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Home My WebLink AboutEx 11_Drainage Report PRELIMINARY TECHNICAL INFORMATION REPORT FOR Towns on 12th CITY OF RENTON, WASHINGTON 2/18/2022 Prepared by: Andrew Oh, E.I.T., Katie Lane, E.I.T. Approved by: Holli Heavrin, P.E. Date: 2/18/2022 Revised: Core No.: 21008 RECEIVED 05/17/2022 AMorganroth PLANNING DIVISION DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH i TOWNS ON 12TH Table of Contents 1. PROJECT OVERVIEW .......................................................................................................................... 1-1 2. CONDITIONS AND REQUIREMENTS SUMMARY ................................................................................ 2-2 2.1 Core Requirements .......................................................................................................................... 2-3 2.1.1 Core Requirement #1: Discharge at the Natural Location ...................................................... 2-3 2.1.2 Core Requirement #2: Offsite Analysis .................................................................................... 2-3 2.1.3 Core Requirement #3: Flow Control......................................................................................... 2-3 2.1.4 Core Requirement #4: Conveyance System ............................................................................. 2-3 2.1.5 Core Requirements #5: Erosion and Sediment Control ........................................................... 2-3 2.1.6 Core Requirement #6: Maintenance and Operations .............................................................. 2-3 2.1.7 Core Requirement #7: Financial Guarantees and Liability ....................................................... 2-3 2.1.8 Core Requirement #8: Water Quality ...................................................................................... 2-5 2.1.9 Core Requirement #9: On-Site BMPs ....................................................................................... 2-6 2.2 Special Requirements ...................................................................................................................... 2-6 2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements .................................... 2-6 2.2.2 Special Requirement #2: Flood Hazard Area Delineation ........................................................ 2-6 2.2.3 Special Requirement #3: Flood Protection Facilities ................................................................ 2-6 2.2.4 Special Requirement #4: Source Control ................................................................................. 2-6 2.2.5 Special Requirement #5: Oil Control ........................................................................................ 2-6 2.2.6 Special Requirement #6: Aquifer Protection Area ................................................................... 2-6 3. OFFSITE ANALYSIS ............................................................................................................................. 3-1 3.1 Resource Review .............................................................................................................................. 3-1 3.1.1 Sensitive Areas ......................................................................................................................... 3-1 3.2 Field Investigation ............................................................................................................................ 3-4 4. FLOW CONTROL AND WATER QUALITY DESIGN ............................................................................... 4-7 4.1 Hydraulic Analysis ............................................................................................................................ 4-7 4.1.1 Existing Conditions ................................................................................................................. 4-10 4.1.2 Upstream Conditions.............................................................................................................. 4-10 4.1.3 Developed Conditions ............................................................................................................ 4-12 4.2 Water Quality Treatment Analysis and Design .............................................................................. 4-19 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH ii 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ................................................................................. 5-1 6. SPECIAL REPORTS AND STUDIES ....................................................................................................... 6-1 7. OTHER PERMITS ................................................................................................................................ 7-1 8. ESC ANALYSIS AND DESIGN ............................................................................................................... 8-1 9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT ............................. 9-1 10. OPERATIONS AND MAINTENANCE .................................................................................................. 10-1 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 1-1 1. PROJECT OVERVIEW The proposed Towns on 12th project is located to the north of the intersection of NE 12th Street and NE 11th Place Renton, Washington within Section 4, Township 23 North, Range 5 East, W.M. See Vicinity Map below. The site is composed of seven parcels #042305-9063, #042305-9099, #042305-9067, #042305-9140, #042305-9100, #042305-9101 and #032405-9247 with an area of approximately 6.55 acres. The site is bordered by NE Sunset Blvd. (which is a major arterial) to the north, NE 12th Street (collector) to the south, residential developed parcels to the west, and commercial developed parcels to the east. The site has a high point in the middle of the site and slopes down in all directions ranging from 5-20%. Proposed development of the property will include the construction of multiple townhomes, private roads, utilities, and minor frontage improvements. All existing structures and hard surfaces will be demolished and removed. The subject project’s drainage facilities were designed using the guidelines and requirements established in the 2017 City of Renton Surface Water Design Manual (RSWDM). Flow Control Duration Standard (Matching Forested) and Enhanced Basic Water Quality Treatment are required for this project. