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Stormwater Report recd 4-10-18.pdfit7 23135 75TH A Ds, WA 98026 Technical Information Report 23135 — 751h Place West; Edmonds, WA 98026 I certify that this technical information report and all attachments were prepared either by me or my technical staff working directly under my supervision. REVISIONS March 27, 2018 Ori inal Submission TABLE OF CONTENTS MR 1 — PREPARATION OF STORMWATER SITE PLAN...........................................................................................2 PROJECTOVERVIEW ........................ ................... ......., .....,».»...,.., ,... ...._.......2 SITEINFORMATION ......................... ....... ............ ..... ...,....,. .. .................. ................. .........,2 SOILSINFORMATION ......... ......... ........... .. .... ......... ._ .....,,. ,........ ................... . .........3 EXISTING CONDITIONS ......... ......... . ,,..... ............ .........3 DEVELOPEDCONDITIONS...................................................................................................................................3 THRESHOLDDETERMINATION............................................................................................................................4 MR2 — CSWPPP...................................................................................................................................................5 CONSTRUCTION SEQUENCE AND PROCEDURE ......... .................. ......... .......... . : ...... ......... ......5 TRAPPING SEDIMENT ..................................... ___ .... ...,,,... .. ....... .......... .........5 MR3— SOURCE CONTROL OF POLLUTION........................................................................................................... 6 MR4 — PRESERVATION OF NATURAL DRAINAGE SYSTEMS & OUTFALLS..........................................................6 MR5 — ON -SITE STORM WATER MANAGEMENT..................................................................................................6 Appendices • Appendix A: Soil Data • Appendix B: Civil Plans Trang Short Plat -Technical Information Report March 2018 MR 1 - PREPARATION OF STORMWATER SITE PLAN Project Overview The property is located at 23135 — 751" Place West in the City of Edmonds, Washington. The site is currently developed with a SFR that will remain. The project proposes to short plat the property into a total of two (2) parcels and construct a single-family structure, driveway and associated utilities on the newly platted lot. This Technical Information Report and attached Stormwater Site Plan submittal is required for the approval of the short plat and building permit for the above referenced subject property. The current land -use designation of the property is single-family residential. The proposed land -use is single-family residential. The Stormwater Site Plan has been prepared in accordance the City of Edmonds 2017 Stormwater Addendum to Ecology's 2014 Stromwater Management Manual for Western Washington. Site Information Address: 23135 — 751" Place West; Edmonds, WA City, County, State: Edmonds, King County, Washington Governing Agency: City of Edmonds Figure 1 — Vicinity Map / Site Location Not -to -Scale The Concept Group Page 2 Trang Short Plat -Technical Information Report March 2018 Soils Information Soils Survey The Soils Conservation Service (SCS) mapped the soils information in the buildable project area as as Alderwood/Urban Land (5/6). Refer to Figure 2 below and Appendix A for the detailed soil map. Figure 2 Soils Map Not -to -Scale Existing Conditions The site is currently developed with a SFR. The general topography of the site slopes from west to east. Elevations on the site vary from a high point of 306 feet at the western o portion of the property to 296 feet at the eastern property line. Developed Conditions The project proposes to construct a single-family structure, driveway and associated utilities. Table 1 details the proposed impervious surface areas. Table 1 — Im ervious Surface (SF) Roof 3,000 Driveway680 Total _ 3,680 The Concept Group Page 3 Trang Short Plat -Technical Information Report March 2018 Threshold Determination The City of Edmonds 2017 Stormwater Addendum to Ecology's 2014 Stromwater Management Manual for Western Washington sets forth the drainage requirements for this project. Based on the flowchart of Figure 3.1, "Flow Chart for Determining Requirements for Development," Minimum Requirements (MR) 1 through 5 will apply to the new and replaced impervious surfaces and converted pervious surfaces. Next Question Does the project. add 5,000 square feet or more of new phr replaced Bard surfaces? OR Convert 4_7 5 acres or more of vegetation to lawn of landscaped areas? OR Convert 2.5 acres or more of natixv vegetation to pasture? No Yes Is this a road related project? .All Minimum Requirements apply to the new and replaced des hard