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Civil Review 2 - Drainage Report Recd 5-10-19.pdf9215 Bowdoin Way 2-Lot Short Plat Edmonds, WA 98020 Stormwater Site Plan Report Prepared for: Select Homes Date: August 1, 2018 Revised: January 4, 2019 Revised: May 10, 2019 �go WAVr l� �� 36508 w \�EGISTEg��/ IONTAT . 5/10/201J° Prepared by: Rob Long, PE RAM Engineering, Inc. 16531 13t" Ave W, Suite A108, Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc.com Job No. 17-018 9215 Bowdoin Way 2-Lot Short Plat Introduction: This summary report provides site design information for a 2-Lot Short Plat residential development by Select Homes. This report includes storm drainage analysis to support permit review and approval. The property is located at 9215 Bowdoin Way, in the northeast 1/4 of Section 25, T 27 N, R 3 E, W.M. Site Address: 9215 Bowdoin Way Applicant: Select Homes Edmonds, WA 98020 16513 13th Ave W, Suite A108, Lynnwood WA 98037 Tax Parcel Number: 008364 000 00100 Contact: Kayla Clark (425) 742-6044 TABLE OF CONTENTS SECTION PAGES A. Project Overview.............................................................................................................................. 4 B. Existing Conditions Summary........................................................................................................... 1 C. Developed Site Hydrology................................................................................................................ 9 D. Soils Report....................................................................................................................................... 1 E. Construction SWPPPP Requirements................................................................................................ 2 F. Operation and Maintenance Guidelines............................................................................................ 5 RAM Engineering, Inc. Select Homes: 9215 Bowdoin Way RAM No. 17-018 Stormwater Site Plan Report A. PROJECT OVERVIEW. This report provides engineering information for the proposed construction of a single-family residential 2- Lot short plat on a 16,434 sf (0.38 ac) lot; the project is located at 9215 Bowdoin Way in the City of Edmonds. The applicant, Select Homes proposes to remove the existing residence and all existing hard surface to construct two single family residences on the project site. This report provides the evaluation for the proposed 2-lot single-family residence development. Summary of Minimum Requirements for Category 1 Project: Minimum Requirement #1 —Preparation of Storm water Site Plan. The proposed site development consists of disturbing about 0.38 acres of land and creating/replacing about 8,030 sf (greater than 5,000 sf) of hard surface area. Thus, the project is classified as a Category 2 project per the City's classification system. Per ECDC 18.30, Category 2 projects must comply with Minimum Requirements No. 1 through No. 9. The civil site development plans and this report have been prepared to address the projects impacts. Minimum Requirement #2 — Construction Stormwater Pollution Prevention (SWPPP). A construction Stormwater Pollution Prevention Plan (SWPPP) has be incorporated into the site development plans. A summary of the site's erosion control measures that evaluates the typical 13 elements of a SWPPP are included in section E. The total site disturbance area of the project is less than one acre, thus a formal Notice of Intent application for NPDES coverage will not be made to the Department of Ecology. The grading quantities are estimated to be 350 CY of cut and 50 CY of fill. Minimum Requirement #3 —Source Control of Pollution. Specific source controls are not required for single family residential sites. General requirements for these sites include preventing the discharge of pollutants to the City's storm drainage system per Edmonds City Code Chapter 7.200 (Illicit Discharges). This includes common household items such as pesticides, herbicides, fertilizers, detergents and fluids from vehicle maintenance. Single family residences shall incorporate DOE's S411 BMPs for landscape and lawn vegetation management. Lawn and vegetation management can include control of objectionable weeds, insects, mold, bacteria, and other pests with pesticides. Examples include weed control on golf course lawns, access roads, and utility corridors and during landscaping; sap stain and insect control on lumber and logs; rooftop moss removal; killing nuisance rodents; fungicide application to patio decks, and residential lawn/plant care. It is possible to release toxic pesticides such as pentachlorophenol, carbamates, and organometallics to the environment by leaching and dripping from treated parts, container leaks, product misuse, and outside storage of pesticide contaminated materials and equipment. Poor management of the vegetation and poor application of pesticides or fertilizers can cause appreciable stormwater contamination. RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page A-1 Minimum Requirement #4 — Preservation of Natural Drainage Systems and Outfalls. The site generally slopes in a westerly direction, stormwater leaving the site would sheet flow westerly to 92nd PI W and enter the stormwater pipe and catch basin system along 92nd PI W (near the southwest corner of the site). Discharge flows for the developed site will either continue to sheet flow (lower lawn areas) or will be tight -line (roof and yard drains) into a storm system catch basin in 92nd PI W near the southwest corner of the site. The proposed site development will maintain the natural drainage outfall of the site (catch basin in 92nd PI W). The local stormwater system within 92nd PI W discharges south to Bowdion Way which directs the stormwater westerly. The site is located within the upper reach of the Shell Creek watershed basin per the City of Edmonds maps (see section C). Minimum Requirement #5 — Onsite Stormwater Management. A site specific geotechnical evaluation of the site recommends two stormwater BMPs for the site; BMP T5.13: Post -Construction Soil Quality and Depth and BMP T5.10C: Perforated Stub -out Connections based on the site's soil conditions. In accordance with Edmonds Code, lawn and landscape areas will be managed by post -construction soil quality and depth (soil amendments); roof tops will be managed by perforated stub -out connections; and other hard surfaces will be managed by detention a pipe designed in accordance with the Edmonds Storm water Addendum. See civil site development plans and Section C for additional stormwater BMP evaluation and construction details. 