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 2-2 2. CONDITIONS AND REQUIREMENTS SUMMARY The proposed project is classified as requiring “Full Drainage Review” per the 2017 RSWDM. Therefore, all nine core requirements and six special requirements will be addressed per Section 1.2 and 1.3 of the 2017 RSWDM. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 2-3 2.1 Core Requirements 2.1.1 Core Requirement #1: Discharge at the Natural Location The project discharges its drainage at the natural location which is to the City’s conveyance system located along the south side of NE Sunset Blvd. 2.1.2 Core Requirement #2: Offsite Analysis See Section 3 of this Report for the downstream analysis. 2.1.3 Core Requirement #3: Flow Control The site falls within the City’s Flow Control Duration Standard (Matching Forested). See City’s Flow Control Application Map on the following page. This flow control standard requires matching forested conditions over the range of flows extending from 50% of 2-year up to the full 50-year flow AND matches peaks for the 2- and 10-year return periods. See Section 4 of this Report for details on the Flow Control Analysis. 2.1.4 Core Requirement #4: Conveyance System This core requirement will be addressed during final design. 2.1.5 Core Requirements #5: Erosion and Sediment Control This core requirement will be addressed during final design. 2.1.6 Core Requirement #6: Maintenance and Operations This core requirement will be addressed during final design. 2.1.7 Core Requirement #7: Financial Guarantees and Liability This core requirement will be addressed at the time the permit is issued. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Lake Desire Shady Lake (Mud Lake) Panther Lake Lake Youngs Lake Washington B l a c k Ri ve r Gr een Ri v e r Ce darRi verUV900 UV167 UV515 UV169 UV900 UV169 UV167BN IncBN IncBBNNIInnccSSEE RReennttoonn IIssss aa qquuaahh RR dd RReennttoonn MMaappllee VVaalllleeyyRRdd MMaapplleeVVaalllleeyyHHwwyy 110088tthhAAvveeSSEESSWW SSuunnsseettBBllvv dd RRaaiinnii eerrAAvveeNNNE 3rd S t NE 3rd S t SW 43rd StSW 43rd St SS EE CCaarrrrRR dd NE 4th StNE 4th St SSEE RReennttoonn MMaappllee VVaalllleeyy RRddLLooggaannAAvveeNN SR 515SR 515PPaarrkkAAvveeNNOOaakkeessddaalleeAAvveeSSWWSSuunnsseettBBllvvddNN EE DDuuvvaallllAAvveeNNEEI-405 FWYI-405 FWY II--440055FFWWYYSR 167SR 1671144 00tthh WWaayy SS EENNEE 2277tthh SStt 115566tthhAAvveeSSEEUUnniioonnAAvveeNNEE111166tthhAAvveeSSEESW 7th StSW 7th St N 8th StN 8th St PP uuggeettDDrrSSEE RR ee nnttoonnAAvvee SS SSWW 2277tthh SStt BBeennssoonnRRddSSWWiilllliiaammssAAvveeSSMMoonnrrooeeAAvveeNNEESE 128th StSE 128th St II nntt eerr uurr bbaannAA vvee SS HHooqquuiiaammAAvveeNNEE8844tthhAAvveeSSSSEEPPeett rr oovvii tt sskkyyRRddEEVVaalllleeyyHHwwyySE 192nd StSE 192nd St SE 60th StSE 60th St TTaallbboottRRddSSRRee nn tt oo nn AAvveeSS116644tthhAAvveeSSEESE 208th StSE 208th St SE 72nd StSE 72nd St RR aaiinniieerr AA vvee SS 111166tthhAAvveeSSEES 128th StS 128th St NNeewwccaassttllee WWaayy SS 221122tthh SStt SS 118800tthh SStt CCooaall CCrreeeekkPPkkwwyySSEESW 41st StSW 41st St 114400tthhAAvveeSSEE112288tthhAAvveeSSEE6688tthhAAvveeSSSSEE 116688tthh SStt NE 12th StNE 12th St BBeeaaccoonn AA vv ee SS FFoorreesstt DDrr SSEE SSEE 116644tthh SStt 114488tthhAAvveeSSEESSEE MMaayy VVaalllleeyy RRdd SS EE JJ oo nn ee ss RR dd SS EE 22 00 44 tthh WW aayySW 34th StSW 34th St SE 144th StSE 144th St 114488tthhAAvveeSSEE115544tthhPPllSSEELL aa kk ee WWaa sshhii nnggtt oonnBBll vvddNNEEddmmoonnddssAAvveeNNEEAAbbeerrddeeeennAAvveeNNEEEEMM eerrcceerrWWaayyWWeessttVVaalllleeyyHHwwyyEast Valley RdEast Valley Rd,§-405 ,§-405 ,§-405 μ 012 Miles Flow Control Application Map Reference 1-A Date: 01/09/2014 Flow Control Standards Peak Rate Flow Control Standard (Existing Site Conditions) Flow Control Duration Standard (Existing Site Conditions) Flow Control Duration Standard (Forested Conditions) Flood Problem Flow Unincorporated King County Flow Control Standards Renton City Limits Potential Annexation Area PROJECT SITE DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 2-5 2.1.8 Core Requirement #8: Water Quality As the City of Renton does not have Sensitive Lake or Sphagnum Bog Water Quality Treatment Areas, Basic Water Quality Treatment is the only other option available. As the proposed project is a Multifamily Residential development, Enhanced Basic Water Quality Treatment is required. See Section 4 of this Report for details on Water Quality Facility Sizing. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 2-6 2.1.9 Core Requirement #9: On-Site BMPs See Section 4.1.2 of this Report for discussion on how this Core Requirement is addressed. 2.2 Special Requirements 2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements There are no known additional requirements for the subject project. 2.2.2 Special Requirement #2: Flood Hazard Area Delineation Not applicable since the project does not contain nor is adjacent to a flood hazard area. 2.2.3 Special Requirement #3: Flood Protection Facilities Not applicable since the project does not rely on an existing flood protection facility or plans to modify or construct a new flood protection facility. 2.2.4 Special Requirement #4: Source Control Not applicable since the project is not a commercial development. 