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WI <br /> I <br /> I <br /> 4.0 OFF-SITE ANALYSIS REPORT <br /> I This section evaluates the upstream and downstream drainage system. The intent of this section <br /> is to identify any existing or potential drainage impacts created or exacerbated by the proposed <br /> project and drainage facilities. <br /> I <br /> 4.1 Task 1: Study Area Definition <br /> I The offsite analysis study area was identified from a USGS contour map and a site visit on June <br /> 1, 2009. The weather was dry and approximately 70 degrees. <br /> 4.2 Task 2: Review of Available Information <br /> The following resources were utilized in preparation of this section: <br /> I USGS Topographic Map <br /> Site and downstream visit on June 1, 2009 <br /> Snohomish County SnoScape Landscape Imaging Interactive Map <br /> I Washington Department of Fish and Wildlife SalmonScape <br /> National Resources Conservation Service Web Soil Survey 2.2 <br /> Critical Area Study(Wetland Resources, Inc.) <br /> I Geotechnical Report(GeoEngineers) <br /> Record As-Builts <br /> I 4.3 Task 3: Field Inspection <br /> Based on USGS and Snohomish County topographic information it appears that Hwy 526 serves <br /> as a north/south basin boundary. Drainage north of Hwy 526 flows north and drainage south of <br /> I <br /> Hwy 526 flows south. A small amount of drainage may flow north via an existing 24" culvert <br /> crossing under Hwy 526 which outfalls on the south property line. Flow then enters a 24" <br /> underground conveyance system located on-site east of the proposed improvements. As-built <br /> Irecords show this drainage system increasing in size to 36" diameter further north as additional <br /> site drainage enters the system. This conveyance system outfalls into an existing detention pond <br /> I located in the north east corner of the site. <br /> Flow exits the site via an existing control structure crossing under 75th Street SW. A large <br /> wetland north of 75th Street SW forms the headwaters to Narbeck Creek which flows unimpeded <br /> Ibeyond %a mile from the north site boundary. <br /> No evidence of existing flooding or erosion problems were identified by a site visit or by <br /> I <br /> research of the above mentioned resources. The proposed development will have no backwater <br /> effects on surrounding properties. <br /> I <br /> IDAVID EVANS AND ASSOCIATES,INC. <br /> OPERATIONS CENTER ADDITION JUNE,2010 <br /> ' STORMWATER SITE PLAN PAGE 5 <br />