5.6 Minimum Requirement #6 — Runoff Treatment. The project does not propose to create/replace more than 5,000 sf of pollution -generation hard surface onsite. The project will create about 1,300 sf pollution generation hard surfaces (driveways); most the site's hard surface is rooftop and walkway/patio areas (non - pollution generation surfaces). Thus, no basic or enhanced water quality treatment system is required. 5.7 Minimum Requirement #7— Flow Control. The project does not propose to create/replace more than 10,000 sf of effective imperious surfaces, or convert 0.75 acres or more of vegetation to lawn or landscape. The 0.38 acer project proposed to create/replace 7,520 sf of hard surface; thus exempt from flow control per ECDC 18.30.060.D.7.b. As discussed above each lot will implement BMP T5.13: Post - Construction Soil Quality and Depth; BMP T5.10C: Perforated Stub -out Connections; and install a detention a pipe (per Edmonds Stormwater Addendum) onsite stormwater management BMPs. 5.8 Minimum Requirement #8— Wetland Protection. No wetlands are known to exist on or adjacent to the site. 5.9 Minimum Requirement#9—Operation and Maintenance. An operation and maintenance summary is provided in section H. RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page A-2 Parcel (Vicinity) Map: NE 25 27 `. C-W$— LC1 Ft., — s.aoll -- Cp, . _ .. Oot La s..oa,. — ROW— OIIrMr --- TOAM3 1 i..•.•»».1 - MAW 1YY/ ORYr1a ...... Vx ROW ____ 1ff1---. 61111IMe1 , ... »......., a,�/-A.rNrr.•.rr//A7•Ia -tr-hiv- 4ri0r VI4t11 p11t, Swan• — Vic 1.01 _ _ _ _ - I jf I SEaa.2743 _�7Nr?+pl �L . I VJALIaIJT ST Tn 35, 1 II , , • , • , • �f ) r $ t 00S $ 1.� \�� `� 1.001 1 B -=Dy IN IROAD RACTS 02 04 07 0(1-022 W1 6-�{I{.:1J:�•ii�'.'r� ;1 -021 1.019 01 aDw� .06 _ A f �37 - - _ ._... 02 , 3 WAY -0" ;1463 :1-0t2 1035 11-03{ A7 ti , 1 J 1-025 02 -- ' S•7.89 i'� -039 { 1r-033�{{-� ° ! • ! 13 iC w „ 7 }-.- 7 IAUREI tANEI- �17 I 10 17 9 e CL { • 7 '�Q. 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TR 100 — (5491 O1 17 1t 16 /{ 13 �R E :A 7 f 3 { 7 2.059 ( I Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Mrs 1�— Page A-3 Site Plan: ' 1 a 20• APN.�EDMONDS. rWA 98020 � APO:OOJ7J600501006 I - f 98 1100-- � 3!/-48 I tt 11 . ........ RIM 94.50 4" IE 91.00 (IN -NW) 4" SO STUB FOR 18" IE 88.18 (00r-S) 1 1 ��L RWWAr 1 AIN 10' PRIVA IE DRAINAGE 1 EASEMENT SHORT PLAT 1 10' Jr y unclrr S - 97 - 103 1 EASEMENT (A.F. NO. 199908165001) jj$$ 4OOF SD S TUB APIN R o J 2+ E 96.Of 1.0% MIN (rYP) PROVIDE J ' MIN HORIZONTAL ZI SEPARA 17ON BETNEEN 56 y18" P 1 �I Wr 0 DRY U77L RES PIPE PROVIDE DRY UnUrY STUBS TO EACH LOT 1 1 4" SO STUB FOR WIN ROOF FUTURE DRIVEWAY II DRAIN nGHnJNE CHANNEL DRAIN 0 1.OX MIN (rYP) ` \ � B s j ► 1 1 III I EASEMENT TO L1 rr 1 RESTRIC TOR I RIME 8Z 4 4" P ROOF 1 - \ av 18" IE 87.90 (IN-N) � IN O 4" 1£ 68.50 (IN-W) 1.OX STUB 0 \�i 6" IE 8Z90 (OUT-S) ' e 1 SD S7UB \ 15'-12' CPEP IE 95.O1 EX CB 0 0.60X = 1 21-d PI 8" IE 8J.90 (IN-N) SO/C0 O 16.5 � 12" IE 83.90 (IN -SE) � ` AN DSO 6- IE 83.80 (OUT-S) N, I - RIM 92.50 \ 6" IE 8Z50 (W-N) 6" 1E 8750 mqI r-s ) EX O%VEWAY CUT TO BE REMOVED _ \ AND REPLACED WITH STANDARD CURB/CUMA STREET /NTX.; AND REPLACE RAMPS (ADA C0MPUANI)�' y"•• •:' ��'/yp{� i`• 7HRU-CURB INLET 1 \\ _`! \•'µy'. .'� \` \ \ RIAI 8Z56 1Dom 6" IE 83.99 (IN -NE) 12" IE 83.99 (OU1-NW) --- _ �{ � __ INr 1+00.00 92ND PL W 10+00.00 80N00'N WAY I I L l S, - _ O 1�[ -- - �+�•. Y. �`4�. I ?~ AZzq�p v �•�1P� �� r:,. 3 xr nN F/Rf HYDRANT RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page A-4 B. EXISTING CONDITIONS SUMMARY Existing Conditions. The total site consists of about 16,434 sf (0.38 acre) and is currently occupied by a single-family residence construction around 1957. There is an access driveway for the existing onsite residence that cuts across the neighboring property to the east that access Bowdoin Way (to be removed and not used for the site development). The site is surrounded by single family residences to the north, vacant lots to the east, 92nd PI W to the west and Bowdoin Way to the south (see attached aerial photo). The site is generally slopes in a westerly direction in a range between 2% and 5%; with a steep bank/rockery along a portion of the westerly boundary and southerly boundary. No define drainage courses are found on -site and stormwater runoff would sheet flow in a westerly direction. No stream or wetlands were discovered on or immediately adjacent to the site. Additional discussion of the local drainage basin and downstream path is discussed in Section C of this report. Soils: In accordance with the project's site specific geotechnical engineering study by Earth Solutions NW, LLC the on -site soils consist of silty sand (USCS: SM) glacial till deposits. The subject site is underlain by glacial till deposits (Qvt, Alderwood series soils). Full soils description is included in the project's site specific geotechnical engineering study by Earth Solutions NW, LLC. Aerial Photo (City of Edmonds GIS, 2015 photo): AW 4 ; --J Ad It .41 RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page B-1 C. DEVELOPED SITE HYDROLOTY. Developed Conditions. The proposed project includes the construction of two single residence and associated driveway and utilities. All existing structures and impervious surface onsite will be removed/replaced with the site development; landscaping and grass lawn around the new residence will stabilize the site upon building construction. The project proposes to create and/or replace 8,030 sf of hard surface with the complete site development (Lot 1 =3,245 sf and Lot 2 = 3,300 sf). All existing onsite hard surface will be removed with the project site development. The following is a summary of the proposed