2.2.5 Special Requirement #5: Oil Control Not applicable since the project is not a high use site. The expected average daily traffic is less than 100 vehicles per 1,000 square feet of gross building area. 2.2.6 Special Requirement #6: Aquifer Protection Area Not applicable since the project is not in an Aquifer Protection Area. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-1 3. OFFSITE ANALYSIS 3.1 Resource Review The proposed project is located within the Lake Washington Cedar River Watershed. 3.1.1 Sensitive Areas Renton GIS was reviewed for sensitive areas. The proposed project site falls within the following sensitive areas: landslide and regulated slopes. The proposed project site does not fall within the following sensitive areas: coal mines, erosion hazard, flood hazard, floodway, channel migration zone, seismic hazard, regulated stream, or wellhead protection. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-2 According to the Renton GIS Map, there is a portion of the site along the north boundary which falls within a landslide hazard area. See picture below. Landslide Hazard (Per Renton GIS Data) DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-3 According to the Renton GIS Map, there is a portion of the site along the north boundary which falls within 15%-90% regulated slope. See picture below. Regulated Slope: 15% to 90% Slope Area (Per Renton GIS Data) DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-4 3.2 Field Investigation A field investigation was completed on June 9, 2021. Upstream Tributary Analysis There is a neighborhood directly west of the project site. In this neighborhood there are three side roads that are located to the northwest of the project boundary that point towards the site: NE 14th Place, NE 14th Court, and NE 14th Street.. The southern-most road (NE 14th Street) slopes east towards the site, but has a mound and catch basin located at the low point of the road. This would prevent any runoff from this road from entering the project site. The other two roads slope west away from the project site. We will assume that the site’s western boundary acts as a high point for the purposes of the basin boundary. The rest of the homes located to the south of these three side roads drain directly into the existing detention pond located in this neighborhood. There is a storm system located in the backyard of these homes that will collect all backyard drainage and direct it to the detention pond as well. We can assume that this portion of the site’s western boundary is also a high point for the purposes of the basin boundary. There is additional upstream area to the site that is accounted for in the existing wetland located along the west boundary of the site. The area from four existing homes to the south of the wetland, drains to the wetland and discharges into the site. Along the south property line lies NE 12th Street. No runoff from NE 12th Street will enter the project site as there is an existing mound along the north edge of NE 12th Street that prevents any runoff from entering into the site. Downstream Drainage Complaints Drainage complaints were researched within a quarter mile of the project site. City of Renton does not list any current complaints along the project’s downstream route. Onsite Drainage System Description The site consists of seven connected parcels. The center of the project site contains a single-family residence and shed. A dirt roadway creates a passage between the west and east portions of the site which connects through the center of the site. There are two additional single-family homes in the southern portion of the site. The remainder of the site is undeveloped consisting of thick forest cover. The northwestern portion of the site contains a large meadow with thick grass cover. Large trees are found in abundance throughout the project site. The entire site is located within the May Creek drainage basin. The topography of the entire site generally slopes to the north. Steep slopes are found along the northern boundary of the parcel bordering NE Sunset Blvd. The site also contains steep slopes along the eastern boundary of the project site. These slopes lead downhill eastward towards the adjacent businesses on NE Sunset Blvd. These slopes lead to the existing stormwater collection system along the southern border of NE Sunset Blvd. The meadow in the northwestern region of the map is a localized DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-5 low point for site runoff. The slope is directed in a northeastern direction towards the adjacent road from this point. Downstream Drainage System Description Runoff travels over the site via sheet flow over thick forested ground cover. All flow onsite is eventually directed towards NE Sunset Blvd. which is located north of the site. An existing catch basin network along the southern boundary of NE Sunset Blvd. serves to collect runoff and continue the flow eastward (see downstream map). All the receiving catch basins north of site are located along the southern edge of NE Sunset Blvd. All runoff is directed to the point where it is conveyed across the highway and then directed northward into Honey Dew Creek. Honey Dew Creek flows northward and eventually joins May Creek beyond the ¼ mile extent of the analysis. At this point the downstream analysis is terminated. See downstream map for details. Downstream Reach Description to a quarter mile Pipe Material Pipe Diameter (in) Overall Length (ft) Cumulative Length (ft) Concrete 18 262 262 Concrete 18 55 317 Concrete 18 53 370 Unknown 18 47 417 Honey Dew Creek N/A 1,273 1,320 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 3-6 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-7 4. FLOW CONTROL AND WATER QUALITY DESIGN A flow control facility is proposed as the project is required to meet the City’s Flow Control Duration Standard (Matching Forested). This flow control standard requires matching forested conditions over the range of flows extending from 50% of 2-year up to the full 50-year flow and matching peaks for the 2- and 10-year return periods. A water quality treatment facility is proposed as the project is required to provide enhanced basic water quality treatment as delineated in Section 4.2 of this Report. 