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STORMWATER SITE PLAN FEBRUARY 26, 2018 <br />MUKILTEO SCHOOL DISTRICT- TRANSPORTATION PAGE 2 <br />STORMWATER TREATMENT BMP'S <br />With 4,400 sf of pollution generating surface, the project does not require stormwater <br />treatment. <br />PROTECTION OF WETLANDS <br />There are critical areas on -site. Buffers are proposed to be reduced to allow for the <br />construction of the added parking. Buffer review and mitigation is part of the project under the <br />direction of the site critical areas consultant. <br />OPERATIONS AND MAINTENANCE <br />Maintenance requirements match those currently occurring on -site. <br />EXISTING CONDITIONS SUMMARY <br />DESCRIPTION <br />The project area is to the south of the existing bus parking area. The bus parking area consists <br />of pavement with curb & gutter and fencing. The area for the new paving is currently brush <br />with some trees. Grades adjacent to development are steep man made slopes (2:1 max) <br />including a rockery wall at one point. Grades past the previous disturbance line are generally 5- <br />10 percent. See Figure 2: Existing Site Map. <br />SOILS DESCRIPTION <br />GeoTest Services, Inc has performed several test pits on the site. The underlying soils were <br />determined to be Topsoil over Weathered Till over Glacial Till. Groundwater was encountered <br />from 2.8 feet to 7 feet below grade. <br />UPSTREAM ANALYSIS <br />There are no upstream areas that impact this project. <br />DOWNSTREAM ANALYSIS <br />Runoff will enter the existing storm drainage system on -site. The new pavement is tributary to <br />two systems. To the east, the actual parking (3,380 sf) will flow to an existing catchbasin that <br />flows to the on -site detention pond where it will be released to the wetland system to the east. <br />The western portion of the new pavement (1,020 sf) will flow to an existing catchbasin and be <br />released to the wetlands to the south. <br />Once in the wetlands, the flow follows a complicated system of natural and man-made <br />drainages that ultimately discharge to the Swamp Creek system to the east. There are no <br />known limitations in the discharge system. Swamp Creek is not listed on the 303d list until past <br />Lake Stickney approximately 3 miles downstream of the site. <br />PAWork\Projects\2012\12-185 Mukilteo SMCMDOMTransportation Faciliq, Drainage 12-185.doc <br />