surfaces and converted vegetation areas_ All blinimiun Requirements apply to the new hard swfaces and converted vegetation areas. Yes Does ttie pio)eci a d 5,0�00 square feet or No more of new hard uu°faces° Yes. Do new bard surfaces add 50°co- or nuue to the existing hard siufaces within the project limits? No Edmonds Stormwater Addendum, Figure 3.1 No additional requirements. The Concept Group Page 4 Trang Short Plat -Technical Information Report March 2018 MR 2 — CSWPPP A Construction Stormwater Pollution Prevention Plan (CSWPPP) has been prepared for this project under a separate cover. The project will comply with the thirteen (13) SWPPP elements during construction. All erosion and sediment control measures shall be governed by the requirements of the City of Edmonds. Construction Sequence and Procedure Prior to the start of any grading activity upon the site, all erosion control measures, including installation of a stabilized construction entrance, shall be installed in accordance with the construction documents. The best construction practice will be employed to properly clear and grade the site and to schedule construction activities. The planned construction sequence for the construction of the site is as follows: 1. Flag or fence clearing limits. 2. Install catch basin protection if required. 3. Grade and install construction entrance(s). 4. Install perimeter protection (silt fence, brush barrier, etc.). 5. Construct sediment ponds and traps. 6. Grade and stabilize construction roads. 7. Construct surface water controls (interceptor dikes, pipe slope drains, etc.) simultaneously with clearing and grading for project development. 8. Maintain erosion control measures in accordance with City of Edmonds standards and manufacturer's recommendations. 9. Relocate erosion control measures or install new measures so that as site conditions change the erosion and sediment control is always in accordance with the City of Edmonds Erosion and Sediment Control Standards. 10. Cover all areas that will be unworked for more than seven days during the dry season or two days during the wet season with straw, wood fiber mulch, compost, plastic sheeting or equivalent. 11.Stabilize all areas that reach final grade within seven days. 12.Seed or sod any areas to remain unworked for more than 30 days. 13. Upon completion of the project, all disturbed areas must be stabilized and BMPs removed if appropriate. Trapping Sediment Structural control measures will be used to reduce erosion and retain sediment on the construction site. The control measures will be selected to fit specific site and seasonal conditions. The following items will be used to control erosion and sedimentation processes: • Temporary gravel construction entrance • Filter fabric fences (Silt fences) • Ground cover measures such as straw cover and/or hydroseeding • Inlet protection The Concept Group Page 5 Trang Short Plat —Technical Information Report March 2018 Vehicle tracking of mud off -site shall be avoided. Installation of a gravel construction entrance will be installed at a location to enter the site. The entrances are a minimum requirement and may be supplemented if tracking of mud onto public streets becomes excessive. MR3 - SOURCE CONTROL OF POLLUTION BMPs for mobile fueling of vehicles and heavy equipment and for concrete handling will is included in the CSWPPP, prepared under a separate cover. MR4 - PRESERVATION OF NATURAL DRAINAGE SYSTEMS & OUTFALLS The property is located within the Hall Creek basin which drains to Lake Ballinger. The entire project site currently drains to this one basin. In the developed condition, stormwater will continue to drain to Lake Washington. MR5 - ON -SITE STORM WATER MANAGEMENT The Project utilizes Better Site Design by generally matching existing grades. Existing trees and other vegetation around the Site perimeter will be preserved where possible to help minimize disturbance to the hydrologic cycle. Native soils in all disturbed pervious areas will be amended with compost. Roof Surface 1. Full dispersion of runoff from impervious surfaces is not feasible due to the lack of native vegetation on the site. 2. Full infiltration of runoff from impervious surfaces is feasible becaused on -site infiltration test yielded measured infiltration rates of 20.0 inches per hour. This infiltration rate is equivalent to coarse sand designation, suitable for full -infiltration BMPs. No other BMPs are required to be evaluated. Infiltration Trench Design New Impervious Roof Required Trench Provided Trench 3,000SF 60LFx2'Wx2'D 30LFx2'Wx2'D Driveway Surface 1. Runoff from the driveway is mitiaged with permeable pavement surfaces to infiltrate stormwater runoff. Lawn and Landscape Area Soul Management Plan 1. Within the limits of site disturbance, duff and topsoil will be retained in an undisturbed state and stockpiled for later use to stabilize and amend soils throughout the Site. Postconstruction