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APPROVED_DRAINAGE REPORT_2020-04-29Select Homes 2131082nd pl W Edmonds, WA 98026 Stormwater Site Plan Report Prepared for: Select Homes Date: January 9, 2020 Revised: April 29, 2020 COMPLIES WITH APPLICABLE CIT 36508w \'; �'GIST��'�� IOnreI . Prepared by: Rob Long, PE RAM Engineering, Inc. 16531 13t" Ave W, Suite A108, Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc.corn Job No. 19-009 Select Homes 21310 82nd PI W Introduction: This summary report provides site design information for a single-family residential lot development by Select Homes. This report includes storm drainage analysis to support permit review and approval. The property is located at 21310 82nd Place W, in the northeast 1/4 of Section 25, T 27 N, R 3 E, W.M. Site Address: 21310 82nd Place W Applicant: Select Homes Edmonds, WA 98026 16513 13th Ave W, Suite A108, Lynnwood WA 98037 Tax Parcel Number: New BLA Parcel Number Contact: Kayla Clark not assigned, old = 004674 000 01100 (425) 742-6044 TABLE OF CONTENTS SECTION PAGES A. Project Overview.............................................................................................................................. 5 B. Existing Conditions Summary........................................................................................................... 1 C. Developed Site Hydrology................................................................................................................ 9 D. Soils Report .................................................................................... 20 E. Construction SWPPPP Requirements................................................................................................ 2 F. Operation and Maintenance Guidelines............................................................................................ 3 RAM-noineering, Inc. Select Homes, 21310 82nd PI W RAM No. 19-009 Stormwater Site Plan Report A. PROJECT OVERVIEW: This report provides engineering information for the proposed construction of a single-family residence on an 8,960 sf (0.21 ac) lot; the project is located at 21310 82"d Place W in the City of Edmonds. The applicant, Select Homes proposes to remove the existing residence. This report provides the evaluation for the proposed 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 0.21 acres of land and creating/replacing about 4,660 sf of hard surface area onsite and along the site's frontage of 83nd PI W. Thus, the project is classified as a Category 1 project per the City's classification system. Per ECDC 18.30, Category 1 projects must comply with Minimum Requirements No. 1 through No. 5. 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 200 CY of cut and 200 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. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page A-1 Minimum Requirement #4 — Preservation of Natural Drainage Systems and Outfalls. The site generally slopes in an easterly direction towards 82nd PI W; there are no concentrated flows or existing defined conveyance systems located onsite; any site runoff leaves the site as sheet -flow. Stormwater runoff leaving the site collects in the existing stormwater systems (ditch and pipe/catch basins) along the west side of 82nd PI W and flows north. The 82nd PI W stormwater system routes runoff norther under 2121h St SW and eventually outfalls to Good Hope Pond about 0.36 miles north of the site. The proposed development will discharge the collected stormwater discharge to the existing ditch along the west side of 82nd PI W thus maintaining the existing stormwater outfall of the site. The site is located within the upper reach of the Good Hope Pond watershed basin per the City of Edmonds maps (see section C). Minimum Requirement #5 — Onsite Stormwater Management. A site specific geotechnical evaluation (see section D) of the site demonstrates that based on the site soils characteristics, the available and applicable stormwater BMPs for the site are BMP T5.13: Post -Construction Soil Quality and Depth and BMP T5.10C: Perforated Stub -out Connections. Roof areas from the new house (2,720 sf) and will not exceed the 5,000 sf capacity of a single perforated stub -out connection for the site; all of the new house downspouts shall be connected to a perforated stub -out connection prior to leaving the site. The new onsite hard surface (driveway + walkway = 974 sf) will be mitigated by a stormwater detention tank BMP (per Edmonds Stormwater Addendum 6.3.1). The frontage improvements (390 sf frontage sidewalk and 730 sf pavement widening) within the 82n1 PI W right-of-way are not feasibly controlled by "onsite stormwater management" BMPs A LID BMP feasibility evaluation table (below) has been prepared based on recommendations from the geotechnical engineer and the stormwater site plan evaluation of the project. See the civil site development plans and Section Cfor additional stormwater BMP evaluation and construction details. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page A-2 LID BMP Performance Standards Evaluation Lawn and landscaped areas: BMP Viable Limitations / Infeasibility Criteria BMP T5.13: Post -Construction Soil Quality and Depth Yes BMP T5.13 shall be applied to the site post construction. (Volume V, Chapter 5) Roofs: BMP T5.30: Full Dispersion Infeasibility: Large vegetation retention and flow path areas (Volume V, Chapter 5) No (100 ft) are not available on the project site. BMP T5.10A: Downspout Full Infiltration Systems Infeasibility: Due to soil variability and very low permeability. (Volume III, Chapter 3) No See additional discussion in the geotechnical evaluation by Earth Solutions NW. Infeasibility: Due to soil variability and very low permeability. Bioretention No See additional discussion in the geotechnical evaluation by (Volume V, Chapter 7) Earth Solutions NW. BMP T5.106: Downspout Dispersion Systems No Not feasible due to the required expansive trench length and (Volume III, Chapter 3) dispersion area that is required. BMP T5.10C: Perforated Stub -out Perforated stub -out will be included in the construction (Volume III, Chapter 3) Yes plans. Detention vaults or pipes in accordance with the A shallow detention tank system is feasible onsite and will be Edmonds Stormwater Addendum Yes utilized for the "other hard surfaces" (driveway and walk areas. Other Hard Surfaces: BMP T5.30: Full Dispersion Infeasibility: Large vegetation retention and flow path areas (Volume V, Chapter 5) No are not available on the project site. BMP T5.15: Permeable Pavement Infeasibility: Due to soil variability and very low permeability. (Volume V, Chapter 5) No See additional discussion in the geotechnical evaluation by Earth Solutions NW. Infeasibility: Due to soil variability and very low permeability. Bioretention (Volume V, Chapter 7) No See additional discussion in the geotechnical evaluation by Earth Solutions NW. BMP T5.12: Sheet Flow Dispersion Infeasibility: Vegetation retention and flow path areas are BMP T5.11: Concentrated Flow Dispersion Yes not available on the project site. (Volume V, Chapter 5) RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page A-3 Parcel (Vicinity) Map: NE 30 27 4 ce.Wine 31-1, — $-If- Ci y Urnit. Cow lot Subdiv ROW — Q..rt. 7-A= W]OF Water Offior Uft ------- Vac RQ1W - - - - Idth Easement ............. Minor Water Other rpubdiv — Vac WE SE 19.27-D4 JIM 7 CONDO 6.057.E 052 2 AIRE NE 01 : ROSE� CONDOMONTE 0 CONDO F, Iii, T 167) (8884 ONDO E ROSE� D MONTE CONDO 1 67� (8884) ZJ 6 47 S LAIR .1202 L"'I'll, 13 �613G4) N N T LI�FN 09-A-03 8141 �UC,111)08G270 8- 212TR TED FR= 7p,3E TOWN SQUIRE �11 �i,, 11 1,17 5 "WANOR (A CONDO) 01 U "A la 7� a N I ' &E I R (3j36) 1 " S ITE 5 �'2 T 2G 06 � 1 1.1 T. 0201 7 7 25 SI�STA+ PARK 7 — 11_� j 5 .1. 