4.1 Hydraulic Analysis The drainage analysis was modeled using WWHM. Per the City of Renton Soil Survey, the site soil is Alderwood (AgC), hydrologic soil group “C”, Till for modeling. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E BeD BeD BeC AkF AmC KpD AmB AmB AgC AgC AgC KpB AgC AkF BeD AgC KpD AgCKpBKpBAgD EvC Bh BeC AgB KpB AgD UrEwC EvB AmC Or AgC BeC BeD BeD BeC AgC KpD BeC KpCKpB Sk AmC BeC UrBeCUr AgD KpB Ur AgC EvC KpC AgC OvD SkAgB AgB AgD EvC W OvD OvD No AgC AgD EvC RdC Sk KpB No AgC RdEAgCEvC BeC InC Bh BeD RdC AkF AgD BeD BhSm AgC RdE AgC AgDEvC Sh Sh PITS PyAgC AgD AgC EvC PITS EvB BeC OvC PITS EvB AgC InA AgD AgD Ur Bh AgC RdCEvCEvD AmB AmC AgD BeD EvC AkF AgD AgC Sm AgD AmC EvB AgD Sm AgC EwC PITSPu Bh Bh AgC KpD Tu AgC EvC EvC Ur RdC KpB RdCRdE W AgD AgD No AgB AgC No An AkF EvB InD AgD EvD InC Sk BeC PITS Ur WBeC W BeD An EvC BeC Pc EvC Rh AmC BeCBeD BeD Ur BeC Ur W W Rh EvC AkF PITS AgD AmCPITS Wo Py Tu BeD EvB Py AmC Py An InC Ng AgC AgC W EvB AgD AgCWo Ur W Ur RdC Ur Ur EvC Rh Pc PyPy AgD AmC AgD MaRh Pc BeD Py Ng AmC Ng BeC AgC Rh AgD Py Ma PyRh Rh RhPu Pu Ur Pu PITS BeD Rh EvC Py Wo Wo AgC Ma AgD AgB So Ng AgD Tu Py AgC AgC AgC AgC Ng Ur AgD Sk AmB AgB AgD PuUr Py Sk Ur No AgB MaBeC W AkFBeD Pu Py No No AmC AgC EvC Ma Sk EvC AgB Py InC SkAmC AmC AgC AgB SkAgB PITS AgC No Pu AmB EvC PITS AmB AgB AkF AkF Ng AgD AgD AgB EvBAmB AmC AmC Wo Ng PITS Nk AgD SkNk SkAgB Ur AgD Wo AmC No AgD No Ur AgB Ur Br Py Os So EvB Ur Nk Wo AmB W Pc OsRe UrAmB Sk W AmC Tu No Pu AgD AmBAkFAgCNo UrPk AgB AgC Ur AmB NoAmC NoAmC AmB PITS UrSkAgCAgB AgC Re AgB W InC No AgC Wo EvC NoAmC Wo AgB AgB Ur Ng SkAmB ReOs AgC No AgD No Ur No InC AgD EvC Tu AmB EvB AgD Pu InC EvCAmC NoEvC Ur EvB AkF Br Os AgD AmC AgB AgD AmB Os Os NoAmC Pk AkF No AmC EvC NoNo AgDBr AgBAgC AgBAmC Re Re KpBAmBNoPyPySkNgAgBEvC EvC Sk NoEvBNo Pu BrPuOsOsWo AmC Panther Lake Lake Youngs Lake Washington B l a c k Ri v er Gr een R iverCeda rRiverDuwamish W a t erw ayUV167 UV900 UV515 UV169 UV900 UV167BN IncBN IncBBNNIInnccSSEE RReennttoonn IIss ss aa qq uuaahhRRdd S 2nd StS 2nd St RReennttoonn MMaappllee VVaalllleeyyRRdd MMaapplleeVVaalllleeyyHHwwyy 110088tthhAAvveeSSEEMMaaiinnAAvveeSSMMaa rrttiinn LL KKiinnggJJrrWWaayySS SSWW SSuunnsseett BBllvv dd RRaaiinnii eerrAAvveeNNNE 3rd S t NE 3rd S t II--440055FFWWYYSW 43rd StSW 43rd St SSEE CCaarrrrRR dd NE 4th StNE 4th St SS GG rr aa dd yy WW aa yy SSEE RReennttoonn MMaappllee VVaalllleeyy RRddLLooggaannAAvveeNN SR 515SR 515PPaarrkkAAvveeNNBBeennssoonnDDrrSSSSuunnsseettBBllvvddNN OOaakkeessddaalleeAAvveeSSWWSSuunnsseettBBllvvddNN EE DDuuvvaallllAAvveeNNEESR 167SR 16711 4400tthh WWaayySSEEWWaa tt ee rrss AA vv ee SS NNEE 2277tthh SStt HH oouusseerrWWaayyNN115566tthhAAvveeSSEEUUnniioonnAAvveeNNEE111166tthhAAvveeSSEESW 7th StSW 7th St PPuuggeettDDrrSSEERR ee nnttoonnAAvvee SS GGaarrddeennAAvveeNNSSWW 2277tthh SStt BBeennssoonnRRddSSWWiilllliiaammssAAvveeSSMMoonnrrooeeAAvveeNNEEII nntt eerr uurr bbaannAAvveeSS HHooqquuiiaammAAvveeNNEE8844tthhAAvveeSSSSEEPPeettrroovviittsskkyyRRdd SSoouutthhcceenn tt ee rr BB llvvdd EEVVaalllleeyyHHwwyySShhaattttuucckkAAvveeSSRR aaiinniieerr AAvvee SS TTaallbboottRRddSSRRee nn tt oo nn AAvv ee SS116644tthhAAvveeSSEESE 208th StSE 208th St SSWWLLaannggssttoonnRRdd SE 72nd StSE 72nd St SE 128th StSE 128th St 112244tthhAAvveeSSEES 128th StS 128th St NNeewwccaassttllee WWaayy SS 221122tthh SStt SS 118800tthh SStt CCooaallCCrr eeeekkPPkkwwyySSEESW 41st StSW 41st St N 30th StN 30th St T a y l o r P l NW T a y l o r P l NW114400tthhAAvveeSSEE112288tthhAAvveeSSEE111166tthhAAvveeSSEE6688tthhAAvveeSSSSEE 116688tthh SStt NE 12th StNE 12th St BB ee aa cc oo nn AAvv ee SSFFoorreesstt DDrr SSEE UUnniioonnAAvveeSSEESE 164th StSE 164th St NNiilleeAAvveeNNEE114488tthhAAvveeSSEESSEE MMaayy VVaalllleeyy RRdd SS EE 22 00 44 tthh WW aayySW 34th StSW 34th St SS EE JJ oo nnee ss RR dd SE 144th StSE 144th StEEMMeerrcceerrWWaayy 114488tthhAAvveeSSEEWWMMeerrcceerrWWaayy115544tthhPPllSSEEEEddmmoonnddssAAvveeNNEEAAbbeerrddeeeennAAvveeNNEEWWeessttVVaalllleeyyHHwwyyEast Valley RdEast Valley Rd,§-405 ,§-405 Reference 1-C Renton City Limits Potential Annexation Area Groundwater Protection Area Boundary Aquifer Protection Area Zone 1 Aquifer Protection Area Zone 1 Modified Soil Type AgB AgC AgD AkF AmB AmC An BeC BeD Bh Br EvB EvC EvD EwC InA InC InD KpB KpC KpD Ma Ng Nk No Or Os OvC OvD PITS Pc Pk Pu Py RdC RdE Re Rh Sh Sk Sm So Tu Ur W Wo Date: 01/09/2014 012 MilesμSoil Survey PROJECT SITE DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-9 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-10 4.1.1 Existing Conditions See Existing/Upstream Conditions exhibit on the following pages. The existing basin boundary area, 6.70 acres, will include the property area, 6.55 acre, plus the proposed frontage improvement area along NE 12th Street, 0.15 acre. As part of the project, the City has requested the existing sidewalk along NE Sunset Blvd. be relocated to create a planter strip. As the project is simply moving the existing sidewalk, this sidewalk construction will not be considered as a targeted impervious surface for the purpose of drainage design. Some areas of the subject site discharge west offsite providing recharge to the existing offsite wetland. During final design, an area will be designated to discharge to this offsite wetland. Drainage collected by this wetland though, discharges back onto the project site and will be conveyed through the stormwater flow control and treatment facilities. The following information was used for generating flow frequencies. EXISTING CONDITIONS Total Area = 6.70 acres GROUND COVER AREA (acres) Till-Forest 6.70 4.1.2 Upstream Conditions