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Home My WebLink About03306 - Technical Information Report - Drainage . ' LPD Engineering, PLLC Civil Engineering Consulting � � � � �-S� � �� , � m � QI c.-c� ��Q e x s February 8, 2006 City of Renton PlanningBuilding/Public Works Department 10�5 S Grady Way Renton,WA 98055 Attn.Mike Dotson, PE Re: Development of two dup[exes at 617 SW 4`�Pl City File#PRE05-64 Drainage Narrative Project Description The development proposal is to construct two residential duplexes on each of two legal lots at 617 SW 4"'Place in Renton. Parcel numbers: 2143701710 and 2143701715 Owner: Joshua and Jennifer Steele, contact David Steele ((206)715-0383). Existing Conditions Two separate legal lots. Each is 40'x100' (4,000 sfl. One of lots is vacant, the other has an unoccupied 900 sf single family residence. Both lots are overgrown with grass,weeds and blackberry bushes.The lots have frontage on SW 4`� Pl and abut a public alley in the rear. The two lots slope down from SW 4`�Pl toward the alley at an average slope of approximately 7%. Surface runoff from the existing site sheet flows to the alley at the south end of the properties.There are no formal drainage facilities in the alley. Surface water in the alley drains to the parking area for the adjacent apartment complex. Runoff in the adjacent parking lot is collected in catch basins and routed towards SW 5`�Ct via 12"PVC conveyance pipes.There are no signs of flooding or drainage problems in the described portions of the downstream system. Proposed Conditions Each lot will construct one duplex structure with two driveways (front and rear). Each lot will add approximately 1,400 sf of impervious roof area and 830 sf of impervious driveway area. Total of new impervious area will be 2,230 sf per lot. In accordance with City of Renton requirements and 2005 King County Surface Water Design Manual (KCSWDM), Stormwater Best Management Practices (BMPs)are a part of the proposed development. Full dispersion of roof runoff was evaluated for each lot,however it is not feasible for the projects given that there is no way to achieve 100 foot native vegetated flowpath segment downstream of the proposed improvements within the 100 ft by 40 ft lot. 7936 Se�vard Park Avenue South LPD Engineering, PLLC Phone: 206. 725.1211 Seattle, Washington 98118 Fax: 206.793.5344 33e� , Drainage\arrative Steele Duplexes January 3, 2006 Page 2 of 2 Full infiltration of runoff was evaluated. A soils report has not been completed for the project, SCS King County Soil Survey indicates that the subject properties contain Beausite gravelly sandy loam which is considered Type 4 soil in the Soil Textural Classification system indicating that the soils w-ill not be suitable for full infiltration. The BMPs proposed will take advantage of any infiltrative capacity available. Given the small size of the lots there is not sufficient area available to construct a gravel filled trench or a drywell for limited infiltration. The drainage system design proposed for the site incorporates a combination of BVIPs including a rain garden and downspout splash blocks to provide basic dispersion. Designs of the BMPs are based on the KCSWDM. The rain garden will provide 380 cf of storage and will take advantage of any infiltration capacity in the site soils. The downspouts will have a 701f native vegetated flow path prior to leaving the site. Any flow that is not infiltrated on site will drain by overland flow to the alley and then on to the storm drainage system in the parking lot of the adjacent apartment complex to match the existing drainage patterns. The size of the site and the dispersion measures provided should ensure that any change in runoff rates due to the proposed development will be negligible. Multiple catch basins in the adjacent parking lot provide redundancy in the downstream drainage system. The proposed development should not create or aggravate any downstream drainage problems. Details and maintenance guidelines for the proposed B�1Ps are attached. To prevent erosion and trap sediments during construction the followin�, BN1Ps �vill be used. • Clearing limits will be marked by fencing • The driveways will be constructed and graveled immediately. • Silt fencing will be placed along slope contours at the downslope limit of clearin�T. • Mulch will be spread over areas of the site not being worked. � �'�t va�`�'�� ¢{� 4�;'l�.