for drainage ditches or culvert headwaters, the base flood elevation and extent of the floodplain shall be determined using the Direct Step backwater method, Standard Step backwater method, or the King County Backwater computer program, as described in Section 4.4.1.2. For lakes,wetlands,and closed depressions without an approved floodplain or flood hazard study,the base flood elevation and the extent of the floodplain shall be determined using the"point of compliance technique"described in Section 3.3.6. 4.4.2.4 MAJOR FLOODPLAIN/FLOODWAY STUDY The floodplain analysis shall be based on the 100-year storm event using existing land use hydrology except as noted in the paragraph titled"Flood Flows from Adopted Basin Plan Information." IF the proposed project site is on land that is partially or fully within an already delineated floodplain of a river or stream, or determined by a Minor Floodplain Study to be partially or fully within the floodplain of a river or stream,THEN a Major Floodplain/Floodway Study is required to determine the floodplain, floodway, and base flood elevation in accordance with the methods and procedures presented in this section. This information will be used by the City to evaluate the project's compliance with regulations for development or improvements within the floodplain. Major Floodplain/Floodway Studies must conform to FEMA regulations described in Part 65 of 44 Code of Federal Regulations(CFR). In addition,the following information must be provided and procedures performed. ❑ INFORMATION REQUIRED The applicant shall submit the following information for review of a floodplain/floodway analysis in addition to that required for the drainage plan of a proposed project. This analysis shall extend upstream and downstream a sufficient distance to adequately include all backwater conditions that may affect flooding at the site and all reaches that may be affected by alterations to the site. Floodplain/Floodway Map A Major Floodplain/Floodway Study requires submittal of five copies of a separate floodplain/floodway map stamped by a licensed civil engineer and a professional land surveyor registered in the State of Washington(for the base survey). The map must accurately locate any proposed development with respect to the floodplain and floodway,the channel of the stream, and existing development in the floodplain;it must also supply all pertinent information such as the nature of any proposed project,legal description of the property on which the project would be located, fill quantity,limits and elevation,the building floor elevations, flood-proofing measures, and any use of compensatory storage. 2017 City of Renton Surface Water Design Manual 12/12/2016 4-73 SECTION 4.4 OPEN CHANNELS,FLOODPLAINS,AND FLOODWAYS The map must show elevation contours at a minimum of 2-foot vertical intervals and shall comply with survey and map guidelines published in the FEMA publication Guidelines and Specifications for Flood Hazard Mapping Partners.The map must show the following: • Existing elevations and ground contours; • Locations, elevations and dimensions of existing structures, and fills; • Size,location, elevation, and spatial arrangement of all proposed structures, fills and excavations, including proposed compensatory storage areas,with final grades on the site; • Location and elevations of roadways,water supply lines, and sanitary sewer facilities,both existing and proposed. Study Report A Major Floodplain/Floodway Study also requires submittal of two copies of a study report, stamped by a licensed civil engineer,which must include calculations or any computer analysis input and output information as well as the following additional information: 1. Valley cross sections showing the channel of the river or stream,the floodplain adjoining each side of the channel,the computed FEMA floodway,the cross-sectional area to be occupied by any proposed development, and all historic high water information. 2. Profiles showing the bottom of the channel,the top of both left and right banks, and existing and proposed base flood water surfaces. 3. Plans and specifications for flood-proofing any structures and fills,construction areas,materials storage areas,water supply, and sanitary facilities within the floodplain. 4. Complete printout of input and output(including any error messages) for HEC-RAS. Liberal use of comments will assist in understanding model logic and prevent review delays. 5. One ready-to-run digital copy of the HEC-RAS input file used in the study. Data shall be submitted in an electronic format. 6. The applicant shall prepare a written summary describing the model development calibration, hydraulic analysis, and floodway delineation. The summary shall also include an explanation of modeling assumptions and any key uncertainties. ❑ DETERMINING FLOOD FLOWS The three techniques used to determine the flows used in the analysis depend on whether gage data is available or whether a basin plan has been adopted.The first technique is for basins in adopted basin plan areas. The second technique is used if a gage station exists on the stream. The third technique is used on ungaged catchments or those with an insufficient length of record. In all cases,the design engineer shall be responsible for assuring that the hydrologic methods used are technically reasonable and conservative, conform to the Guidelines and Specifications for Flood Hazard Mapping Partners, and are acceptable by FEMA. Flood Flows from Adopted Basin Plan Information For those areas where the City or King County has adopted a basin plan since 1986, flood flows may be determined using information from the adopted basin plan. The hydrologic model used in the basin plan shall be updated to include the latest changes in zoning,or any additional information regarding the basin that has been acquired