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I <br /> I <br /> � Section One <br /> ( <br /> Project Overview Seaway Lot 3B is a 6.16 acre parcel, of���hich 1.71 acres will be <br /> � developed as commercial in the Seaway Center development. The <br /> proposed site will consist of one building and associated parking. <br /> Sanitary sewer and water mains will connect to the esisting sewer <br /> Iand water system located in 23`d Dri��e West. <br /> 7he site sits on a ridge line, spPtting drainage beriveen the Po�vder <br /> I Mill Gulch Creek and Narbeck Creek. For the Powder Mill Gulch <br /> Geek drainage (Basin A), �n existing regional storni wa?er <br /> detention facility is located w�est of the property. Storm �vater <br /> I deten'ion volume for the developed site is provided in die existing <br /> faciliry. A stormfilter manhole is to be installed at the northwest <br /> corner of the site to treat storm runoff for up to the 6 month event <br /> from the paved areas. The water quality facility has capacity (1.8 <br /> cfs) to pass the peak 100-year flow of 0.24 cfs without a flow <br /> splittcr. Thc proposed building roof drains ���i�l be tightlined in a <br /> � seplrate conveyance pipe and connect downstream of the water <br /> quality facility. Therefore, the stonnfilter design is based on <br /> � treating only the ���ater quality stonn from the onsite paved areas <br /> west of the ridge line. <br /> The drainage to the east of the ridgeline drains to the Narbeck <br /> Creek basin (Basin B). This area is cullected and sent to a detention <br /> vault, which also acts as pre-settliug prior to releasing through a <br /> stormfilter manhole. The stormfilter manhole�vas sized by Contech <br /> Stonmvater Solutions, Inr„ per the City of Everett design criteria. <br /> � Oncc the drainage passes through the storn�filter, it is wnveyed <br /> offsite to an eaisting Narbeck Creek drainage pipe. <br /> i P�@I11711113f�/ COI1dItIOf1S 'Che pre-developed condition ofthc 1.71-acre site is second growth <br /> � Summary forest with dense underbrush. The westem 13? acres of the <br /> development(Basin A) that flows to the Po�vder Mill Gulch Creek <br /> I Dasin sheet tlows to 23`d Drive West, ��liich is then conveyed to <br /> the regional detention facility. The eastern 038 acres (Basin B) <br /> sl�eet Ilows to Narbeck c:eek directly. <br /> I 'f�ie underlying soils are classified as Vashon Till. For stormwater <br /> analysis ���e modeled [his as Aldenvood till or hydrologic soil <br /> group "C," which has a rioderately high runoff potentiaL These <br /> soils also have n moderate rate of water transmission or interflaw, <br /> tliat flo���s to�vards dic ���est, a�vay from the top of the eastern <br /> ravine slope. This inforniation was obtained from the Geologic <br /> Recomtaissance Reporl prepared for 7'OC by Terra Associates o�i <br /> Seple�nb•rr• ld, 2006. A copy of the report is attached in Appendix <br /> A. <br /> , $P[!1V[!}�Cen(er Lot 3/3—Storm Urniari,�e Reporr 17'lOC.t7o;,ogemenr5ervices'033/79Drsiqri'RrpomV007-Ol-09�emm�mpoe�REb75ED.dnc <br /> I li�c@NPacific, hre. November6, 2706/Page 1 <br />