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STORMWATER SITE PLAN <br />EVERETT COMMUNITY COLLEGE HOUSING - BROADWAY <br />MR 1: PREPARATION OF STORMWATER SITE PLANS <br />SEPTEMBER 11, 2015 <br />PAGE 2 <br />DRAINAGE PLAN DESCRIPTION <br />Direct precipitation is the source of runoff. The building takes up most of the lot area with a <br />central concrete courtyard and walks and vegetated surfaces along the boundaries of the <br />property. Vegetated surfaces will continue to sheetflow to the east towards Broadway Avenue <br />along the existing grade. Nine parking stalls will be constructed along the alley to the west. The <br />Roof and pavement runoff will be collected and discharged to the proposed storm detention <br />system along the north side of the building then to the storm in Broadway. <br />WATER QUALITY MEASURES <br />Following is a list of the proposed construction water quality BMPs. See MR 3: Water Pollution <br />Source Control for more information. The proposed BMPs are as follows: <br />BMP C103, High Visibility Fence: <br />BMP C107, Construction Road/Parking Area Stabilization: <br />BMP C120, Temporary and Permanent Seeding: <br />BMP C121, Mulching: <br />BMP C123, Plastic Covering <br />BMP C125, Topsoiling/Composting: <br />BMP C140, Dust Control: <br />BMP C220, Storm Inlet Protection: <br />BMP C233, Silt Fence <br />DETENTION SIZING <br />The site does not meet the thresholds for runoff detention as the increase in runoff rate is less <br />than 0.1 cfs. In addition, the site is part of the combined sewer basin in Everett and according <br />to discussions with City personnel, is not in an area of CSO's or basement flooding. However, <br />the City had made a request that there be some level of stormwater management on-site to <br />provide as much reduction of flow into the combined sewer as the site allows. To that end, a <br />40 foot long detention pipe will be installed to detain runoff and release it at rates below the <br />current conditions. <br />CONVEYANCE CALCULATIONS <br />The downstream pipe is 10" concrete. With installation of the proposed detention pipe there is <br />20% decrease in runoff from the site. The minimal onsite storm piping will be 6" and 8" in <br />diameter and is adequate by inspection. <br />STORMWATER TREATMENT BMP'S <br />The site does not meet the thresholds for runoff treatment. <br />PROTECTION OF WETLANDS <br />There are no wetlands on or near the site. <br />_ __ __-- --- _.----- ---------- -- _ . — - _ _ <br />--- <br />I: A��n{. �'i� j���l. �fll'� I� 17f1 I,�v ISii.iL�aj ( � I1O(�S hi�i Iti�.,�io„i� I)i.iiii,i�,i, �� �-•��_il��.� <br />