5 1 1 a 1 21 (4674) 8 213T�H ST SW 01 9 T i 2 FW 1-051 2 00 1� 14 12 9 10 11 12 31 4D) N ADD (61 DEX10 No A61 S3TA Fl 06 01 1-0550 "1 I. --2..47H -PL 214TH PIL 8- _H PL S1 5 214TH PIL SW 4 15 16 IT 1-048 01 3. 7 8 90, 1-01311 11 25. 6E-dk LAWN 2 FATS'AN P 4 1 3 jr 5 3 1 02 215TH ST. -sw 7 711rT 55 1� JT}16 1, 47 1 8 0,i 10 11 14 11 ... . ........ ........... 7 ISIESTA 11LEN -4 601_2115TH PL SIN 1213,, 1 1 - 02 11 01 i12113 0. LC LC 02 1 01 10 1(5 14 Sp ki 46TH--------------------------- tY PAF3K A 2 1 _B z to 14 2 7 I-W4 > F . .......... GOLDEN , 13 01 1 014 1_07�1 1-005 LEAVES �z (A CON DO) 3 (6798) 4 1.119 4 EA W 12 J LEGEND ISLES 111D0 024 112 ESTATES PH I I-D 0 '2-021 1-008 �J OZ CONDO 1_010� C 04 PH 2 (10330) ............. . ...... a -04 14)21 7 i 01 uu 02 7 c.j 91- V A �2-050 1-026 2 [11",2 . _j 7 6 2-047 2 4 nt .1.- 3 03 v, 02 m -0 8 2-046 6 1 2 5 02 Z% ()1: 02 03 E, 2 -1-4070 If) J -2119TH ST t144". 2-0132 1-1341 1-020 N -, �78� A DITIJO -01 P2 12 2 9 4 5 �2. 6 1-047 Al 17 IV 4�_ 2 F� A 02 ID 14 15� C4 � ul� N&TJA__ST RAM Engineering, Inc. RAM No. 19-009 SE-30-27-04 WEE 6 ANf (A C,ONDO) f787' ROSE POINTS (ACONI)Gj DC, IA OON11714 DO) ',.T,.. 21 ---------- - - - - - - - - - - z x 02 02 Select Homes: 21310 82nd pl W Stormwater Site Plan Report Page A-4 e 73�ara - wrov a am;. a+ec 40Rll+OC'�e.K9 w'�-,p 99 39 p# b{ '70A VAPVM.L Naun 'Mom d H '� P wz 9 CL B. EXISTING CONDITIONS SUMMARY Existing Conditions. The total site consists of about 8,960 sf (0.21 acres) and is currently occupy by an existing home. The lot has been recently reconfigured by a boundary line adjustment with the neighboring church property. The site is vegetated with grass lawn a few mature trees. The subject property is bordered by 82nd Place W along the east boundary, single family lot to the north and south, and the church property to the west (see attached aerial photo). The site generally slopes in an easterly direction towards 82nd PI W in a range between 2% and 5%. No define drainage courses are found on -site and stormwater runoff would sheet flow easterly direction to a roadside ditch along the westerly side of 82nd PI W. 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 evaluation by Earth Solutions NW, LLC the on -site soils consist of silty sand with gravel (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 evaluation by Earth Solutions NW, LLC. Aerial Photo (City of Edmonds GIS, 2017 photo): RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page 8-1 C. DEVELOPED SITE HYDROLOTY. Developed Conditions. The proposed project includes the construction of a single residence and associated driveway, frontage improvements and utilities. The new residence will access 82"d PI W along the subject site's easterly boundary. Landscaping and grass lawn around the new residence will stabilize the site upon building construction. In accordance with the Edmonds City Code (18.30) Category 1 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. 1" or meet the LID performance standard. The propose development will follow and apply on -site stormwater management BMPs from List NO. 1. The attached geotechnical evaluation letter assesses the feasibility criteria of each stormwater BMP; per the evaluation only BMP T5.10C: Perforated Stub -out Connections, detention pipe (Edmonds BMP) and BMP T5.13: Post - Construction Soil Quality and Depth are considered feasible (dispersion, infiltration, rain gardens, and bioretention are all considered infeasible). Additionally, a detention pipe (per Edmonds Stormwater Addendum, 6.3.1 ) will be installed to mitigate the additional hard surfaces of the sites driveway and walk/patio areas. The project proposes to create and/or replace 4,660 sf of hard surface with the complete site development, 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: New and Replaced Hard Surface: Total Hard Surface Proposed = 4,660 sf Building Rooftop = 2,720 sf sf Uncovered Patio and Walk = 30 sf Uncovered Driveway = 790 sf ROW Sidewalk = 390 sf ROW Pavement = 730 sf The 2,720 sf roof area (less than 5,000 sf), will be collected and routed through a 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 driveway and onsite walk/patio areas ("other hard surfaces") will total 820 sf and will be managed by an onsite detention pipe (per Edmonds Stormwater Addendum, 6.3.1). The 820 sf of "other" hard surface will require 28 ft of 18" pipe (820 sf x 0.034) for BMP mitigation. The detention pipe will be constructed onsite, under the access driveway; see the Civil Site plan for layout and construction details of the detention pipe system. No other BMPs are feasible to mitigate the frontage sidewalk (390 sf) and pavement widening (730 sf) areas within the public right-of-way. 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). Attached are the DOE summary criteria of BMP T5.10C: Perforated Stub -out Connections and BMP T5.13: Post -Construction Soil Quality and Depth. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-1 BMP T5.10C: Perforated Stub -out Connections 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 V-4.7: Per- forated Stub -Out Connection illustrates a perforated stub out connection. These systems are inten- ded to provide some infiltration during drier months. During the wet winter months, they may provide little or no Flow Control. 2019 Storm water Management Manual for Western Washington Volume V - Chapter - Page 719 Applications & Limitations Perforated stub -outs are not appropriate when the seasonal water table is less than one foot below the trench bottom. 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 per- forated pipe portion of the system under impervious or heavily compacted (e.g., driveways and park- ing areas) surfaces. Use the same setbacks asfor infiltration trenches in BMP T5.10A: Downspout Full Infiltration. Have a licensed geologist, hydrogeologist, or engineering geologist evaluate potential runoff dis- charges 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 licensed engineer in the state of Washington 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 drain - field primary and reserve areas. This requirement can be waived if site topography will clearly pro- hibit 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 wide 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 V-4.7: Perforated Stub -Out Con- nection). Runoff Model Representation Any flow reduction is variable and unpredictable. No computer modeling techniques are allowed that would predict any reduction in flow rates and volumes from the connected area. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-2 Figure V-4.7: Perforated Stub -Out Connection DEPARTMENT OF ECOLOGY State of Washington RAMEngineering, Inc. RAM No. 19-009 random fill Trench X-Section slope Plan View of Roof to road drainage system x 10' vel trench Vperf pipe NOT TO SCALE Perforated Stub -Out Connection Revised June 2016 Please see httpl/unvw.ecywa.gov/copyright.html for copyright notice including permissions, limitation of liability, and disclaimer. 