See Existing/Upstream Conditions exhibit on the following pages. The project site has two upstream areas. The first upstream area is located along the southern half of the west boundary line of the site. This upstream area is tributary to the existing wetland located on site. The second upstream area is located at the southwest corner of the site. This area is tributary to the site. Impervious coverage from all these areas were delineated through the use of aerial images at correct scales to outline the impervious areas. The following information was used for generating flow frequencies. UPSTREAM CONDITIONS Total Area = 1.19 acres GROUND COVER AREA (acres) Till-Grass 0.78 Impervious 0.41 The below inputs into WWHM for the basin include both the existing area (6.70 acres) and the upstream areas draining to the site (1.19 acres). DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E DESIGNE N G I N E E R I N G P L A N N I N G S U R V E Y I N G12100 NE 195th St, Suite 300Bothell, Washington 98011425.885.7877 Fax 425.885.796321008DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-12 4.1.3 Developed Conditions See Developed/Upstream Conditions exhibit on the following pages. For preliminary design, the developed basin boundary area is equal to and the same as the existing basin boundary area, 6.70 acres. For the final design, some area swapping will be incorporated into the flow control and water quality treatment design. Drainage along NE 12th Street will be collected and discharged to the existing conveyance system along NE 12th Street. Modeling of upstream areas will be adjusted in final design to account for the runoff from NE 12th Street which will not be collected by the proposed system. Impervious areas for the site were calculated based on the proposed site plan per the project’s designation as multi-family. The impervious areas are summarized in the table below. The site areas for the upstream condition are provided in Section 4.1.2 of this report. Credits received for Onsite BMPs are not considered for this preliminary design. Onsite BMP credits will be reviewed and incorporated during final design. DEVELOPED CONDITIONS Total Area Impervious Area Pervious Area Sidewalk 16,750 16,750 0 Road/Driveway 64,392 64,392 0 Roof 78,646 78,646 0 Grass 132,199 0 132,199 Total Area (SF) 291,987 159,788 132,199 Total Area (AC) 6.70 3.67 3.03 The inputs into WWHM for the basin include both the developed basin area (6.70 acres) and the upstream areas draining to the site (1.19 acres). DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E 04230590630423059099042305906704230591400423059100DESIGNE N G I N E E R I N G P L A N N I N G S U R V E Y I N G12100 NE 195th St, Suite 300Bothell, Washington 98011425.885.7877 Fax 425.885.796321008DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-14 FLOW CONTROL FACILITY ANALYSIS Flow control facilities have been designed using WWHM. Proposed vault dimensions are given below along with WWHM generated modeling reports. The project will utilize a standard detention vault to provide flow control. The required surface area of the vault is 7,392 square feet. Using the maximum water surface of the 50-year storm (Elev. 405.82) as calculated in the vault overflow analysis below under VAULT OVERFLOW ANALYSIS, along with the normal water surface elevation (Elev. 393.50), the minimum required volume will be the surface area times the live storage depth, 7,392 square feet x (405.82 – 393.50) = 91,069 cubic feet. The proposed detention vault will provide a volume of 95,234 cubic feet, exceeding the required volume, and therefore providing adequate flow control. The full WWHM generated report is provided in Appendix A. VAULT OVERFLOW ANALYSIS The water surface elevations within the vault are conservatively calculated based on the equation below as WWHM does not accurately calculate the water surface when the water surface starts to exceed the riser elevation. The primary overflow for the vault is the riser pipe on the control structure. The water surface elevation above the riser for the 100-year developed flow is calculated assuming all orifices are plugged. The 100- year, 15-minute return period storm for the developed tributary area is 5.82 cfs. See snip from WWHM model below which shows the developed site undetained flows. To pass the 100-year, 15-minute return period storm, 5.82 cfs, through a 36-inch overflow riser, 0.34 feet of head is required per the following equation: = 9.739 / 5.82 = 9.739 3 / The primary overflow elevation, elevation 405.84, would therefore, be equal to the elevation of the top of the riser, elevation 405.50, plus the amount of head required to pass the 100-year return period storm, 0.34 feet. This primary overflow elevation will be utilized as the vault’s tailwater elevation for sizing of the conveyance system during final design. The 50-year maximum water surface for the vault is determined in the same way as the 100-year overflow. The water surface elevation above the riser for the 50-year developed flow is calculated assuming all orifices are plugged. The 50-year, 15-minute return period storm for the developed tributary area is 5.18 cfs. See snip from WWHM model below which shows the developed site undetained flows. To pass the 50-year, 15-minute return period storm, 5.18 cfs, through a 36-inch overflow riser, 0.32 feet of head is required per the following equation: = 9.739 / 5.18 = 9.739 3 / The 50-year maximum water surface elevation, elevation 405.82, would therefore, be equal to the elevation of the top of the riser, elevation 405.50, plus the amount of head required to pass the 100- year return period storm, 0.32 feet. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-15 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-16 ON-SITE BMPs Per Section 1.2.9.1 in the RSWDM, projects subject to Core Requirement #9 must apply flow control BMPs to either supplement the flow mitigation provided by required flow control facilities or provide flow mitigation where flow control facilities are not required. Flow control BMPs must be implemented per the requirements and approach detailed in Sections 1.2.9.2 and 1.2.9.3 for individual lots and subdivisions or road improvement projects, respectively. The Section applicable to this project is Section 1.2.9.2. Per Section 1.2.9.2, projects on individual sites/lots, flow control BMPs must be selected and applied according to the individual lot BMP requirements. The category of requirements applicable to the subject project is the Large Lot BMP Requirements (for sites/lots >22,000 square feet). 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, large lot full dispersion will be applicable only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet the minimum requirements for full dispersion in Appendix C, Section C.2.1.1 Full Dispersion is not feasible due to being unable to meet the minimum required flow path with the appropriate slopes. 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. Full Infiltration is not feasible due to existing onsite non-infiltrating till soils. 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 Appendix C, Section C.2.2, or per Section 5.2, whichever is applicable Full Infiltration is not feasible due to existing onsite non-infiltrating till soils • Limited Infiltration per Appendix C, Section C.2.3, Opportunities for Limited Infiltration will be further reviewed during final design. At this time, limited infiltration is not feasible per the project’s geotechnical engineer. Per the Geotechnical Report prepared by Terra Associates, Inc, dated 8/5/21, soils observed on site are not suitable for Flow Control BMPs due to the presence of weathered glacial till at shallow depths. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-17 • 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, Opportunities for Bioretention will be further reviewed during final design. Bioretention is not feasible per the project’s geotechnical engineer. Per the Geotechnical Report prepared by Terra Associates, Inc, dated 8/5/21, soils observed on site are not suitable for Flow Control BMPs due to the presence of weathered glacial till at shallow depths. • Permeable Pavement per Appendix C, Section C.2.7 Opportunities for Permeable Pavement will be further reviewed during final design. Permeable Pavement is not feasible per the project’s geotechnical engineer. Per the Geotechnical Report prepared by Terra Associates, Inc, dated 8/5/21, soils observed on site are not suitable for Flow Control BMPs due to the presence of weathered glacial till at shallow depths. 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 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, Basic dispersion is not feasible due to being unable to meet the minimum required flow path with the appropriate slopes and the steep slope adjacent to the site. 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. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-18 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 The impervious area requiring mitigation is equal to 10% of the Site Area. The Site Area (285,453 square feet) is defined as the property area (289,606 square feet) less the ROW dedication area (4,153 square feet). The site includes buffer areas which totals to 19.268 square feet. This leaves the buildable area at 266,185 square feet, with the proposed impervious of 163,622 square feet The impervious area requiring mitigation is, therefore, equal to 10% of 266,185 square feet, or 26,619 square feet. Opportunities for Reduced Impervious Surface Credit will be reviewed during final design. The only feasible options possible under this option are open grid decking over pervious surface and wheel strip driveways. Since the proposed project will develop the entire site with the exception of sensitive areas and associated buffers, a Native Growth Retention Credit is not feasible. Tree Retention Credits will be applied during final design. 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. Soil amendment will be incorporated for disturbed areas not covered with hard surfaces. 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. All proposed connections to the local drainage system will be provided via a perforated pipe connection. Exhibit 11 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 4-19 4.2 Water Quality Treatment Analysis and Design Per Core Requirement #8: Water Quality, the project is required to provide a water quality system to treat stormwater runoff from pollution-generating surfaces. The site is located within an Enhanced Basic Water Quality Area and is therefore required to implement a system from the Enhanced Basic Water Quality Menu described in Section 6.1.2 of the 2017 RSWDM. The project proposes to implement a Biopod proprietary water quality media filter manufactured by Oldcastle as per Enhanced Basic Option 5 in the 2017 RSWDM. The Biopod filter has General Use Level Designation (GULD) Approval from the Department of Ecology. Water Quality calculations are shown below. The Biopod system is located downstream of the vault, therefore the design flow for sizing of the system is the 2-year flow rate from the vault. The flow rate is 0.2635 as provided from WWHM. Using this table provided by Oldcastle, the 8’x16’ Biopod has sufficient capacity for the flow of 0.2635 cfs out of the vault. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 5-1 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN Conveyance system analysis and design will be addressed during final design. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 6-1 6. SPECIAL REPORTS AND STUDIES The following reports and assessments are provided for reference and informational purposes only. Core Design takes no responsibility or liability for these reports, assessments, or designs as they were not completed under the direct supervision of Core Design. • Geotechnical Report by Terra Associates, Inc, dated 8/5/21 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 7-1 7. OTHER PERMITS No other permits relevant to this Preliminary TIR are known to be required at this time. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 8-1 8. ESC ANALYSIS AND DESIGN The ESC analysis and design will be addressed during final design. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 9-1 9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT The bond quantities, facility summary, and declaration of covenant will be addressed during final design. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-1 10. OPERATIONS AND MAINTENANCE The operations and maintenance manual will be addressed during final design. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-2 Appendix A WWHM Report DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-4 WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: New Vault Site Name: Site Address: City : Report Date: 4/11/2022 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.17 Version Date: 2021/08/18 Version : 4.2.18 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year ___________________________________________________________________ High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Mod 6.7 C, Lawn, Mod .78 Pervious Total 7.48 Impervious Land Use acre ROOF TOPS FLAT 0.41 Impervious Total 0.41 Basin Total 7.89 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : Basin 1 Bypass: No DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-5 GroundWater: No Pervious Land Use acre C, Lawn, Mod 3.81 Pervious Total 3.81 Impervious Land Use acre ROOF TOPS FLAT 4.08 Impervious Total 4.08 Basin Total 7.89 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Vault 1 Vault 1 ___________________________________________________________________ Name : Vault 1 Width : 84 ft. Length : 88 ft. Depth: 13 ft. Discharge Structure Riser Height: 12 ft. Riser Diameter: 18 in. Orifice 1 Diameter: 2 in. Elevation: 0 ft. Orifice 2 Diameter: 1.5 in. Elevation: 7.2 ft. Orifice 3 Diameter: 2.25 in. Elevation: 10.5 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Vault Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.169 0.000 0.000 0.000 0.1444 0.169 0.024 0.041 0.000 0.2889 0.169 0.049 0.058 0.000 0.4333 0.169 0.073 0.071 0.000 0.5778 0.169 0.098 0.082 0.000 0.7222 0.169 0.122 0.092 0.000 0.8667 0.169 0.147 0.101 0.000 1.0111 0.169 0.171 0.109 0.000 1.1556 0.169 0.196 0.116 0.000 1.3000 0.169 0.220 0.123 0.000 1.4444 0.169 0.245 0.130 0.000 1.5889 0.169 0.269 0.136 0.000 1.7333 0.169 0.294 0.142 0.000 1.8778 0.169 0.318 0.148 0.000 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-6 2.0222 0.169 0.343 0.154 0.000 2.1667 0.169 0.367 0.159 0.000 2.3111 0.169 0.392 0.165 0.000 2.4556 0.169 0.416 0.170 0.000 2.6000 0.169 0.441 0.175 0.000 2.7444 0.169 0.465 0.179 0.000 2.8889 0.169 0.490 0.184 0.000 3.0333 0.169 0.514 0.189 0.000 3.1778 0.169 0.539 0.193 0.000 3.3222 0.169 0.563 0.197 0.000 3.4667 0.169 0.588 0.202 0.000 3.6111 0.169 0.612 0.206 0.000 3.7556 0.169 0.637 0.210 0.000 3.9000 0.169 0.661 0.214 0.000 4.0444 0.169 0.686 0.218 0.000 4.1889 0.169 0.710 0.222 0.000 4.3333 0.169 0.735 0.226 0.000 4.4778 0.169 0.759 0.229 0.000 4.6222 0.169 0.784 0.233 0.000 4.7667 0.169 0.808 0.237 0.000 4.9111 0.169 0.833 0.240 0.000 5.0556 0.169 0.857 0.244 0.000 5.2000 0.169 0.882 0.247 0.000 5.3444 0.169 0.906 0.250 0.000 5.4889 0.169 0.931 0.254 0.000 5.6333 0.169 0.956 0.257 0.000 5.7778 0.169 0.980 0.260 0.000 5.9222 0.169 1.005 0.264 0.000 6.0667 0.169 1.029 0.267 0.000 6.2111 0.169 1.054 0.270 0.000 6.3556 0.169 1.078 0.273 0.000 6.5000 0.169 1.103 0.276 0.000 6.6444 0.169 1.127 0.279 0.000 6.7889 0.169 1.152 0.282 0.000 6.9333 0.169 1.176 0.285 0.000 7.0778 0.169 1.201 0.288 0.000 7.2222 0.169 1.225 0.300 0.000 7.3667 0.169 1.250 0.319 0.000 7.5111 0.169 1.274 0.331 0.000 7.6556 0.169 1.299 0.341 0.000 7.8000 0.169 1.323 0.350 0.000 7.9444 0.169 1.348 0.358 0.000 8.0889 0.169 1.372 0.366 0.000 8.2333 0.169 1.397 0.373 0.000 8.3778 0.169 1.421 0.380 0.000 8.5222 0.169 1.446 0.387 0.000 8.6667 0.169 1.470 0.393 0.000 8.8111 0.169 1.495 0.399 0.000 8.9556 0.169 1.519 0.405 0.000 9.1000 0.169 1.544 0.411 0.000 9.2444 0.169 1.568 0.417 0.000 9.3889 0.169 1.593 0.422 0.000 9.5333 0.169 1.617 0.428 0.000 9.6778 0.169 1.642 0.433 0.000 9.8222 0.169 1.666 0.439 0.000 9.9667 0.169 1.691 0.444 0.000 10.111 0.169 1.715 0.449 0.000 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-7 10.256 0.169 1.740 0.454 0.000 10.400 0.169 1.764 0.459 0.000 10.544 0.169 1.789 0.493 0.000 10.689 0.169 1.813 0.528 0.000 10.833 0.169 1.838 0.553 0.000 10.978 0.169 1.862 0.573 0.000 11.122 0.169 1.887 0.591 0.000 11.267 0.169 1.911 0.607 0.000 11.411 0.169 1.936 0.623 0.000 11.556 0.169 1.960 0.637 0.000 11.700 0.169 1.985 0.651 0.000 11.844 0.169 2.010 0.664 0.000 11.989 0.169 2.034 0.677 0.000 12.133 0.169 2.059 1.460 0.000 12.278 0.169 2.083 2.949 0.000 12.422 0.169 2.108 4.584 0.000 12.567 0.169 2.132 5.902 0.000 12.711 0.169 2.157 6.669 0.000 12.856 0.169 2.181 7.300 0.000 13.000 0.169 2.206 7.842 0.000 13.144 0.169 2.230 8.347 0.000 13.289 0.000 0.000 8.821 0.000 ___________________________________________________________________ ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:7.48 Total Impervious Area:0.41 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:3.81 Total Impervious Area:4.08 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.525386 5 year 0.821257 10 year 1.052694 25 year 1.387507 50 year 1.669031 100 year 1.97922 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.261109 5 year 0.360508 10 year 0.437405 25 year 0.548134 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-8 50 year 0.641141 100 year 0.743756 ___________________________________________________________________ Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.876 