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D. STORMWATER FLOW CONTROL <br />The site contains 0.39 Acres. The site is currently developed with a home and a gravel <br />driveway from Wetmore Ave. The remainder of the site exists as lawn and native <br />vegetation. The site contains one drainage basin that that slopes slightly to the east. Please <br />refer to the downstream analysis map for more details. Per NRCS survey of Snohomish <br />County, the project site contains Alderwood soils that have a hydrologic classification of <br />Type "C". Please refer to the soils map and descriptions attached later in this report for <br />more details. <br />The proposal is to subdivide the site into two lots and build a garage on Lot 1 and a <br />single-family residence on Lot 2 with associated utilities. The access to this site will be <br />from Wetmore Ave. The proposal also includes frontage improvements along Wetmore <br />Ave. <br />Per Figure I-3.1, (flow chart for new development requirements) Volume I <br />SWMMWW, Minimum requirements #1 through 9 shall apply for this project. See <br />Minimum Requirements Summary included later in this report. Flow control requirements <br />are exempt for this project because it will not convert 3/4 acres of native vegetation to lawn, <br />the new effective impervious area is only 3,823 SF, which is less than the threshold of <br />5,000 SF, and the 100-year flow frequency increase is 0.01 cfs, which is less than the <br />threshold of 0.15 cfs. Water quality requirements are exempt for this project because the <br />total PGHS is only 2,175 SF, which is less than the threshold of 5,000 SF. <br />Per Minimum Requirement #5 (Section I-3.4.5 of the SWMMWW), the following <br />NPGIS BMPs shall be applied to provide onsite stormwater management and must be <br />considered in the following order per List #2: Full Dispersion, Infiltration, Bioretention, <br />Basic Dispersion, and then Perforated Stub -Out Connections. Full Dispersion is infeasible <br />because the required native vegetation preservation could not be achieved for the LDMR <br />zoned project. Full Infiltration is also not feasible as the design infiltration rate is only 0.21 <br />in/hr which is less than the recommended 0.3 in/hr. Also, Glacial Till was observed at 1.5- <br />Insight Engineering Co. — Stormwater Site Plan 05/31/2022 <br />-27- <br />