2019 Stormwater Management Manual for Western Washington Volume V - Chapter 4 - Page 721 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-3 V-11 Miscellaneous LID BMPs V-11.1 Introduction to Miscellaneous LID BMPs BMPs in this chapter have been grouped because they have the following in common: . They employ Low Impact Development (LID) Principles . They cannot be used to meet 1-3.4.6 MR6: Runoff Treatment . They cannot, by themselves, be used to meet the Flow Control Performance Standard or the LID Performance Standard. o Some of the BMPs in this chapter do allow for some amount of Flow Control credit. See the guidance for each individual BMP for details. . The design methods for each BMP in this chapter are unique. They do not have strong enough design similarities to other BMPs in this volume to place them in the other BMP cat- egories identified in this volume. BMP T5.13: Post -Construction Soil Quality and Depth Purpose and Definition Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions includ- ing: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltra- tion; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal top- soil and sod. Not only are these important stormwater functions lost, but such landscapes them- selves become pollution generating pervious surfaces due to increased use of pesticides, fertilizers and other landscaping and household/industrial chemicals, the concentration of pet wastes, and pol- lutants that accompany roadside litter. Establishing soil quality and depth regains greater stormwater functions in the post development landscape, provides increased treatment of pollutants and sediments that result from development and habitation, and minimizes the need for some landscaping chemicals, thus reducing pollution through prevention. Applications and Limitations Establishing a minimum soil quality and depth is not the same as preservation of naturally occurring soil and vegetation. However, establishing a minimum soil quality and depth will provide improved on -site management of stormwater 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 RAMEngineering, Inc. RAM No. 19-009 2019 Stormwater Management Manual for Western Washington Volume V - Chapter 11- Page 927 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-4 meet this 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 fines. This BMP can be considered infeasible on till soil slopes greater than 33 percent. Design Guidelines Soil Retention Retain, in an undisturbed state, the duff layer and native topsoil to the maximum extent practicable. In any areas requiring grading, remove and stockpile the duff layer and topsoil on site in a des- ignated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible. Soil Quality All areas subject to clearing and grading that have not been covered by impervious surface, incor- porated into a drainage facility or engineered as structural fill or slope shall, at project completion, demonstrate the following: A topsoil layer with a minimum organic matter content of 10% dry weight in planting beds, and 5% organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the undisturbed soil. The topsoil layer shall have a minimum depth of eight inches 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 inches of organic material. 3. Use compost and other materials that meet the following organic content requirements: a. The organic content for "pre -approved" amendment rates can be met only using com- post meeting the compost specification for BMP T7.30: Bioretention, with the exception that the compost may have up to 35% biosolids or manure. 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 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region. Calculated amendment rates may be met through use of composted material meeting (a.) above; or other organic materials amended to meet the carbon to nitrogen ratio requirements, and not exceeding the contaminant limits identified in Table 220-B, Test- ing Parameters, in WAC 173-350-220. The resulting soil should be conducive to the type of vegetation to be established. RAMEngineering, Inc. RAM No. 19-009 2019 Stormwater Management Manual for Western Washington Volume V - Chapter 11- Page 928 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-5 Implementation Options The soil quality design guidelines listed above can be met by using one of the methods listed below: 1. Leave undisturbed native vegetation and soil, and protectfrom compaction during con- struction. 2. Amend existing site topsoil or subsoil either at default "pre -approved" rates, or at custom cal- culated 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 same site. Soil that already meets the depth and organic matter quality standards, and is not compacted, does not need to be amended. Planning/Permitting/inspection/Verification Guidelines & Procedures Local governments are encouraged to adopt guidelines and procedures similar to those recom- mended in Building Soil: Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington (Stenn et al., 2016). Maintenance • Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from 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, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices. Runoff Model Representation All areas meeting the soil quality and depth design criteria may be entered into approved runoff mod- els as "Pasture" rather than "Lawn/Landscaping". RAMEngineering, Inc. RAM No. 19-009 2019 Stormwater Management Manual for Western Washington Volume V - Chapter 11- Page 929 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-6 Offsite Analysis. The site is located within the City of Edmonds Good Hope Pond watershed basin. Good Hope Pond basin is a centralized urban basin that collects stormwater runoff from primarily residential areas in the City Edmonds and ultimately discharges flows westerly to Shell Creek and on to the Puget Sound (see attached watershed map). Site runoff leaves the site as sheet -flow runoff that flows in an easterly direction. Runoff leaving the site sheet -flows across the site and enter the roadside ditch and culvert system along the west side of 82nd PI W. The storm system along 82nd PI W crosses to the east side of 82nd PI W near the northeast corner of the project site and continues north in a pipe and catch basin system along 82nd PI W. The stormwater system continues north under 2121h St SW through the Solaire Condominium complex and back in to the 82nd PI W right-of-way north of the condominium property. The stormwater system continues north and shifts west to 83rd Ave W at 210th PI SW; at 208th St SW the system turns west and discharges into Good Hope Pond about 0.36 miles downstream of the site. Good Hope Pond discharges westerly to Shell Creek and eventually discharges to the Puget Sound about 2.6 miles from the site. The downstream system primarily consists of a man-made pipe and catch basin system within the 0.36 miles from the site to Good Hope Pond. 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. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-7 Watershed Basin Map FIGURE B-1 CITY OF EDMONDS WATERSHEDS W: Deer Creek Perrinville Edmonds Marsh Puget Sound Edmonds Way Puget Sound Piped k Pruitdaie i�i Shell Creek Good Hope Pond Shellabarger f Halls Creek Southwest Edmonds Hindley Creek Southwest Edmonds B Lake Ballinger L Stilthouse Creek l� Lund's Gulch = Talbot Park A Mead owdale A Talbot Park B Mead owdale B f�l I Terrace Creek Northstream Westgate Pond 0kafall Creek Willow Creek 0 1.000 2000 I'mo 6,00 1 in = 2,000 ft No Wa A"ty or any son, nd.ding arcurary, Mr,—, m mercharambdity Ku pny this godurr. M—h 30, 2010 N A. N A Meadowdale A M o owdale B an T I I SRI, 51° "N�------- til Creek5tilthouse Creek !` l , 1 I Terrace Creek rinville l / i 3 e' i%rh St SW e rthstream [asper 5[ G 20o -Sr SW G m. " rs __._• ..._.. x-;�'•'_': Good Hope Pond I zsP� ~� •�� 206th St SW Puget Sound PipedAR ,,;n.s, s s �FM PUgej Sound, ,.� .. Talbot Park A Aalbbt Park 8 3.l gE \Frustdaie w EdniandslNarsh 1—do,nwy Shell Creek 212�n Sc SW � a Shel�a�aj -------• ----- Pine 5x._ — Z� i J S` illQW, Creek I — I iDeer Creek fa9��n& wv RAMEngineering, Inc. RAM No. 19-009 3 9hd' P I aWestgate Pon Halls Creek /I a i -----270rh 5t 5W--- I 3 ! 