new/replaced hard surface areas: Hard Surface Summary: Lot 1 (House 2,415 + Driveway 780 + Sidewalk/Patio 50) = 3,245 sf Lot 2 (House 2,500 + Driveway 750 + Sidewalk/Patio 50) = 3,300 sf Lot Hard Surface = 6,545 sf (0.150 ac) Frontage/ROW New + Replaced Hard Surface = 1,485 sf (0.034 ac) Total New + Replaced Hard Surface = 8,030 sf (0.183 ac) In accordance with the Edmonds City Code (18.30) Category 2 projects shall meet MR 5 and provide on - site stormwater management. To meet these criteria, the proposed hard surfaces of the site shall implement BMPs from "List No. 2" or meet the LID performance standard. The attached geotechnical evaluation letter assesses the feasibility criteria of each stormwater BMP; per the evaluation only BMP T5.10C: Perforated Stub -out Connections, and BMP T5.13: Post -Construction Soil Quality and Depth are considered feasible (dispersion, infiltration, rain gardens, and bioretention are all considered infeasible). The proposed roof areas of each lot (less than 5,000 sf each), will be collected and routed through individual lot perforated stub -out prior to discharging from the site; and thus, will be considered a manage hard surface and no other BMPs are necessary for the roof area. The lot driveway, patio and walk ways and frontage improvement areas ("other hard surfaces") will total 1,630 sf (lot 1 = 830 sf and lot 2 = 800). A detention a pipe designed in accordance with the Edmonds Stormwater Addendum will be installed to manage the "other hard surfaces" proposed. In accordance with the Edmonds Stormwater Addendum (Section 6.3), 56 feet of 18" detention pipe will mitigate the 1,500 sf of other hard surface area (L = 0.034 x 1,630 = 55.4 ft). A detention pipe will be installed along the westerly boundary of the site to manage the additional proposed "other hard surfaces". Additionally, all disturbed pervious (lawn and landscape) areas will preserve or restore the health and water - holding capacity of the soils by compost -amending (Per DOE BMP T5.13: Post -Construction Soil Quality and Depth). RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-1 Flow Control Summary. The project site development is exempt from full flow control due to the fact the project proposes less than 10,000 sf of hard surface area (8,030 sf), will not convert more than % of an acre of native vegetation (0.377 ac site), and will not increase the peak 100-year flow rate by more than 0.1 cfs. Individual lot perforated sub -outs and a joint detention tank for the "other hard surfaces" will mitigate the hard surfaces for the individual lots (Lot 1 =3,245 sf, Lot 2 = 3,300 sf). Due to site constraints (lower elevation of ROW areas) the right-of-way (ROW) improvements/replacement (1,485 sf) will by-pass the proposed lot systems. 100-year peak flow Analysis: The following calculations demonstrates the site development effective impervious surface and converted pervious surfaces does not causes a 0.1 cubic feet per second or greater increase in the 100-year recurrence peak flow from a threshold discharge area as estimated by an Ecology - approved continuous simulation model. The following hydrologic analysis uses WWHM2012 software to model the peak 100-yr storm event. The following is a summary of the input and output data from the WWHM2012 model: Project Area Summary: Project Site = 16,434 sf (0.377 ac) ROW Improvement/Replacement Area = 1,485 sf (0.034 ad Total Site Development Area = 17,919 sf (0.411 ac) Existing Conditions Areas: Site Pre-existing Conditions.............................................................................. 0.411 ac. (17,919 sf) Modeled as Forested Developed Conditions Areas: Lot Impervious routed to roof stub -outs and detention tank ..................... 0.150 Ac. (6,545 sf) Non -effective Hard Surface, modeled as Lawn ROW Improvement/Replacement Area ......................................................... 0.034 Ac. (1,485 sf) Hard Surface, modeled as Impervious LawnAreas......................................................................................................... 0.227 Ac. (9,889 sf) Pervious Areas with BMP T5.13 treated, modeled as Pasture WWHM2012 PIZOJI',C'I' IZI',POIZ'I' Project Name: 17-018 Site Site Name: Rowdoin 2-1.oL Site Address: 17-018 City Report Date: 5/9/2019 MGS Regoin : lugeL I�:asL Data Start 1901/10/1 Data End : 2058/09/30 DOT Data Number: 03 Version Date: 2018/10/10 Version : 4.2.16 RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-2 PREDEVELOPED LAND USE MITIGATED LAND USE Name : Basin 1 Name : Basin 1 Bypass: No Bypass: No GroundWater: No GroundWater: No Pervious Land Use acre Pervious Land Use acre C, Forest, Flat .411 C, Lawn, Flat .15 Pervious Total 0.411 C, Pasture, Flat .227 Pervious Total 0.377 Impervious Land Use acre Impervious Total 0 Impervious Land Use acre ROADS FLAT 0.023 Basin Total 0.411 DRIVEWAYS STEEP 0.007 SIDEWALKS MOD 0.004 Impervious Total 0.034 Basin Total 0.411 Element Flows To: Surface Interflow Groundwater ANALYSIS RESULTS Stream Protection Duration Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.411 Total Impervious Area:O Mitigated Landuse Totals for POC #1 Total Pervious Area:0.377 Total Impervious Area:0.034 Flow Frequency Return Periods for Predeveloped. POC 41 Return Period Flow(cfs) 2 year 0.006911 5 year 0.010678 10 year 0.012716 25 year 0.014785 50 year 0.016013 100 year 0.017021 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.017486 5 year 0.027872 10 year 0.037046 25 year 0.051828 50 year 0.065567 100 year 0.082025 Existing Conditions 100-year Peak Flow = 0.017 cfs Developed Conditions 100-year Peak Flow = 0.082 cfs Increase in 100-year Peak Flow = 0.065 cfs < 0.10 cfs RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-3 New + Replace Hard Surface Exhibit: Hard Surface Summary: Lot 1 (House 2,415 + Driveway 780 + Sidewalk/Patio 50) = 3,245 sf Lot 2 (House 2,500 + Driveway 750 + Sidewalk/Patio 50) = 3,300 sf Lot Hard Surface = 6,545 sf (0.150 ac) Frontage/ROW New + Replaced Hard Surface = 1,485 sf (0.034 ac) Total New + Replaced Hard Surface = 8,030 sf (0.183 ac) I w I iI I I I �I it I N 8830'31 119.30, Lot 2 = 3.300 sf I I i � I � I •III ` I I 3 I 2 8.020 SF 01 ao "i IQ I IIII — -�r --9 -- I IZI 3 `\ I ICI I $ z I o ROW= 1,485 sf Lot 1 = 3,2 sf r 3 l i 11 4/4 SF 1 I �W� • \ w -Z. ' W-_ :,z 2U, RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-4 Attached below are the DOE summary criteria of BMP T5.10C: Perforated Stub -out Connections and BMP T5.13: Post -Construction Soil Quality and Depth. 