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Western 9740 Evergreen Way Everett, Washington 98204 <br />Engineers (425) 356-2700 FAX (425) 356-2708 <br />Inc. <br />SURVEYORS • PLANNERS • ENGINEERS <br />be provided in the report assuming the information provided in the report is sufficient to <br />show the jurisdiction the general site design is in compliance with the drainage manual. <br />Please keep this in mind for the future so that reports don't get saturated with redundant <br />and non -helpful information making reports harder to read and understand. <br />• Exhibits - Vicinity Map updated with downstream flow path and a note of no know <br />significant geologic or sensitive areas located between site and Silver Lake. Additionally, <br />downstream analysis description updated with notation of no known geological features <br />or sensitive areas downstream of the site prior to reaching Silver Lake. References to <br />applicable basin maps, grading and SWPPP plan along with drainage plan added to <br />upstream and downstream analysis sections for depiction of existing and developed site <br />hydrology in regards to section 3.1.7 of the drainage manual. <br />• French Drain has been removed. Upstream runoff traveling into the site is now designed <br />to sheet flow into new underground storm system and conveyed to the new underground <br />detention facility. Detention facility designed to account for upstream runoff entering <br />into the site. <br />• MR#6 - Large Linear Sand Filter is acceptable for Enhanced Water Quality Treatment per <br />verification with WADOE and City. <br />• MR#7 — Bypass map along with additional descriptions of bypass area added to MR#&. <br />Detention sizing has been revised to include existing upstream runoff flows into the site. <br />Note that resizing of the 6' diameter Detention Pipe with the existing upstream flows to <br />the east of the site results in lower orifice sizing able to meet the 0.5" minimum sizing <br />with the 6' diameter pipe. <br />• Wetland Protection section updated with no known wetlands in general area or between <br />site and Silver Lake. <br />• Separate O &M Manual has been updated and provided with the resubmittal package. <br />b. Design Plans: <br />• Sheet 5 <br />o French Drain has been removed from site design. All upstream runoff now <br />designed to sheet flow into new parking area drainage system and detention <br />system. <br />• Sheet 6 <br />o Large Linear Sand Filter approved by City for enhanced treatment use. <br />• Underdrain specified at 0.50% slope. <br />• Gravel drain rock depths increased to 9" total depth to contain <br />under drain within rock layer while also providing a 0.50% slope. <br />• Energy dissipation to be provided by sediment chamber. Note that <br />due to sand filter being located downstream of detention, 100 year <br />flow rates into the chamber are only expected to be about 0.05 cfs. <br />• Outlet baffle wall specified at a height of 3" above inlet baffle wall. <br />Note that calculations show that the max height of water in the inlet <br />chamber are expected to only be about 1.5" to 2" above inlet baffle <br />wall for the 100 year detained storm event. Additionally, an <br />overflow outlet from the system has been added at a height 6" <br />above inlet baffle wall and 12" above top of sand layer resulting in <br />a max ponding depth above sand layer of 12". <br />LAND USE CONSULTANTS * CIVIL ENGINEERS * LAND SURVEYORS * ENGINEERING CORPORATION <br />"'j <br />RAIDSERVER\2015\15-1415-A SILVER PLAZA\ENGINEERING\PUBLIC WORKS REVIEW RESPONSE LETTER <br />