3 m = n l i PP4th 5[ 5w I I I a - Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page C-8 Downstream Drainage Facility Map (City of Edmonds GIS) _o 4 ❑ b, w 00 O ° Good ° ° ° Hope O PDy 11 Pond 09. 17 7 F S� MM ry,?-1 O + G FA gPD&,z2 OQ Q O o 00,6 o a 0 1 O O 0 i PDt 3.21 1 8 O Q O Ir 13.32 I RAM =ngineering, Inc. RAM No. 19-009 LVbin51 — Discharge to Good Hope Pond about 0.36 miles downstream of site. Select Homes: 21310 82nd PI W Stormwater Site Plan Report a Oa C Page C-9 D. SOILS REPORT: 1. Geotechnical Evaluation Letter Prepared by: Earth Solutions NW, LLC Date: January 8, 2020 2. Geotechnical Response to Stormwater Review Comments Prepared by: Earth Solutions NW, LLC Date: April 29, 2020 RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page D-1 January 8, 2020 ES-6999 Select Homes, Inc. 16531 — 13t" Avenue West, A107 Lynnwood, Washington 98037 Attention: Ms. Kayla Nichols Subject: Geotechnical Evaluation Proposed Single -Family Residences 21222 — 82" d Place West Edmonds, Washington L h ons «C Earth Solutions NW LLC Geotechnical Engineering, Construction Observationjesting and Environmental Services Reference: Department of Ecology Stormwater Management Manual for Western Washington Amended December 2014 1F.r,I-MalTAffo eI Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles Washington, 1983 Edmonds City Code Chapter 23.80 (Geologically Hazard Areas) Pacific Coast Surveys, Inc. Record of Survey & Boundary Line Adjustment Dated August 26, 2019 Dear Ms. Nichols: As requested, Earth Solutions NW, LLC (ESNW) has prepared this geotechnical evaluation report for the proposed single-family residences. As part of our scope of services, we completed a subsurface exploration, in -situ infiltration testing, laboratory and engineering analyses, and prepared this written report with our findings and recommendations for the proposed project. Based on our evaluation, the proposed project is feasible from a geotechnical standpoint. 15365 N.E. 90th Street, Suite 100 0 Redmond, WA 98052 0 (425) 449-4704 9 FAX (425) 449-4711 Select Homes, Inc. January 8, 2020 Project Description ES-6999 Page 2 The subject site is located at 21222 — 82nd Place West in Edmonds, Washington, as illustrated on the attached Vicinity Map (Plate 1). The site consists of one tax parcel (Snohomish County parcel number 0046740000-1100) totaling approximately 1.14 acres of land. Based on the referenced survey, the boundary lines will be adjusted to create two new parcels. The property is currently developed with a single-family residence and associated improvements. The subject site generally descends towards the east with total elevation change on the order of 10 feet or less. The subject site is bordered to the north and south by single-family residences, to the east by 82n1 Place West, and to the west by Calvary Chapel. We understand the existing single-family residential structure and associated improvements will be demolished and two new single-family residential structures will be constructed. We anticipate grading activities will include cuts and fills of about two to four feet to establish the planned building alignments. However, grading plans were not available at the time this report was prepared. We understand infiltration and low impact development methods are being evaluated for stormwater management. Site improvements will also include underground utility installations. At the time this report was prepared, specific building load values were not available. However, we anticipate the proposed residential structures will consist of relatively lightly loaded wood framing supported on conventional foundations. Based on our experience with similar developments, we estimate wall loads on the order of one to two kips per linear foot and slab -on - grade loading of 150 pounds per square foot (psf). If the above design assumptions are incorrect or change, ESNW should be contacted to review the recommendations in this report. ESNW should review the final design to verify the geotechnical recommendations provided in this report have been incorporated into the plans. Subsurface Conditions As part of this geotechnical evaluation, an ESNW representative observed, logged, and sampled six test pits on November 19, 2019, excavated at accessible locations within the proposed development area, using a mini-trackhoe and operator provided by the client. The approximate locations of the test pits are depicted on the Test Pit Location Plan (Plate 2). Please refer to the test pit logs provided as an attachment to this report for a more detailed description of subsurface conditions. Representative soil samples collected at the test pit locations were analyzed in general accordance with the Unified Soil Classification System (USCS) and United States Department of Agriculture (USDA) methods and procedures. Topsoil and Fill Native topsoil was observed extending to depths of approximately six to eight inches below existing ground surface (bgs). The topsoil was characterized by dark brown color and fine organic material. Fill was encountered at test pit location TP-1 during our fieldwork, extending to a depth of approximately two feet bgs. Select Homes, Inc. January 8, 2020 Native Soil ES-6999 Page 3 Underlying the topsoil and limited fill, soil conditions at the test pit locations were observed to consist primarily of silty sand (USCS: SM). Silty gravel (USCS: GM) was observed near -surface at TP-4 which transitioned to silty sand at depth. Poorly graded sand was observed between five and seven feet bgs at TP-3 and TP-6, which transitioned to silty sand at depth. Overall soil relative density increased with depth, extending to the maximum exploration depth of about eight feet bgs. The native soil was generally observed in a weakly to moderately cemented condition at about four to five feet bgs. Geologic Setting The referenced geologic map resource identifies Vashon till (Qvt) across the site and surrounding areas. As reported on the geologic map resource, Vashon till typically consists of a nonsorted mixture of clay, silt, sand, pebbles, cobbles, and boulders. The till was deposited directly by ice advanced over previously deposited sediment and rocks. In addition, the referenced WSS resource identifies Alderwood-Urban land complex (Map Unit Symbol: 5) as the primary soil units underlying the subject site. The Alderwood series was formed in glacial till plains. Based on our field observations, native soils on the subject site are generally consistent with the geologic setting outlined in this section. Groundwater Groundwater seepage was not observed at the test pit locations during the fieldwork (November 2019). However, seepage should be expected in deeper excavations at this site, particularly during the winter, spring, and early summer months. Groundwater seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. Geologically Hazardous Areas Assessment As part of this geotechnical evaluation, the referenced chapter of the Edmonds City Code was reviewed. Based on our investigation and review, there are no geologically hazardous areas present on or adjacent to the site. Foundations The proposed single-family structures can be supported on conventional spread and continuous footings bearing on undisturbed, competent native soil, compacted native soil, or new structural fill. Competent native soils, suitable for support of the foundation, should be encountered beginning at depths of approximately two to three feet bgs. Where loose or unsuitable soil conditions are encountered at foundation subgrade elevations during site preparation activities, compaction of the soils to the specifications of structural