�i;'*�+��,7 r,,<-� `'1��;�- ;� ;1. 1,� i� =; � :..�.r•. � s.��.2 l� ��,��'Q L�a-'-�. �� -+t_ � � w •� 3 3� .� :�.:;° �:�,.:,,a � T�C� �n 32?`_=7 �S� �-�`'r.'a. `'��rr,`s1�.c:1r�''-��J �,�, `:, .� �� ..:i!'�W ��:15.:��•"� 7�� y �.�1� ti. �—'.sfi'ft1�J i �J' �• rJ " rJ� 1 Jeff Lamourewc Attachments: Info re. proposed BMP's Cc: David Steele 7936 Seward Park Avenue South LPD Engineeri�g, PLLC Phone: 206. 725.1211 Seattle, Washington 98118 Fax: 206.793.5344 � SECTfON C? FL01V CO�,'TROL BMPs 0 TEXT OF INSTRUCTIONS FOR BASIC DISPERSION Your property contains a stormwater management flow control BMP (best management practice)called "basic dispersion,"which was insta{led to mitigate the stormwater quantity and quality impacts of some all of the impervious surfaces or non-native pervious surfaces on your p�operty. Basic dispersion is a strategy for utilizing any available capacity of onsite vegetated areas to retain, absorb, and filter the rurn from developed surfaces. This flow control BMP has two primary components that must be maintained: the devices that disperse runoff from the developed surfaces and (2)the vegetated area over which run is dispersed. Dispersion Devices The dispersion devices used on your property include the following as indicated on the flow control BMf site plan: ❑ splash blocks, 0 rock pads, ❑ gravel filled trenches, ❑ sheet flow. The size, placement, composition,and downstream flowpaths of these devices as depicted by the flow control BMP site plan and design details must be maintained and may not be changed without written approval either from the King County Water and Land Resources Division or through a future development permit from King County. Dispersion devices must be inspected annually and after major storm events to identify and repair any physical defects. When native soil is exposed or erosion channels are present, the sources of the erosi or concentrated flow need to be ident�ed and mitigated. Concentrated flow can be mitigated by IeveGnc the edge of the penrious area and/or realigning or replenishing the rocks in the dispersion device, such: in rock pads and gravel filled trenches. Vegetated Flowpaths The vegetated area over which runoff is dispersed must be maintained in good condition free of bare sp and obstructions that would concentrate flows. � FIGURE C1.4.A TYPICAL SPLASHBLOCK FOR BASIC D[SPERSION house roof downspout serves up to 700 s.f. of roof 50'min. vegetated flow path splash block \downspout extension NTS splash block 1/24/2005 2005 Surface Water Design Manual—Appendi: C-52 • • SECT[ON C.2 FLOW CONTROL BMPs � FIGURE C.2.S.A TYPiCAL RAIV GARDEN(BIORETENTION POND) � iq . . .. - .'{ , . •a , __ � � _ ., , . #�. ,. ' �: .. � , f�� c� J� �_� ` � ��. ,� �- • , . . , t�ees,shrubs ' f:�, � �� - -.. BlORETENTION AR- _ ''� . . % , .. �n - _`--- � .. ��. ` ` ground cover , , 2'wide g�ass filter strip existing edge I of pavement � A . � � _ - . ' � sheet flow ; PLAN VIEW (not to scale) trees trees - "-� __ ;_.\ - overflow to suitable 2'wide grass `�� - surface discharge area fifter strip '�, ;i sheet flow `��:� ,V�'�',,,,�`-' 3:1 maximum siope ��\ 1 ground shrubs �' cover �``�Y-� ^ , .. - 12"maximum min. 12" = water depth : amended �%� -- soii with 4" -- - of compost - - � _, � � BIORETENTION AREA SECTION VIEW A-A (not to scale) I 1i24/200� 2005 Surface Water Design Manual-Appendix C C-58 C.2.5 RAIN GARDEN Rain gardens,also known as "bioretention,"are excavated or otherwise formed depressions in the landscape that provide for storage,treatment,and infiltration of stormwater runoff. The soil in the depression is enhanced to promote infiltration and plant growth. Plants adapted to wet conditions are planted in the enhanced soil. Figure C.2.S.A(p.C-58)shows a plan view and section of a typical rain garden system. Applicable