3.1.3 Perforated Stub -Out Connections (BMP T5.10C) A perforated stub -out connection is a length of perforated pipe within a gravel -filled trench that is placed between roof downspouts and a stub -out to the local drainage system. Figure 3.1.8 illustrates a perforated stub -out connection. These systems are intended to provide some infiltration during drier months. During the wet winter months. they may provide little or no flow control. Applications & Perforated stub -outs are not appropriate when seasonal water table is less Limitations than one foot below trench bottom. In projects subject to Minimum Requirement 45 (see Volume 1). perforated stub -out connections may be used only when all other higher priority on -site stormwater management BMPs are not feasible. per the criteria for each of those BMPs. Select the location of the connection to allow a maximum amount of runoff to infiltrate into the ground (ideally a dry, relatively well drained location). To facilitate maintenance, do not locate the perforated pipe portion of the system under imperious or heavily compacted (e.g.. driveways and parking areas) surfaces. Use the same setbacks as for infiltration trenches in Section 3.1.1. Have a licensed geologist, hydrogeologist, or engineering geologist evaluate potential runoff discharges towards landslide hazard areas. Do not place the perforated portion of the pipe on or above slopes greater than 20% or above erosion hazard areas without evaluation by a professional engineer with geotechnical expertise or qualified geologist and jurisdiction approval. For sites with septic systems, the perforated portion of the pipe must be downgradient of the drainfield primary and reserve areas. This requirement can be waived if site topography will clearly prohibit flows from intersecting the drainfield or where site conditions (soil permeability. distance between systems, etc.) indicate that this is unnecessary. Design Criteria Perforated stub -out connections consist of at least 10 feet of perforated pipe per 5,000 square feet of roof area laid in a level. 2-foot %vide trench backfilled with washed drain rock. Extend the drain rock to a depth of at least 8 inches below the bottom of the pipe and cover the pipe. Lay the pipe level and cover the rock trench with filter fabric and 6 inches of fill (see Figure 3.1.8). Runoff Nodel Any flow reduction is variable and unpredictable. No computer modeling Representation techniques are allowed that would predict any reduction in flow rates and volumes from the connected area. RAM Engineering, Inc. RAM No. 17-018 Volume HI — Hydrologic Analysis and Flow Control BAfPs — December 2014 3-17 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-5 gndoin 1! g� L raw Ub& • • 4• wa ow :.: ..o I%Y- �,nw roa TRENCH X-SECTION NTS sbpe r $0 mi Jr...i.v eyltem ,7xta vW4 Vvwh PLAN VIEW OF ROOF NTS Source: King County Figure 3.1.8 - Perforated Stub -Out Connection Volume HI - Hydrologic Analysis and Flow Control BMPs - December 2014 3-18 RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-6 BMP T5.13: Post -Construction Soil Quality and Depth Purpose and Naturally occurring (undisturbed) soil and vegetation provide important Definition stonnmater fiunctions including: water infiltration: nutrient. sediment. and pollutant adsorption: sediment and pollutant biofiltration: water i nterflow storage and transmission: and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with nruninnal topsoil and sod. Not only are these important stonnnivater fiuictions lost. but such landscapes themselves become pollution generatnng pervious surfaces due to increased use of pesticides. fertilizers and other landscaping and household Industrial chemicals. the concentration of pet wastes. and pollutants that accompany roadside litter. Establishing soil quality and depth regales greater stommnvater functions in the post development landscape. provides increased treatment of pollutants and sediments that result fi-onn development and habitation. and nnunnmizes the need for some landscaping clnernnicals. thus reduucnng pollution through prevention. Applications and Establishing a nninunuun soil quality and depth is not the same as Limitations preservation of naturally occurring soil and vegetation. However. establishing a rnniniunnurnn soil quality and depth will provide Improved on - site management of stonnm-ater flow and water quality. Soil organic matter can be attained through numerous materials such as compost. composted woody material. biosolids, and forest product residuals. It is Important that the materials used to meet the soil quality and depth BMP be appropriate and beneficial to the plant cover to be established. Likewise. it is Important that Imported topsoils Improve soil conditions and do not have an excessive percent of clay fuses. This BMP can be considered Infeasible on till soil slopes greater than 33 percent. Design Soil retention. Retain. in an undisturbed state. the duff layer and Guidelines native topsoil to the maxumuum extent practicable. In any areas requiring grading remove and stockpile the duff layer and topsoil on site in a designated. controlled area. not adjacent to public resources and critical areas. to be reapplied to other portions of the site where feasible. RAM Engineering, Inc. RAM No. 17-018 • Soil quality. All areas subject to clearing and grading that have not been covered by impervious surface. Incorporated into a drainage facility or engineered as structural fill or slope shall. at project completion. demonstrate the following: 1. A topsoil layer with a minimum organic matter content of 10% dry weight m planting beds. and S% organic matter content in turf areas. and a pH fi-orn 6.0 to 8.0 or matching the pH of the undisturbed soil. The topsoil layer shall have a rnnininnuurnn depth of Volume V — Runoff Treatment BMPs — December 2014 S-8 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-7 RAM Engineering, Inc. RAM No. 17-018 eight finches except where tree roots limit the depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches -,with some incorporation of the upper material to avoid stratified layers. where feasible. 