soil amendment will meet the requirements of BMP T5.13 Post -Construction Soil Quality and Depth. The Concept Group Page 6 Trang Short Plat -Technical Information Report Appendix A Soils Data The Concept Group Appendices Z. 4701 SW Admiral Way, Ste 353 o Seattle, WA 98116 . T 206.446.1291 o www.ConceptBusinessGroup.com Date: March 8, 2018 To: City of Edmonds Public Works Department From: Anna Nguyen, P.E. Subject: Drainage Calculations & Subsurface Report Project Address: 23135 75`h Place W -lamed bu: L�DWGWAA4134CS �J� /— & " Geologic Setting The Web Soil Survey, provided by the USDA Natural Resources Conservation Service was referenced for the geologic and soil conditions of the project site. According to this publication, the surficial soil unit at the project site and its immediate vicinity is mapped as Alderwood/Urban Land (5/6). Figure 1 — Geologic Map (Not -to -Scale) ANEW DIMENSION OF DESIGN PERCEPTION 23135 71h Place West Page 2 of 4 Small Scale Pilot Infiltration Test On March 7, 2018, we performed a field infiltration test. The test pit location is shown in Figure 2. The small-scale infiltration test was performed in accordance with the specifications provided in the 2014 DOE Stormwater Management Manual for Western Washington. The test consists of excavating a test pit to the design bottom of the proposed infiltration facility (5 feet deep) with a minimum of 12 square feet bottom area. The test pit is then pre-soaked with at least 12 inches of water above the bottom for a period of 6 hours. At the end of pre-soak period, the pit is filled with 12 inches of water and a constant head is maintained for the constant head testing period of one hour. The volume of water per time unit needed to maintain 12 inches of constant head is recorded. The field infiltration rate is then calculated based on the final measured volume per time unit and the surface area of the hole. In addition, at the end of the constant head test, we shut the water off and measured the falling head infiltration rate by recording the drop in water level between regular time intervals. An additional test pit was excavated to a depth of 7 feet below the existing grade to verify the any presence of groundwater and soil conditions. Based on the surface area of the test hole, and the measured volume of water per hour needed to maintain the constant head, we calculated a field infiltration rate of 20 inches per hour. Groundwater/ Hydraulic Restrictive Layer This test was conducted during the wet -season. The test pit was excavated to a total depth of 7 FT. The subsurface exploration did not encounter a restrictive/hardpan clay layer or groundwater. ,T Figure 2 - Soils Test Pit Location (Not -to -Scale) THE CONCEPT GROUP 23135 71h Place West Page 3 of 4 Figure 3 - Soils Log (Not -to -Scale) Correction Factor The correction factor was calculated based on Table B.1 of the 2017 City of Edmonds Stormwater Manual. The test was conducted during the wet -season in the vicinity of the proposed stormwater BMP; therefore, the minimum correction factor is recommended for this site. Design Infiltration Rate = Measured Infiltration Rate x CFv + CFT + CF. Where: CFv = 0.50 CFT=0.50 CFm = 0.90 Design Infiltration Rate = 20.0 x 0.50 x 0.50 x 0.90 = 4.5 inches/hour Table B.I. Correction Factors to be Used With In -Situ Saturated Hydraulic Conductivity Measurements to Estimate Desi n Rates. Issue Partial Correction Factor Site variability and number of locations tested (CF.) CFv = 0.33 to 1.0 Test Method (CFi) CFT • Large-scale PIT • CFr = 0.75 for Large-scale PIT • Small-scale PIT • CFT = 0.5 for Small-scale PIT • other small-scale (e.g., Double ring, falling head) • CFT = 0,4 for other small-scale test • Grain Size Method • CFT = 0.4 for Grain Slze Method Deme of influent control to grevent siltation and bo-bulldu Um = 0.9 Figure 4 — Table B.1 (2017 Edmonds Stormwater Addendum) THE CONCEPT GROUP 23135 7`h Place West Page 4 of 4 Recommendations Based on our infiltration testing on the site, it is our opinion infiltration of the development storm water on the site is feasible. Limitations The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test pits are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking soil samples in test pits. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. This report has been prepared for the exclusive use of property owners and its representatives for specific application to this project and site. Our recommendations and conclusions are based on observed site materials and engineering analyses. Our conclusions and recommendations are professional opinions derived in accordance with current standards of practice within the scope -of our services and within budget and time constraints. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct• the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in stormwater design. 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