0.219 1950 0.818 0.262 1951 0.717 0.499 1952 0.343 0.188 1953 0.259 0.201 1954 0.423 0.243 1955 0.510 0.263 1956 0.536 0.277 1957 0.614 0.237 1958 0.369 0.243 1959 0.342 0.219 1960 0.714 0.431 1961 0.457 0.234 1962 0.231 0.177 1963 0.466 0.240 1964 0.492 0.227 1965 0.531 0.248 1966 0.348 0.215 1967 0.885 0.255 1968 0.542 0.216 1969 0.455 0.217 1970 0.485 0.224 1971 0.566 0.259 1972 0.724 0.353 1973 0.315 0.226 1974 0.563 0.243 1975 0.677 0.256 1976 0.489 0.250 1977 0.329 0.183 1978 0.419 0.235 1979 0.330 0.177 1980 1.266 0.399 1981 0.445 0.209 1982 0.948 0.405 1983 0.448 0.256 1984 0.324 0.195 1985 0.318 0.216 1986 0.655 0.352 1987 0.651 0.392 1988 0.262 0.205 1989 0.214 0.195 1990 2.029 0.436 1991 1.158 0.438 1992 0.437 0.231 1993 0.314 0.216 1994 0.193 0.164 1995 0.423 0.264 1996 1.096 0.512 1997 0.732 0.455 1998 0.457 0.219 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-9 1999 1.385 0.356 2000 0.485 0.256 2001 0.275 0.177 2002 0.674 0.374 2003 0.849 0.209 2004 0.851 0.628 2005 0.552 0.258 2006 0.534 0.245 2007 1.667 0.845 2008 1.440 0.669 2009 0.771 0.333 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 2.0286 0.8446 2 1.6669 0.6690 3 1.4402 0.6278 4 1.3853 0.5122 5 1.2656 0.4989 6 1.1576 0.4549 7 1.0956 0.4379 8 0.9482 0.4363 9 0.8848 0.4311 10 0.8764 0.4050 11 0.8514 0.3986 12 0.8486 0.3918 13 0.8184 0.3737 14 0.7713 0.3557 15 0.7317 0.3526 16 0.7235 0.3516 17 0.7174 0.3333 18 0.7143 0.2774 19 0.6766 0.2643 20 0.6735 0.2633 21 0.6552 0.2620 22 0.6505 0.2588 23 0.6145 0.2580 24 0.5656 0.2565 25 0.5627 0.2562 26 0.5523 0.2559 27 0.5419 0.2552 28 0.5357 0.2505 29 0.5336 0.2481 30 0.5308 0.2450 31 0.5098 0.2430 32 0.4917 0.2430 33 0.4887 0.2428 34 0.4854 0.2404 35 0.4853 0.2372 36 0.4656 0.2350 37 0.4573 0.2335 38 0.4565 0.2311 39 0.4550 0.2266 40 0.4475 0.2265 41 0.4448 0.2238 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-10 42 0.4373 0.2191 43 0.4234 0.2191 44 0.4225 0.2189 45 0.4187 0.2168 46 0.3692 0.2164 47 0.3484 0.2162 48 0.3434 0.2160 49 0.3425 0.2149 50 0.3300 0.2093 51 0.3294 0.2085 52 0.3237 0.2049 53 0.3183 0.2013 54 0.3153 0.1948 55 0.3139 0.1948 56 0.2746 0.1884 57 0.2622 0.1830 58 0.2591 0.1772 59 0.2306 0.1771 60 0.2144 0.1771 61 0.1930 0.1642 ___________________________________________________________________ Stream Protection Duration POC #1 The Facility PASSED The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.2627 5950 5805 97 Pass 0.2769 5159 4472 86 Pass 0.2911 4436 3328 75 Pass 0.3053 3882 3104 79 Pass 0.3195 3364 2911 86 Pass 0.3337 2922 2588 88 Pass 0.3479 2545 2271 89 Pass 0.3621 2199 1931 87 Pass 0.3763 1948 1655 84 Pass 0.3905 1706 1434 84 Pass 0.4047 1501 1175 78 Pass 0.4190 1308 923 70 Pass 0.4332 1167 591 50 Pass 0.4474 1025 415 40 Pass 0.4616 911 302 33 Pass 0.4758 819 283 34 Pass 0.4900 719 257 35 Pass 0.5042 608 231 37 Pass 0.5184 527 218 41 Pass 0.5326 455 205 45 Pass 0.5468 396 190 47 Pass 0.5610 341 171 50 Pass 0.5752 293 145 49 Pass 0.5894 255 127 49 Pass 0.6036 217 109 50 Pass 0.6178 193 92 47 Pass 0.6320 171 64 37 Pass 0.6462 143 53 37 Pass DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-11 0.6604 126 34 26 Pass 0.6747 112 13 11 Pass 0.6889 97 9 9 Pass 0.7031 85 6 7 Pass 0.7173 78 5 6 Pass 0.7315 63 4 6 Pass 0.7457 58 4 6 Pass 0.7599 55 4 7 Pass 0.7741 50 2 4 Pass 0.7883 48 2 4 Pass 0.8025 43 2 4 Pass 0.8167 41 2 4 Pass 0.8309 34 2 5 Pass 0.8451 33 0 0 Pass 0.8593 28 0 0 Pass 0.8735 27 0 0 Pass 0.8877 24 0 0 Pass 0.9019 24 0 0 Pass 0.9161 23 0 0 Pass 0.9303 22 0 0 Pass 0.9446 20 0 0 Pass 0.9588 19 0 0 Pass 0.9730 19 0 0 Pass 0.9872 15 0 0 Pass 1.0014 14 0 0 Pass 1.0156 13 0 0 Pass 1.0298 13 0 0 Pass 1.0440 13 0 0 Pass 1.0582 13 0 0 Pass 1.0724 13 0 0 Pass 1.0866 13 0 0 Pass 1.1008 11 0 0 Pass 1.1150 10 0 0 Pass 1.1292 10 0 0 Pass 1.1434 9 0 0 Pass 1.1576 9 0 0 Pass 1.1718 8 0 0 Pass 1.1860 8 0 0 Pass 1.2003 8 0 0 Pass 1.2145 8 0 0 Pass 1.2287 8 0 0 Pass 1.2429 8 0 0 Pass 1.2571 8 0 0 Pass 1.2713 6 0 0 Pass 1.2855 6 0 0 Pass 1.2997 6 0 0 Pass 1.3139 6 0 0 Pass 1.3281 6 0 0 Pass 1.3423 6 0 0 Pass 1.3565 6 0 0 Pass 1.3707 6 0 0 Pass 1.3849 6 0 0 Pass 1.3991 5 0 0 Pass 1.4133 5 0 0 Pass 1.4275 5 0 0 Pass 1.4417 4 0 0 Pass 1.4559 4 0 0 Pass DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-12 1.4702 4 0 0 Pass 1.4844 4 0 0 Pass 1.4986 3 0 0 Pass 1.5128 3 0 0 Pass 1.5270 3 0 0 Pass 1.5412 3 0 0 Pass 1.5554 2 0 0 Pass 1.5696 2 0 0 Pass 1.5838 2 0 0 Pass 1.5980 2 0 0 Pass 1.6122 2 0 0 Pass 1.6264 2 0 0 Pass 1.6406 2 0 0 Pass 1.6548 2 0 0 Pass 1.6690 1 0 0 Pass _____________________________________________________ ___________________________________________________________________ Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0.4133 acre-feet On-line facility target flow: 0.2116 cfs. Adjusted for 15 min: 0.2116 cfs. Off-line facility target flow: 0.1323 cfs. Adjusted for 15 min: 0.1323 cfs. ___________________________________________________________________ LID Report LID Technique Used for Total Volume Volume Infiltration Cumulative Percent Water Quality Percent Comment Treatment? Needs Through Volume Volume Volume Water Quality Treatment Facility (ac-ft.) Infiltration Infiltrated Treated (ac-ft) (ac-ft) Credit Vault 1 POC N 1066.13 N 0.00 Total Volume Infiltrated 1066.13 0.00 0.00 0.00 0.00 0% No Treat. Credit Compliance with LID Standard 8 Duration Analysis Result = Failed ___________________________________________________________________ Perlnd and Implnd Changes No changes have been made. ___________________________________________________________________ This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by : Clear Creek Solutions, Inc. 2005-2022; All Rights Reserved. DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-13 Appendix B Special Reports DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E Core Design, Inc. TOWNS ON 12TH Page 10-14 Insert reports referenced in section 6 DocuSign Envelope ID: CB3FEE37-E3FD-4E9A-BBDA-38822A5CC20E