2. Mulch planting beds with 2 filches of organic material 3. Use compost and other materials that meet these organic content requirements: a. The organic content for -'pre-approved" amendment rates can be met only using compost meeting the compost specification for Bioretention (BMP T7.30). with the exception that the compost may have tip to 35% biosolids or immure. The compost must also have an organic matter content of 40% to 65%. and a carbon to nitrogen ratio below 25:1. The carbon to nitrogen ratio may be as high as 3 5: 1 for plant mgs composed entirely of plants native to the Puget Sound Lowlands region. b. Calculated amendment rates may be met through use of connposted material meeting (a.) above: or other organic materials amended to meet the carbon to nitrogen ratio requirements. and not exceeding the contaminant lfinnits identified fin Table 220-B. Testing Parameters. in WAC 173- 350-220. ` The resulting soil should be conducive to the type of vegetation to be established. • Implementation Options: The soil quality design guidelines listed above can be met by using one of the methods listed below: 1. Leave urndistu bed native vegetation and soil, and protect fionn compaction during construction. 2. Annend existing site topsoil or subsoil either at default "pre - approved" rates. or at custom calculated rates based on tests of the soil and amendment. 3. Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements. either at a default "pre -approved" rate or at a custom calculated rate. 4. Import topsoil mix of sufficient organic content and depth to meet the requirements. More than one method may be used on different portions of the smile site. Soil that already nnects the depth and organic matter quality standards, mid is not compacted, does not need to be amended. Volume V — Runoff D eannent BMPs — December 2014 S-9 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-8 PlanningPermitt Local goverimnents are encouraged to adopt guuidelunes and inglnspectionlVe procedures similar to those reconmmended un Guidelines and rification Resources For Implementing Soil Quality and Depth B.'l1P T5.13 in Guidelines & WDOE Stor►nwater Management Manual for Western Washington, Procedures This document is available at: lnttp: xN-N%-N%-.soilsforsahmon.or}pdf Soil_ BMP_Manual.pdf .Vaintenance • Establish soil quality and depth toward the end of construction and once established. protect fi•onn compaction. such as from lacge machinery use. and fiom erosion. • Plant vegetation and mulch the amended soil area after installation. • Leave plant debris or its equivalent on the soil surface to replenish organic matter. • Reduce and adjust. ,x-liere possible. the use of irrigation. fertilizers. herbicides and pesticides. rather than contumuing to implement formerly established practices. Runoff _Model Areas meeting the design guidelines may be entered into approved runoff Representation models as "Pasture" rather than " Lawn." Flog reduction credits can be taken in runoff modeling when BMP T5.13 is used as part of a dispersion design under the conditions described in: BMP T5.IOB Downspout Dispersion BMP T5.11 Concentrated Flow Dispersion BMP T5.12 Sheet Floe- Dispersion BMP T5.18 Reverse Slope Sidewalks BMP T5.30 Full Dispersion (for public road projects) RAM Engineering, Inc. RAM No. 17-018 Volume V — Runoff Treatment BMPs — December 2014 5-10 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-9 Offsite Analysis. The site is located within the City of Edmonds Shell Creek watershed basin. The Shell Creek basin is a centralized urban basin that collects stormwater runoff from primarily residential areas in the City Edmonds and ultimately discharges flows northwesterly to Puget Sound via Shell Creek (see attached watershed map). Site runoff sheet flows off the site in a westerly direction and collects in 92"d PI W in a catch basin near the southwest corner of the site. The stormwater system in 92nd PI W flows south to Bowdoin Way The Bowdoin Way storm system flows northwesterly to 96t" Ave W and then is directed north along the unopen right-of- way of 96th Ave W and discharges to Shell Creek about 2,100 ft from the site. At this point Shell Creek generally flows in a northwesterly direction and eventually discharges to the Puget Sound about 1.6 miles from the site. No apparent flooding or system inadequacies within downstream of the site were discovered during site visits or research of the downstream system. See attached watershed and downstream drainage facility map for the local downstream path. RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-10 Watershed Basin Map av FIGURE B-1 Lebwx CITY OF EDMONDS WATERSHEDS Deer Creek ® Perrinville Edmonds Marsh - Puget Sound Edmonds Way ® Puget Sound Piped Fruitdale - Shell Creek Good Hope Pond Shellabarger Halls Creek - Southwest Edmonds A Hindley Creek Southwest Edmonds B ® take Ballinger Stihhouse Creek Lund's Gulch Talbot Park A Meadowdale A �� Talbot Park B Meadowdale B Terrace Creek Northstream Outtall Creek 0 1.000 2.000 Westgate Pond Willow Creek kow 600c 1 in - 2,000It N _--_- A Lund's Gulch .cam Meadowdale A, Meadowdale B } 1 I lath sr u Outfall CreekStilthouse Creek ' i L Terrace Creek + L_ i i u Perrimrille i Puget Sound Talbot Park A "4 Talbot Park B ho .■•t v at ry sort rcW" xcurxb. rT■t; a mon Mn bOSy y xco+ WN this w0owl Ms �, 30. 2010 Fruitdale •the4 —M Dry 19 + itn st sw 'tt Lhstream man st sw I Hindley r I Good Hope Pond .6tuget Sound Piped �1,Edmonds Marsh Shellabarger RAM Engineering, Inc. RAM No. 17-018 stsw 9215 Bowdoin Way Shell Cre tM Westgate Pond Hails Creek J u■a st sw Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page C-11 Downstream Drainage Facility Map (City of Edmonds GIS) w y o-.