Surfaces Subject to the minimum design requirements and specifications in this section,rain gardens may be applied to any impervious surface such as a roof,driveway,parking area,road,or sidewalk, and to any non-native pervious surface such as a lawn,landscaped area,or pasture. I Design Considerations �i Rain gardens may overflow occasionally in large storm events or more frequently if located in very poorly drained soils or areas with very high water tables. In very well-drained soils,water may drain too quickly to support water loving plants and ponding may be of short duration. Conditioning the soil in the rain garden with abundant compost will enhance the growth of plants and help the soil to more readily admii water. I Operation and Maintenance I See Section C.2.5.3 (p. C-60}. I � C.2.5.1 MINIMUM DESIGN REQUIREMENTS I All of the following requirements must be met in order for a rain garden to be applicable to a target impervious surface or new pervious surface: 1. A minimum water storage volume equivalent to 3 inches(0.25 feet)of tunoff depth from the impervious surface area served is required. ln other words, the volume in cubic feet shall equa10.25 times the square footage of the impervious surface area served(see example caIculation in Section C.2.5.2 below). For rain gardens servina new pervious surface,a minimum water storage valume eQuivalent to 0.5 inches(0.04 feet)of runoff depth is required. 2. The water storage area,containing the minimum required storage volume,shall be 12 inches deep at �� overflow and have side slopes no steeper than 3 horizontal Eo 1 vertical. ?he overflow point of the � water storage area shall be at least 6 inches below any adjacent pavement area. The overflow point ' must be situated so that overflow does not cause erosion damage or unplanned inundation_ I3. If a containment berm is used to form the water storage area,the berm must be at least 2 feet wide and 6 inches above the 12 inches of water depth. A catch basin or rock pad must be provided to release water when the pond's water level exceeds the 12 inches of water depth. The catch basin may � discharge to the local drainage system or other acceptable discharge location via a 4-inch rigid pipe. � The rock pad may be used with or without a constructed drainage system downstream. If a rock pad is I used,it must be composed of crushed rock,6-inches deep and 2 feet wide(perpendicular to flow}and must extend at least 4 feet or beyond the containment berm,whichever is greater. The rock pad mnst Ibe situated so that overflow does not cause erosion damage or unplanned inundation. � 4. Amended soil consisting of minimum of 4 inebes of compost tilled into the upper 12 inches of soil or I12 inches of imported sand/compost blend is required in the rain garden. Tilling and amending to � greater depth is desirable. �� 5. Water tolerant plants such as those in Table C.2_S.A shall be planted in the pond bottom. Plants ' II native to Westem Washington are preferred. , 1/24/2005 ?005 Surface Water Design Manual—Appendix C � G56 I � � I ' C.25 RAIN GARDEN II 6. A minimum 5-foot setback shall be maintained between any part of a rain garden and any structurc or ', property line. ' � 7. Rain gardens are noi allowed in critical area buffers or on slopes steeper than 20%. Rain gardens � � : proposed on slopes steeper than 15%or within 50 feet of a steep slope ha�ard area or landslide haT.ard area must be approved by a geotechnical engineer or engineerbig genl�gist unless otherw�sc approved by the DDES staff geologist. 