fill, or overexcavation and replacement with granular structural fill will be necessary. Structural fill should consist of suitable granular soils compacted to 95 percent of Modified Proctor (ASTM D1557). Compaction of the soil to the levels necessary for use as structural fill will be difficult during wet weather conditions. Organic material exposed at foundation subgrade elevations must be removed and grades restored with structural fill. I:M-11 .Y.fl ROTI ILTITMIXy Select Homes, Inc. ES-6999 January 8, 2020 Page 4 Provided the structures will be supported as described above, the following parameters can be used for design of the new foundations: • Allowable soil bearing capacity 2,500 psf • Passive earth pressure • Coefficient of friction 300 pcf (equivalent fluid) The passive earth pressure and coefficient of friction values included a safety factor of 1.5. A one-third increase in the allowable soil bearing capacity can be assumed for short-term wind and seismic loading conditions. With structural loading as expected, total settlement in the range of one inch is anticipated, with differential settlement of about one-half inch. The majority of the settlements should occur during construction, as dead loads are applied. Seismic Considerations The 2015 IBC recognizes ASCE for seismic site class definitions. In accordance with Table 20.3- 1 of ASCE, Minimum Design Loads for Buildings and Other Structures, Site Class D, should be used for design. In our opinion, the site susceptibility to liquefaction is low. The native soil relative density and the absence of an established, shallow groundwater table are the primary bases for this opinion. Drainage Temporary measures to control surface water runoff during construction would likely involve passive elements such as interceptor trenches and sumps. ESNW should be consulted during preliminary grading activities to evaluate seepage areas and provide recommendations to reduce the potential for seepage -related instability. Finish grades should be designed to direct surface water away from structures and slopes. Grades adjacent to buildings should be sloped away at a gradient of either at least 2 percent for a horizontal distance of up to 10 feet or the maximum allowed by adjacent structures. In our opinion, foundation drains should be installed along building perimeter footings. Infiltration Evaluation As indicated in the Subsurface Conditions section, native soils encountered during our fieldwork were characterized primarily as silty sand with gravel and silty gravel with sand glacial till deposits. Based on the results of USDA textural analyses, the native soils were classified primarily as sandy loam with fines contents of about 22 to 31 percent. I:M-11 .Y.fl ROTINUTITMIXy Select Homes, Inc. ES-6999 January 8, 2020 Page 5 In -situ testing was completed in accordance with the Small-scale Pilot Infiltration Test (PIT) procedure, as outlined in Volume III, Chapter 3, Page 526 of the 2014 SMMWW. The testing was completed at a depth of approximately four feet at TP-2. The in -situ rate obtained during testing was 0.5 inches per hour. Based on subsurface conditions observed, the measured infiltration was likely predominantly a result of lateral migration rather than vertical infiltration. Considering the variable and generally very low permeability of the native soils, we do not recommend utilizing infiltration systems for the proposed project. It is our opinion the site is better suited for perforated stub -outs. On -site Stormwater Management Pursuant to City of Edmonds stormwater management requirements, implementation of on -site stormwater BMPs are required for proposed developments in accordance with specified thresholds, standards, and lists. The intent of BMP implementation is to infiltrate, disperse, and retain stormwater runoff on site to the extent feasible. The table below summarizes our evaluation of low impact development methods, as outlined in the referenced stormwater manual, from a geotechnical standpoint. It is instructed in the referenced stormwater manual that BMPs are to be considered in the order listed (from top to bottom) for each surface type, and the first BMP that is determined to be viable should be used. For completeness, however, we have evaluated each listed BMP for the proposed surface types. BMP Viable? Limitations or Infeasibility Criteria Lawns and Landscaped Areas T5.13: Post -construction soil quality and depth (Volume V, Yes None. Chapter 5) Roofs T5.30: Full dispersion (Volume Maybe* T5.30: Adequate vegetative flow paths will likely not be V, Chapter 5) available. T5.10A: Downspout full T5.10A: Not recommended due to soil variability and infiltration systems (Volume III, No very low permeability layers. Chapter 3) Bioretention (Volume V, Chapter No Not recommended due to soil variability and very low 7) permeability layers. T5.10B: Downspout dispersion systems (Volume III, Chapter 3) Maybe* Adequate vegetative flow paths are likely not available. T5.10C: Perforated stub -out connections (Volume III, Chapter Yes None. 3) I:M-11 .Y.fl ROTIMO19TAIX01 Select Homes, Inc. January 8, 2020 ES-6999 Page 6 BMP Viable? Limitations or Infeasihilitv Criteria Other Hard Surfaces T5.30: Full dispersion (Volume V, Chapter 5) Maybe` Adequate vegetative flow paths are likely not available. T5.15: Permeable pavement No Not recommended due to soil variability and very low (Volume V, Chapter 5) permeability layers. Bioretention (Volume V, Chapter No Not recommended due to soil variability and very low 7) permeability layers. T5.12: Sheet flow dispersion T5.11: Concentrated flow dispersion (Volume V, Chapter Maybe* Adequate vegetative flow paths are likely not available. 5 * Viability to be determined by storm designer. Limitations The recommendations and conclusions provided in this geotechnical evaluation report are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. A warranty is not expressed or implied. Variations in the soil and groundwater conditions observed at the test pit locations may exist and may not become evident until construction. ESNW should reevaluate the conclusions in this geotechnical evaluation report if variations are encountered. Earth Solutions NW, LLC Select Homes, Inc. January 8, 2020 ES-6999 Page 7 Should you require additional information, or have questions, please call. Sincerely, EARTH SOLUTIONS NW, LLC Adam Z. Shier, L.G. Senior Staff Geologist Attachments: Plate 1 — Vicinity Map Plate 2 — Test Pit Location Plan Test Pit Logs Grain Size Distribution Henry T. Wright, P.E. Senior Project Manager CC' RAM Engineering, Inc. Attention: Mr. Rob Long, P.E. (Email only) Earth Solutions NW, LLC °Z v < I �1 ~� ' rD I �� ac, v 202nd Street Southwest C I S �'� c �1 o < 203rd Street Southwest! r� ` I z �� %� w S Lynnwood-'-4D n (� 1 f � F Municipal I I I I Golf Course _ o J Ca _Str_ee_5_o_uthwest Street 0tCollege - - Place Middle U School ID ^ Maln_Street -� rD J-. N r- rII II V �Q� �pA {ostPark`.,_:�; a� ��P, IJ �� c — b o. S� g3, (D m rD YLI 'El v o2 °� SI Edmonds/ � �� ">' Woodwy Bo Q' po a i wd�in 7 igh School. 