- I HELL ST It 1''111� ra ALDER ST -sp SPRUCE ST� L c: '� 0 sTN Pz Ic IAURLL L m rim RAM Engineering, Inc. RAM No. 17-018 SITE N o NM PL Sw3 LLJ Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report 14 4- Page C-12 D. SOILS REPORT. Geotechnical Evaluation Letter Prepared by: Earth Solutions NW, LLC Date: October 31, 2017 Pages: 31 (Report submitted to City under separate cover.) RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page D-1 E. CONSTRUCTION SWPPP REQUIREMENTS: The project will require grading to construct the proposed buildings and utilities (including the stormwater infiltration trench systems). Standard erosion control measures are proposed to be used during construction. The primary erosion and sediment control BMP during construction will be proper soil stabilization methods. Exposed soils shall be stabilized by application of effective BMPs that protect the soil from the erosive forces of raindrops, flowing water, and wind. Applicable practices include, but not limited to, temporary and permanent seeding, sodding, mulching, plastic covering, erosion control fabrics, matting, soil application of polyacrylamide (pam), the early application of gravel base on areas to be paved, and dust control. The contractor shall select a soil stabilization method best suited for the particular situation. Stock piles must be stabilized and protected with sediment trapping measures. In addition, site containment of exposed soils shall be sustained by using silt fence barriers along the down - slope boundaries of the site's disturbance areas. See the site development plan for details. Soils: In accordance with the project's site specific geotechnical engineering study by Earth Solutions NW, LLC the on -site soils consist of silty sand (Unified Soil Classification System: SM) Vashon till. Per geologic mapping the subject site is underlain by Vashon till (Qvt); Earth Solutions NW, LLC. recommends the limited use of the stormwater BMPs (Post -Construction Soil Quality and Depth BMP T5.13 and Perforated Stub -out Connections BMP T5.10C) due to the existing till soils present on site. A full soils description is included in the site specific geotechnical engineering study by Earth Solutions NW, LLC. Construction Stormwater Pollution Prevention Plan (SWPPP): The following is a summary of the site's erosion control measures that evaluates the typical DOE 13 elements of a SWPPP: Element 1: Mark Clearing Limits: Clearing limits have been delineated on the engineering site development plans. Element 2: Establish Construction Access: A construction access has been delineated on the engineering site development plans. Element3: Control Flow Rates: During construction silt fencing will provide attenuation of site runoff and upon project completion and stabilization (established lawns and landscape of exposed soils), the infiltration storm system will provide flow control. Element 4: Install Sediment Controls: Filter fabric fence has been delineated and detailed on the engineering site development plans. At a minimum, silt fence will be installed along the down gradient perimeter of the disturbed area that will receive sediment -laden runoff. Element 5: Stabilize Soils: Soils will be stabilized per the TESC notes listed on the engineering site development plans. Element 6: Protect Slopes: Exposed slopes shall be stabilized per the TESC notes listed on the engineering site development plans. RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page E-1 Element 7: Protect Drain Inlets: Drain inlet protection will be installed on all catch basins that will receive sediment -laden runoff. See the engineering site development plans for locations and detail. Element 8: Stabilize Channels and Outlets: This element is not applicable since there are no temporary channels or outlets proposed. Element 9: Control Pollutants: Pollutants shall be managed as described in the TESC notes listed on the engineering site development plans. Element 10: Control De -Watering: Highly turbid or contaminated de -watering water shall be handled separately from stormwater. The water from all de -watering systems for trenches and foundations shall be treated or disposed prior to discharging from the site. Element 11: Maintain BMPs: BMPs shall be maintained and removed at the end of the project as follows: i. All temporary and permanent erosion and sediment control BMPs shall be inspected, maintained and repaired in accordance with the Drainage Manual or as approved or required by the City to assure continued performance of their intended function in accordance with BMP specifications. ii. The applicant may remove temporary BMPs when they are no longer needed. iii. All temporary erosion and sediment control BMPs shall be removed within 30 days after construction is completed and the City has determined that the site is stabilized. Element 12: Manage the Project: The owner of the site is responsible for managing the installation and maintenance of the site BMPs. Element 13. Protect Low Impact Development BMPs: Compaction (i.e. vehicle and equipment traffic or storage; and/or placement of stockpiles) to the soils in the area of the stormwater BMPs (Post -Construction Soil Quality and Depth BMP T5.13 and Perforated Stub -out Connections BMP T5.10C) shall be avoided during construction. Grading Quantity Approximation: The below listed grading quantities have been estimated using the AutoCAD 3D modeling of the site; estimated quantities are for permitting purposes only. Estimated grading site development quantities: Cut: 350 CY Fill: 50 CY Conclusion: The final site development construction plans include specific grading and drainage improvement notes and details. With proper installation, maintenance and inspections of the proposed construction the project should have minimal impact to the surrounding environment. RAM Engineering, Inc. RAM No. 17-018 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page E-2 F. OPERATION AND MAINTENANCE GUIDELINES: These guidelines are intended to provide operation and maintenance instructions for the project's storm drainage control facilities. The owners are responsible for maintenance of storm drainage facilities within the property (private property owners system); the owners are not responsible for maintenance within the public right-of-way (City