8. For sites with septic systems,rain gardens must be located down�radient of the primary and resecvc drainfield areas. DDES pecinit review staff can waive this requirement if site topography clearly prohibits subsurface flows from intersecting the drainfield_ 9. The rain garden must not create flooding or erosion impacts as determined by the DDES_ If a rain � garden is proposed near a landslide hazard area,erosion hazard area,steep slope hazard area, or a slope steeper than 15%,DDES may require evaluation and approval of the proposa(by a gentechnical , 0 engineer or engineering geologist. ' TABLE C.2.S.A WATER TOLERANT PLANTS II� Common Name Scientific Name Spacing(on center) f Westem mannagrass Glyceria occidentalis seed I Velvetgrass Nolcus mo!!is seed Shortawn foxtail Alopecurus aequalis seed II Water foxtail Alopecurus geniculatus seed Spike rush Eleochans spp. 4 inches Y. � Slough sedge Carex obnupta 6 inches or seed Sawbeak sedge Carex stipata 6 inches Sedge Carex spp. 6 inches Slender rush Juncus tenuis 6 inches � � Water parsley Oenanthe sarmentosa 6 inches : Hardstem bulrush Scirpus acutus 6 inches Watercress Rorippa nasfurtium-aquaticum 12 inches Small-fruited bulrush Scirpus microcarpus 12 inches G2.5.2 EXAMPLE SIZING CALCULATION Runoff Source:20-foot by 20-foot driveway ', Minimum Storage Required=3 inches(0_25 feet) I Storage Volume Needed=20 feet x 20 feet x OZ5 feet= ]00 cubic feet Pond Design_Choosing a 1 foot depth with 3:1 side slopes and a bottom area of 4 feet x 10 feet provides a storage volume of 100 cubic feet Note that if the soil percolation rate is O.S inches per hoirr, a pond 1 foot deep will take 24 hours to drain when filled. Sinrilarlv, a small storm of 0.�inches woz�ld produce a depth orabout 3 inches and drain in 6 hours. �� ?005 Surface Water Design Manual—Appendix C I�`Z�i3005 G57 • , SECTION C.2 FLOW CONTROL BMPs C.2.5.3 MAINTENANCE INSTRUCTIONS FOR A RAIN GARDEN If the rain garden flow control BMP is proposed for a project, the following maintenance and operation instructions must be recorded as an attachment to the reyuired declaration of covenant and gran#of easement per Requirement 3 of Section C.1.3_3 (p.G18)_ The intent of these instructions is to explain to future property owners, the purpose of the BMP and how it must be maintained and operated. These instructions are intended to be a minimum; DDES may require additional instructions based on site- specific conditions. Also,as the County gains more experience with the maintenance and operation of these BMPs,future updates to the instructions will be posted on King County's Surface Water Design Manual website. ❑ �oF irvsrRucnorvs Your property contains a stormwater management flow control BMP(best management practice)called a "rain garden,"which was installed to mitigate the stormwater quantity and quality impacts of some or afl of the impervious or non-native pervious surfaces on your property. Rain gardens, also known as "bioretention,"are vegetated closed depressions or ponds that retain and filter stormwater from an area of impervious surface or non-native pervious surface. The soi!in the rain garden has been enhanced to encourage and support vigorous plant growth that serves to filter the water and sustain infiltration capacity. Depending on soil conditions,rain gardens may have water in them throughout the wet season and may overflow during major storm events. The size, placement, and design of the rain garden as depicted by the flow control BMP site plan and design detaifs must be maintained and may not be changed without written approval either from the King County W ater and Land Resources Division or through a future development permit from King County. Plant materials may be changed to suit tastes, but chemical fertilizers and pesticides must not be used. Mulch may be added and additional campost shoufd be worked into the soil over time. Rain gardens must be inspected annually for physical defects. After major storm events, the system should be checked to see that the overflow system is working properiy. If erosion channels or bare spots are evident,they should be stabilized with soil, plant material, mulch,or landscape rock. A supplemental watering program may be needed the first year to ensure the long-term survival of the rain garden's vegetation. Vegetation should be maintained as follows: 1)replace all dead vegetation as soon as possible; 2)remove fallen leaves and debris as needed; 3)remove all noxious vegetation when discovered;4)manually weed without herbicides or pesticides; 5)during drought conditions, use mulch to prevent excess solar damage and water loss. 1/24/2005 2005 Surface Water Design Manual-Appendix C , C�0