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1 <br /> 111 <br /> 3.0 BASIN ANALYSIS <br /> 3.1 Soil and Curve Number Information <br /> Soil information at the project site was obtained from the Web Soil Survey website <br /> (http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx) published by the United States <br /> Department of Agriculture Natural Resources Conservation Service (NRCS). The soil series <br /> underlying the tributary area is comprised of Alderwood soil series. The soil is classified as a' <br /> Hydrologic Group C. Appendix A of this report contains copies of the soils map and properties. <br /> A geotechnical investigation of the site was performed by GeoEngineers in October, 2009. The <br /> result of their investigation is provided in their report, Geotechnical Engineering Services, a <br /> copy of which is provided in Appendix A. <br /> 3.2 Predevelopment Condition <br /> Presently, the project development area is developed with an existing parking lot and <br /> landscaping consisting of lawn with some ornamental trees. The site is broken into two on-site <br /> drainage basins converging at the northeast corner of the 52 acre facility. A 24" storm drain <br /> system conveys flows from the wetpond on the south border of the site to the north. The 24" <br /> drainage system outfalls into a detention pond located on the northern portion of the PUD <br /> property. This pond then flows north via a culvert crossing under 75th Street SW into a large <br /> wetland area north of 75`h Street SW. According to Snohomish County Landscape Imaging this <br /> wetland forms the headwaters of Narbeck Creek a non-fish bearing Creek. Off-site flows are <br /> limited. State Route 526 and an off-ramp (Seaway Blvd) from SR526 border the development <br /> area to the south and west. The existing parking lot borders the project area to the north and a <br /> large lawn area borders the site to the east which slopes to the east. <br /> 3.3 Post-Development Condition <br /> The majority of the existing site development is hydraulically isolated from the proposed <br /> impervious surfaces. All runoff in the south basin is collected via an underground conveyance <br /> system and routed to a combined detention and water quality wetpond east of the south storage <br /> yard. The stormwater pond control structure outfalls into an existing 24" conveyance system <br /> east of the project site maintaining the existing site hydrology. <br /> All impervious surfacing within the north basin will be treated by rain gardens or Filterra <br /> according to the Low Impact Development Technical Guidance Manual for Puget Sound prior to <br /> connection into the existing conveyance system. <br /> More detailed information is included in Appendix B, "Hydrologic and Hydraulic Analysis", <br /> attached at the end of this report. <br /> DAVID EVANS AND ASSOCIATES,INC. <br /> OPERATIONS CENTER ADDITION JUNE,2010 <br /> STORMWATER SITE PLAN PAGE 4 <br />