system). Private Property Owners: Private property owners are responsible for properly maintaining the stormwater infrastructure on their property to ensure it operates as designed. The City has developed an inspection program to ensure private property owners are properly maintaining their stormwater systems. City System: City crews perform maintenance activities on the entire storm drainage system, including inspection and cleaning of catch basins, street sweeping, emergency flooding response, creek maintenance, inspection and monitoring of private stormwater detention systems. Operation and Maintenance Requirements for private property owner system: This manual is not comprehensive; although it explains the intended operation of the various components of the drainage system, and suggests a routine of inspection and maintenance, it cannot anticipate every problem. Once a historical record of maintenance is established, it may be prudent to alter the routine. It is recommended that maintenance records be kept, and that the records be reviewed periodically. Concept of Operation: The drainage design is shown and described in the site development engineering plans and report. The approved site development plans and report should be retained by the owner and used as a reference to identify stormwater BMP facilities outlined in this manual. Conveyance Systems. The design objective in pipe sizing was to convey the large storm events without the water surface exceeding the ground elevation. Frequent overtopping of the pipe system in an area might indicate a downstream blockage. Overtopping of the drain systems at the catch -basins is an indication that maintenance is required. The following pages outline standard general maintenance criteria for the project's drainage facilities. For additional and updated maintenance information visit the Washington State Department of Ecology's web -site at: http://www.ecy.wa.gov/programs/wq/wghome.htmI MAINTENANCE REQUIREMENTS ECDC Section 18.30.090 requires privately -owned stormwater management facilities, such as LID BMPs be properly maintained. The owner of the property is the responsible party for such maintenance. The system must be kept in good working order. The entire system should be inspected once per year. An improperly maintained BMP may cause private property or street flooding. RAM Engineering, Inc. RAM No. 17-018 Contact the City Engineering Division for maintenance information. The City may make periodic inspections of BMPs to ensure they are operating properly. ECDC Section 18.30.100 contains the enforcement provisions the City can use to ensure the system is properly maintained. Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page F-1 No. 3 — Closed Detention Systems (Tanks/Vaults) Maintenance Defect Conditions When Maintenance is Needed Results Expected Component When Maintenance is Performed Storage Area Plugged Air Vents One-half of the cross section of a vent is Vents open and blocked at any point or the vent is damaged functioning. Debris and Sediment Accumulated sediment depth exceeds 10% All sediment and of the diameter of the storage area for 1/2 debris removed from length of storage vault or any point depth storage area exceeds 15% of diameter. (Example 72-inch storage tank would require cleaning when sediment reaches depth of 7 inches for more than 1/2 length of tank.) Joints Between Any openings or voids allowing material to All joint between Tank/Pipe Section be transported into facility. tank/pipe sections (Will require engineering analysis to are sealed determine structural stability). Tank Pipe Bent Out Any part of tankipipe is bent out of shape Tank/pipe repaired or of Shape more than 10% of its design shape (Review replaced to design required by engineer to determine structural stability). Vault Structure Cracks wider than 1/2-inch and any Vault replaced or Includes Cracks in evidence of soil particles entering the repaired to design Wall, Bottom, structure through the cracks. or specifications and is Damage to Frame maintenance/inspection personnel structurally sound and/or Top Slab determines that the vault is not structurally sound Cracks wider than 1/2-inch at the joint of any No cracks more than inlet/outlet pipe or any evidence of soil 1/4-inch wide at the particles entering the vault through the walls joint of the inlet/outlet pipe. Manhole Cover Not in Place Cover is missing or only partially in place. Manhole is closed. Any open manhole requires maintenance Locking Kiechanism Mechanism cannot be opened by one Alechanism opens Not Working maintenance person with proper tools. Bolts with proper tools. into frame have less than 1/2 inch of thread (may not apply to self-locking lids). Cover Difficult to One maintenance person cannot remove lid Cover can be Remove after applying normal lifting pressure Intent removed and is to keep cover from sealing off access to reinstalled by one maintenance maintenance person Ladder Rungs Unsafe Ladder is unsafe due to missing rungs, Ladder meets design misalignment, not securely attached to standards. Allows structure wall, rust, or cracks. maintenance person safe access. Catch Basins See 'Catch Banns' See Catch Banns" (No 5) See "Catch Basins" (No ) (No 5). RAM Fnnineering, Inc. RAM No. 17-018 Volume V — Runoff Treatment BMPs — December 201-+ 4-36 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page F-2 No. 4 — Control Structure/Flow Restrictor Maintenance Defect Condition When Maintenance is Needed Results Expected Component When Maintenance is Performed enerai Trash and Debris Material exceeds 25%of sump depth or 1 Control structure (Includes Sediment) foot below orifice plate. orifice is not blocked. All trash and debris removed. Structural Damage Structure is not securely attached to Structure securely manhole wall. attached to wall and outlet pipe. Structure is not in upright position (allow up Structure in correct to 10% from plumb) position. Connections to outlet pipe are not watertight Connections to outlet and show signs of rust. pipe are water tight, structure repaired or replaced and works as designed. Any holes --other than designed holes --in the Structure has no structure