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SAGE HOMES NORTHWEST, LLC <br />HARBORS EDGE DRAINAGE REPORT <br />AUGUST 2022 <br />• Step 5 — Determine if enhanced treatment is required: <br />Enhanced treatment is not required as this is a single residential project. <br />• Step 6 — Select a basic treatment facility: <br />Basic treatment will be provided, by Modular Wetland Units. <br />Subject to the evaluation above, the project will implement two Modular Wetland Units from the <br />basic treatment menu. These units will be upstream of detention. The goal of basic water quality <br />treatment is primarily to remove 80% of total suspended solids. Modular Wetlands have GULD rating <br />from DOE. There is E square feet of pollution generating impervious surface that can not be <br />treated due to the elevation. This area will be traded with 3,096 square feet of existing pavement <br />that will be collected and treated. Calculations are included in the hydrologic modeling calculations <br />provided in Appendix D. The Modular Wetland Unit details are also included in Appendix D. <br />4.3 LOW IMPACT DEVELOPMENT AND ONSITE STORMWATER <br />MANAGEMENT <br />The proposed project occupies less than five acres and is located within the urban growth area. The <br />project is also required to satisfy all nine minimum requirements outlined in the SWMMWW. <br />Therefore, the project is required to either meet the Low Impact Development Performance Standard <br />or evaluate the low -impact stormwater BMPs contained within List #2, as described in Volume I, <br />Section 2.5.5 of the Snohomish County Drainage Manual. <br />This project is electing to evaluate the low -impact BMP options contained within List #2. The required <br />evaluation is provided below: <br />Lawn and Landscape Areas <br />Post -Construction Soil <br />Feasible —The project intends to strip the duff layer, stockpile it, <br />Quality and Depth <br />and re -use it on the completed site. <br />(BMP T5.13) <br />Roofs <br />Full Dispersion <br />Not Feasible — Most of the project was previously cleared of <br />(BMP T5.30) <br />native vegetation, and there are no suitable dispersal flowpaths <br />meeting the applicable requirements. <br />Bioretention <br />Not Feasible —The underlying soil conditions are not conducive to <br />(Vol. T7.30) <br />infiltration. A bioretention facility is considered infeasible if the <br />geotechnical evaluation recommends infiltration not be used due <br />to reasonable concerns of slope failure. <br />Downspout Full <br />Not Feasible —The underlying soil conditions are not conducive to <br />Infiltration (BMP T5.10A) <br />infiltration. . <br />PACE / PAGE 16 <br />