00 u u ; ❑Poo c Do 0 P P4o� F� oopaoppo❑ D0 /0\\00 O00 th Street Southwest A la3_L LL��JJ 216th Street Southwest Ism -—216thStreet Southwest 216 < ❑- U WA99 th Street Southwest (D Edmonds�CD E: Estr 218eet.southwest �i rip o I0th-Street ------- Southwest------- -ICE��ILLI''; 220th_StreetSouthwest -- o._ oICA � I v yam., < D m�222nd Street Southwest i ❑ rD rD� o — e m t L 0 I �+ o StrIIlleet Southwest _ (I�/ 224th_S_treet Southwest W N� � ' W — a Esperance � '® a _ ID �I 226th Street Southwest ---� �F�?6thPlaceSouthw U�i------- N �� Ilf•-,- J _-_ : 228th Street--S.outl wf es17 t --_- E 228thStreet_SouthweSt Reference: NORTH Snohomish County, Washington OpenStreetMap.org Y Eartharth Solutions Solutio NWLLC GeotechniceiEngineering, ' s ' bservation/Testing aid Environmental Services Vicinity Map 21222 — 82nd Place W. Edmonds, Washington NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information Drwn. MRS Date 12/04/2019 Proj. No. 6999 resulting from black & white reproductions of this plate. Checked AZS Date Nov.2019 Plate 1 WI ---------- I � I L� I I 0 I 00 I ------------ I I LEGEND TP-1 Approximate Location of ■ — ESNW Test Pit, Proj. No. — I ES-6999, Nov. 2019 Subject Site Existing Building NORTH 0 30 60 120 1 "=60' Scale in Feet NOTE: The graphics shown on this plate are not intended for design purposes or precise scale measurements, but only to illustrate the approximate test locations relative to the approximate locations of existing and / or proposed site features. The information illustrated is largely based on data provided by the client at the time of our study. ESNW cannot be responsible for subsequent design changes or interpretation of the data by others. NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. Drwn. By MRS Checked By AZS Date 12/04/2019 Proj. No. 6999 Plate 2 Earth Solutions NWLLC SOIL CLASSIFICATION CHART MAJOR DIVISIONS SYMBOLS TYPICAL DESCRIPTIONS GRAPH LETTER GRAVEL AND CLEAN GRAVELS ' �' ' `� • * .0 ■� GW WELL -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES ■C o d o paQ Q Q np GP POORLY-GRADED GRAVELS, GRAVEL- SAND MIXTURES, LITTLE OR NO FINES GRAVELLY SOILS (LITTLE OR NO FINES) COARSE GRAINED SOILS MORE THAN 50% OF COARSE GRAVELS WITH FINES ' Q ° �° ° D a GM SILTY GRAVELS, GRAVEL -SAND - SILT MIXTURES FRACTION GC CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES RETAINED ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) SAND AND CLEAN SANDS SW WELL -GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES MORE THAN 50% OF MATERIAL IS SP POORLY -GRADED SANDS, GRAVELLY SAND, LITTLE OR NO FINES LARGER THAN NO. 200 SIEVE SIZE SANDY SOILS (LITTLE OR NO FINES) SANDS WITH FINES SM SILTY SANDS, SAND - SILT MIXTURES MORE THAN 50% OF COARSE FRACTION S`+ CLAYEY SANDS, SAND - CLAY MIXTURES PASSING ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) INORGANIC SILTS AND VERY FINE ML SANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEY SILTS WITH SLIGHT PLASTICITY FINE GRAINED SOILS SILTS LIQUID LIMIT AND LESS THAN 50 CLAYS CL INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS _- OL ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY MORE THAN 50% OF MATERIAL IS MH INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND OR SMALLER THAN NO.200 SIEVE SILTY SOILS SIZE SILTS LIQUID LIMIT AND CLAYS GREATER THAN 50 CH INORGANIC CLAYS OF HIGH PLASTICITY OH ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY. ORGANIC SILTS HIGHLY ORGANIC SOILS T' ' ' ' ' ' PT PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS DUAL SYMBOLS are used to indicate borderline soil classifications. The discussion in the text of this report is necessary for a proper understanding of the nature of the material presented in the attached logs. Earth Solutions NW TEST PIT NUMBER TP-1 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 6": grass AFTER EXCAVATION - L = H L 2 w Lu g TESTS Q O MATERIAL DESCRIPTION ❑ a- Z W � 0 Q CO 0 TPSL 0.5 Dark brown TOPSOIL, roots (Fill) Brown silty SAND, loose to medium dense, moist (Fill) = MC 17.40% 12.0 SM -large cobbles Gray silty SAND with gravel, medium dense to dense, moist (Unweathered Till) MC = 4.30% -weakly cemented SM 5 MC = 8.50% 7.0 Test pit terminated at 7.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 7.0 feet. Earth Solutions NW TEST PIT NUMBER TP-2 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 6": grass AFTER EXCAVATION - L = H L 2 w Lu g TESTS Q O MATERIAL DESCRIPTION ❑ a- Z � W 0 Q CO 0 T P S L �.. 0.5 Dark brown TOPSOIL Brown silty SAND, loose to medium dense, moist MC = 9.60% SM -roots . 2.0 Gray silty SAND with gravel, medium dense to dense, moist (Unweathered Till) MC = 10.60% Fines = 22.00% -infiltration test at 4', weakly cemented 5 MC = 15.20% SM :: [USDA Classification: gravelly coarse sandy LOAM] 7.5 Test pit terminated at 7.5 feet below existing grade due to refusal on very dense till. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 7.5 feet. Earth Solutions NW TEST PIT NUMBER TP-3 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 6": grass AFTER EXCAVATION - L = H L 2 w g TESTS 0- O MATERIAL DESCRIPTION ❑ 2 Z 0 0 Q CO 0 T P S L �.. 0.5 Dark brown TOPSOIL, roots Brown silty SAND, loose to medium dense, moist SM . 2.0 Gray silty SAND with gravel, medium dense, moist MC = 12.90% SM 5 5.0 Gray poorly graded SAND, medium dense, moist MC = 2.40% SP X7.0 Gray silty SAND, medium dense to dense, moist (Unweathered Till) SM MC = 4.60% 8.0 Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 8.0 feet. Earth Solutions NW TEST PIT NUMBER TP-4 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 6": grass AFTER EXCAVATION - L = H L 2 w Lu g TESTS Q O MATERIAL DESCRIPTION ❑ a- Z W � 0 Q CO 0 TPSL �.. ' 0.5 Dark brown TOPSOIL Brown silty GRAVEL with sand, loose to medium dense, moist MC = 12.50% Fines = 22.20% GM [USDA Classification: very gravelly sandy LOAM] 2.0 Gray silty SAND, medium dense to dense, moist (Unweathered Till) MC = 6.40% -moderately cemented 5 MC = 8.20% SM 7.5 Test pit terminated at 7.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 7.5 feet. Earth Solutions NW TEST PIT NUMBER TP-5 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 8": grass AFTER EXCAVATION - L = H L 2 w Lu g TESTS Q O MATERIAL DESCRIPTION ❑ a- Z W � 0 Q CO 0 TPSL Dark brown TOPSOIL, roots .." 6 0.Brown silty SAND, loose to medium dense, moist SM MC = 10.10% 2.0 Gray silty SAND with gravel, medium dense to dense, moist (Unweathered Till) MC = 8.30% SM -weakly cemented 5 MC = 9.70% 6.5 Test pit terminated at 6.5 feet below existing grade due to refusal on very dense till. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 6.5 feet. Earth Solutions NW TEST PIT NUMBER TP-6 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 PROJECT NAME 21222 — 82nd Place West DATE STARTED 11/19/19 COMPLETED 11/19/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 6": grass AFTER EXCAVATION - L = H L 2 w Lu g TESTS Q O MATERIAL DESCRIPTION ❑ a- Z W � 0 Q CO 0 T P S L �.. 0.5 Dark brown TOPSOIL Brown silty SAND, loose to medium dense, moist MC = 12.00% SM 2.5 Gray silty SAND with gravel, medium dense to dense, damp to moist SM MC = 3.20% 5 MC = 4.60% 5.0 Gray poorly graded SAND, medium dense to dense, moist SP X7.0 -weakly cemented MC = 4.10% [USDA Classification: slightly gravelly coarse SAND] Fines = 3.40% Gray silty SAND, dense, damp (Unweathered Till) SM •" = MC 2.40% 8.0 [USDA Classification: very fine sandy LOAM] Fines = 31.10% Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. Bottom of test pit at 8.0 feet. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 4 2 5-44 9-4 704 Fax: 425-449-4711 PROJECT NUMBER ES-6999 U.S. SIEVE OPENING IN INCHES W m W Z LL z W U w a m�N GRAIN SIZE DISTRIBUTION COBBLES GRAVEL coarse fine SANDI SILT OR CLAY coarse I medium fine Specimen Identification Classification Cc Cu ® TP-02 4.00ft. USDA: Gray Gravelly Coarse Sandy Loam. USCS: SM with Gravel. ® TP-04 1.00ft. USDA: Brown Very Gravelly Sandy Loam. USCS: GM with Sand. ® TP-06 7.00ft. USDA: Gray Slightly Gravelly Coarse Sand. USCS: SP. 1.03 3.79 * TP-06 8.00ft. USDA: Gray Very Fine Sandy Loam. USCS: SM. Specimen Identification D100 D60 D30 D10 LL PL PI %Silt %Clay i TP-02 4.Oft. 37.5 1.139 0.16 22.0 m TP-04 1.0ft. 37.5 4.747 0.19 22.2 • TP-06 7.Oft. 9.5 0.584 0.304 0.154 3.4 * TP-06 8.Oft. 1.18 0.127 31.1 April 29, 2020 ES-6999.02 Select Homes, Inc. 16531 — 1311 Avenue West, A107 Lynnwood, Washington 98037 Attention: Ms. Kayla Nichols Subject: Response to Stormwater Review Comments Proposed Single -Family Residences 21328 — 82nd Place West Edmonds, Washington Reference: City of Edmonds Engineering Division Stormwater Review Comments BLD2020-0045, dated February 28, 2020 BLD2020-0044, dated March 3, 2020 Earth Solutions NW, LLC (ESNW) Geotechnical Evaluation ES-6999, dated January 8, 2020 Dear Ms. Nichols: Earth Solutions N W «C Earth Solutions NW LLC Geotechnical Engineering, Construction ❑bservationjesting and Environmental Services As requested, Earth Solutions NW, LLC (ESNW) has prepared this letter responding to the referenced City of Edmonds Engineering Division stormwater review comments. The comment pertinent to the geotechnical aspects of the project is provided below, followed by our response. ESNW prepared the referenced geotechnical evaluation for the subject project. Review Comment 1. — It is not entirely clear why bioretention is infeasible; slower infiltration soils alone may not be enough to rule out bioretention without additional safety concerns or hardpan/water table separation concerns. Update report as needed to include bioretention, or provide additional explanation as to why it is infeasible on this site, consistent with Edmonds Addendum Appendix A. a. Reviewer notes that the site generally slopes toward the road frontage, with the lowest elevations being at the controlled ditch line which is part of the City MS4; it appears safety concern for perched groundwater is minimal and more mitigation can be achieved with bioretention. 15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 0 (425) 449-4704 • FAX (425) 449-4711 Select Homes, Inc. ES-6999.02 April 29, 2020 Page 2 ESNW Response — The low measured infiltration rate of 0.5 inches per hour (iph) was indicative of lateral infiltration and not vertical infiltration as the test was completed at the contact where the soil becomes cemented (hardpan). The test results indicate the upper approximately four feet of soil exhibits low permeability. Hardpan (weakly to moderately cemented, unweathered glacial till) was generally encountered at about four feet; hardpan was encountered a couple of feet deeper in two of the locations. The long-term performance of infiltration in the site soils would generally be very poor and would provide minor to negligible stormwater mitigation from an infiltration standpoint, particularly during the wet season. While the site conditions may not indicate a distinct infeasibility criterion as outlined in Edmonds Addendum Appendix A, it is our opinion that infiltration at the subject site is not ideal from a geotechnical standpoint and design of a facility with such a low infiltration rate (reduced from 0.5 iph) may prove impractical or infeasible. From a geotechnical standpoint, very shallow perforated stub -outs would be an efficient means to provide minor mitigation of runoff for the upper low permeability soil. Should you require additional information, or have questions, please call. Sincerely, EARTH SOLUTIONS NW, LLC 04/29/2020 Henry T. Wright, P.E. Senior Project Manager cc: RAM Engineering, Inc. Attention: Mr. Rob Long, P.E. (Email only) Earth Solutions NK LLC E. CONSTRUCTION SWPPP REQUIREMENTS: The project will require grading to construct the proposed building and utilities (including the stormwater infiltration trench system). 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, 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. 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. Element 3: 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. 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. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W Stormwater Site Plan Report Page E-1 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. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd PI W 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 owner is responsible for maintenance of storm drainage facilities within the property (private property owner system); the owner is 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.html MAINTENANCF 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. RAMEngineering, Inc. RAM No. 19-009 Contact the City En,ineering Division for maintenance information. The City may make periodic inspections of BMPs to ensure they are operating properly. ECDC Section 18.30.1W contains the enforcement provisions the City can use to ensure the system is properly maintained. Select Homes: 21310 82nd P/ W Stormwater Site Plan Report Page F-1 Appendix V-A: BMP Maintenance Tables Ecology intends the facility -specific maintenance standards contained in this section to be conditions for determining if maintenance actions are required as identified through inspection. Recognizing that Permittees have limited maintenance funds and time, Ecology does not require that a Permittee perform all these maintenance activities on all their stormwater BMPs. We leave the determination of importance of each maintenance activity and its priority within the stormwater program to the Permittee. We do expect, however, that sufficient maintenance will occur to ensure that the BMPs continue to operate as designed to protect ground and surface waters. Ecology doesn't intend that these measures identify the facility's required condition at all times between inspections. In other words, exceedance of these conditions at any time between inspections and/or maintenance does not automatically constitute a violation of these standards. However, based upon inspection observations, the Permittee shall adjust inspection and maintenance schedules to minimize the length of time that a facility is in a condition that requires a maintenance action. Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) Maintenance Results Expected Component Defect Conditions When Maintenance is Needed When Maintenance is Performed Plugged Air Vents One-half of the cross section of a vent is blocked at Vents open and any point or the vent is damaged. functioning. Accumulated sediment depth exceeds 10% of the diameter of the storage area for 1/2 length of storage vault or any point depth exceeds 15% of All sediment and debris Debris and Sediment diameter. removed from storage Storage Area (Example: 72-inch storage tank would require area. cleaning when sediment reaches depth of 7 inches for more than 1/2 length of tank.) Any openings or voids allowing material to be Joints Between transported into facility. All joint between Tank/Pipe Section (Will require engineering analysis to determine tank/pipe sections are sealed. structural stability). Tank Pipe Bent Out of Any part of tank/pipe is bent out of shape more than Tank/pipe repaired or Shape 10% of its design shape. (Review required by replaced to design. engineer to determine structural stability). RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd P/ W Stormwater Site Plan Report Page F-2 Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) Maintenance Results Expected Component Defect Conditions When Maintenance is Needed When Maintenance is Performed Cracks wider than 1/2-inch and any evidence of soil Vault replaced or particles entering the structure through the cracks, repaired to design Vault Structure Includes or maintenance/inspection personnel determines specifications and is Cracks in Wall, Bottom, that the vault is not structurally sound. structurally sound. Damage to Frame and/or Top Slab Cracks wider than 1/2-inch at the joint of any No cracks more than inlet/outlet pipe or any evidence of soil