holes other than designed holes. Cleanout Gate Damaged or Missing Cleanout gate is not watertight or is missing Gate is watertight and works as designed. Gate cannot be moved up and down by one Gate moves up and maintenance person. down easily and is watertight. Chain/rod leading to gate is missing or Chain is in place and damaged works as designed. Gate is rusted over 50% of as surface area. Gate is repaired or replaced to meet design standards. Orifice Plate Damaged or hissing Control device is not working properly due to Plate is in place and missing. out of place. or bent orifice plate. works as designed. Obstructions Any trash. debris. sediment. or vegetation Plate is free of all blocking the plate. obstructions and works as designed. Overflow Pipe Obstructions Any trash or debris blocking (or having the Pipe is free of all potential of blocking) the overflow pipe. obstructions and works as designed. Manhole See "Closed See "Closed Detention Systems" (No. 3). See "Closed Detention Systems" Detention Systems - (No. 3). (No 3) Catch Basin See "Catch Basins" See "Catch Basins" (No. 5). See "Catch Basins - (No. 5). (No. 5). RAM Fnnineering, Inc. RAM No. 17-018 Volume V — Runoff Tmannent B_VPs — December 2014 4-37 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page F-3 No. 5 — Catch Basins Maintenance Defect Conditions When Maintenance is Needed Results Expected When Component Maintenance is performed General Trash & Trash or debris which is located immediately No Trash or debris located Debris in front of the catch basin opening or is immediately in front of blocking inletting capacity of the basin by catch basin or on grate more than 10%. opening. Trash or debris (in the basin) that exceeds 60 No trash or debris in the percent of the sump depth as measured from catch basin. the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of six inches clearance from the debris surface to the invert of the lowest pipe. Trash or debris in any inlet or outlet pipe Inlet and outlet pipes free blocking more than 113 of its height. of trash or debris. Dead animals or vegetation that could No dead animals or generate odors that could cause complaints vegetation present within or dangerous gases (e.g., methane). the catch basin. Sediment Sediment (in the basin) that exceeds 60 No sediment in the catch percent of the sump depth as measured from basin the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of 6 inches clearance from the sediment surface to the invert of the lowest pipe. Structure Top slab has holes larger than 2 square Top slab is free of holes Damage to inches or cracks wider than 114 inch and cracks. Frame and/or Top Slab (Intent is to make sure no material is running into basin). Frame not sitting flush on top slab, i.e., Frame is sitting flush on separation of more than 3/4 inch of the frame the riser rings or top slab from the top slab. Frame not securely and firmly attached. attached Fractures or Maintenance person judges that structure is Basin replaced or repaired Cracks in unsound. to design standards. Basin Walls/ Bottom Grout fillet has separated or cracked wider Pipe is regrouted and than 1/2 inch and longer than 1 foot at the secure at basin wall. joint of any inletoutlet pipe or any evidence of soil particles entering catch basin through cracks. Settlement/ If failure of basin has created a safety, Basin replaced or repaired Misalignment function, or design problem. to design standards. Vegetation Vegetation growing across and blocking more No vegetation blocking than 10% of the basin opening. opening to basin. Vegetation growing in inlet/outlet pipe joints No vegetation or root that is more than six inches tall and less than growth present. six inches apart. Contamination See "Detention Ponds" (No. 1). No pollution present. and Pollution RAM Engineering, Inc. RAM No. 17-018 Volume V — Runoff Treatment BMPs — December 2014 4-38 Select Homes. 9215 Bowdoin Way Stormwater Site Plan Report Page F-4 No. 5 — Catch Basins Maintenance Defect Conditions When Maintenance is Needed Results Expected When Component Maintenance is performed Catch Basin Cover Not in Cover is missing or only partially in place. Catch basin cover is Cover Place Any open catch basin requires maintenance. closed Locking Mechanism cannot be opened by one Mechanism opens with Mechanism maintenance person with proper tools. Bolts proper tools. Not Working into frame have less than 1/2 inch of thread. Cover Difficult One maintenance person cannot remove lid Cover can be removed by to Remove after applying normal lifting pressure. one maintenance person. (Intent is keep cover from sealing off access to maintenance.) Ladder Ladder Rungs Ladder is unsafe due to missing rungs, not Ladder meets design Unsafe securely attached to basin wall, standards and allows misalignment, rust, cracks, or sharp edges. maintenance person safe access. Metal Grates Grate opening Grate with opening wider than 7/8 inch. Grate opening meets (If Applicable) Unsafe design standards. Trash and Trash and debris that is blocking more than Grate free of trash and Debris 20% of grate surface inletting capacity. debris. Damaged or Grate missing or broken member(s) of the Grate is in place and Missing. grate. meets design standards. No. 6 — Debris Barriers (e.g.. Trash Racks) Maintenance Defect Condition When Maintenance is Results Expected When Components Needed Maintenance is Performed General Trash and Trash or debris that is plugging more Barrier cleared to design flow Debris than 20% of the openings in the barrier. capacity. Metal Damaged/ Bars are bent out of shape more than 3 Bars in place with no bends more Missing inches. than 3/4 inch. Bars Bars are missing or entire barrier Bars in place according to design. missing Bars are loose and rust is causing 50% Barrier replaced or repaired to deterioration to any part of barrier. design standards. Inlet/Outlet Debris barrier missing or not attached to Barrier firmly attached to pipe Pipe pipe RAM Fnnineering, Inc. RAM No. 17-018 Volume V — Runoff Tmamtent BVPs — December 2014 4-3 0 Select Homes: 9215 Bowdoin Way Stormwater Site Plan Report Page F-5 ENGINEERING, INC. Civil Engineering /Land Planning