particles 1/4-inch wide at the joint entering the vault through the walls. of the inlet/outlet pipe. Cover Not in Place Cover is missing or only partially in place. Any open Manhole is closed. manhole requires maintenance. Mechanism cannot be opened by one maintenance Locking Mechanism Not person with proper tools. Bolts into frame have less Mechanism opens with Manhole Working than 1/2 inch of thread (may not apply to self-locking proper tools. lids). Cover Difficult to One maintenance person cannot remove lid after Cover can be removed Remove applying normal lifting pressure. Intent is to keep and reinstalled by one cover from sealing off access to maintenance. maintenance person. Ladder is unsafe due to missing rungs, Ladder meets design Ladder Rungs Unsafe misalignment, not securely attached to structure standards. Allows maintenance person wall, rust, or cracks. safe access. See Table V-A.5: See Table V-A.5: Maintenance Standards -Catch See Table V-A.5: Catch Basins Maintenance Standards Basins Maintenance Standards - Catch Basins - Catch Basins Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor Maintenance Defect Condition When Maintenance Results Expected When Component is Needed Maintenance is Performed Trash and Debris (Includes Material exceeds 25% of sump Control structure orifice is not Sediment) depth or 1 foot below orifice blocked. All trash and debris plate. removed. Structure is not securely attached to manhole wall. Structure securely attached to wall and outlet pipe. General Structure is not in upright position (allow up to 10% from Structure in correct position. plumb). Structural Damage Connections to outlet pipe are Connections to outlet pipe are water tight; structure repaired or not watertight and show signs of replaced and works as designed. rust. Structure has no holes other than Any holes - other than designed designed holes. holes - in the structure. Cleanout gate is not watertight Gate is watertight and works as or is missing. designed. Gate cannot be moved up and Gate moves up and down easily Cleanout Gate Damaged or Missing down by one maintenance and is watertight. person. Chain/rod leading to gate is Chain is in place and works as missing or damaged. designed. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd P/ W Stormwater Site Plan Report Page F-3 Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor Maintenance Defect Condition When Maintenance Results Expected When Component is Needed Maintenance is Performed Gate is rusted over 50% of its Gate is repaired or replaced to surface area. meet design standards. Control device is not working Plate is in place and works as Orifice Plate Damaged or Missing properly due to missing, out of designed. place, or bent orifice plate. Obstructions Any trash, debris, sediment, or Plate is free of all obstructions and vegetation blocking the plate. works as designed. Any trash or debris blocking (or Pipe is free of all obstructions and Overflow Pipe Obstructions having the potential of blocking) works as designed. the overflow pipe. See Table V-A.3: Maintenance See Table V-A.3: Maintenance See Table V-A.3: Maintenance Manhole Standards - Closed Detention Standards - Closed Detention Standards - Closed Detention Systems (Tanks/Vaults) Systems (Tanks/Vaults) Systems (Tanks/Vaults) Catch Basin See Table V-A.5: Maintenance See Table V-A.5: Maintenance See Table V-A.5: Maintenance Standards - Catch Basins Standards - Catch Basins Standards - Catch Basins Table V-A.S: Maintenance Standards - Catch Basins Maintenance Results Expected When Component Defect Conditions When Maintenance is Needed Maintenance is performed Trash or debris which is located immediately in front of the catch basin opening or is blocking inletting capacity of the basin by more than 10%. No Trash or debris located immediately in Trash or debris (in the basin) that exceeds 60 percent of front of catch basin or on the sump depth as measured from the bottom of basin grate opening. to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of six inches clearance No trash or debris in the Trash & Debris from the debris surface to the invert of the lowest pipe. catch basin. Trash or debris in any inlet or outlet pipe blocking more Inlet and outlet pipes free than 1/3 of its height. of trash or debris. Dead animals or vegetation that could generate odors No dead animals or that could cause complaints or dangerous gases (e.g., vegetation present within General methane). the catch basin. Sediment (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the basin, but in no No sediment in the catch Sediment case less than a minimum of 6 inches clearance from basin the sediment surface to the invert of the lowest pipe. Top slab is free of holes Structure Damage Top slab has holes larger than 2 square inches or and cracks. to Frame and/or cracks wider than 1/4 inch. (Intent is to make sure no Frame is sitting flush on Top Slab material is running into basin). the riser rings or top slab and firmly attached. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd P/ W Stormwater Site Plan Report Page F-4 Table V-A.S: Maintenance Standards - Catch Basins Maintenance Results Expected When Component Defect Conditions When Maintenance is Needed Maintenance is performed Frame not sitting flush on top slab, i.e., separation of more than 3/4 inch of the frame from the top slab. Frame not securely attached Maintenance person judges that structure is unsound. Basin replaced or Fractures or Grout fillet has separated or cracked wider than 1/2 inch repaired to design standards. Cracks in Basin and longer than 1 foot at the joint of any inlet/outlet pipe Walls/ Bottom or any evidence of soil particles entering catch basin Pipe is regrouted and through cracks. secure at basin wall. Settlement/ If failure of basin has created a safety, function, or Basin replaced or repaired to design Misalignment design problem. standards. Vegetation growing across and blocking more than 10% No vegetation blocking of the basin opening. opening to basin. Vegetation Vegetation growing in inlet/outlet pipe joints that is more No vegetation or root than six inches tall and less than six inches apart. growth present. Contamination and See Table V-A.1: Maintenance Standards - Detention No pollution present. Ponds Pollution Cover is missing or only partially in place. Any open Cover/grate is in place, Cover Not in Place catch basin requires maintenance. meets design standards, and is secured Locking Mechanism cannot be opened by one maintenance Mechanism opens with Catch Basin Mechanism Not person with proper tools. Bolts into frame have less than proper tools. Cover Working 1/2 inch of thread. One maintenance person cannot remove lid after Cover Difficult to applying normal lifting pressure. Cover can be removed by Remove (Intent is keep cover from sealing off access to one maintenance person. maintenance.) Ladder is unsafe due to missing rungs, not securely Ladder meets design Ladder Ladder Rungs attached to basin wall, misalignment, rust, cracks, or standards and allows Unsafe sharp edges. maintenance person safe access. Grate opening Grate with opening wider than 7/8 inch. Grate opening meets Unsafe design standards. Trash and Debris Trash and debris that is blocking more than 20% of Grate free of trash and Metal Grates (If grate surface inletting capacity. debris. Applicable) Grate is in place, meets Damaged or Grate missing or broken member(s) of the grate. the design standards, and Missing. is installed and aligned with the flow path. RAMEngineering, Inc. RAM No. 19-009 Select Homes: 21310 82nd P/ W Stormwater Site Plan Report Page F-5 ENGINEERING, INC. Civil Engineering /Land Planning