10011 3RD AVE SE BASE FILE 2016-01-01 MF Import

MEMORANDUM p0: GORDON WITCNEN FROM: JANE ZI6MERMAN DATE: APRIL 23, 1990 SUBJECP: SEPA N166-88 In the �bove SEPA, under item M4 in the section titled "Information for Developer", it is staTed that the on-site detention volume is required to include the storage lost by fill.ing of the isolated wetland. This point is not taken into account in the detention calculations. The intent of the statement is to address the function of the isolated wetland in reducing the peak discharge from the site during a given storm. A decreased discharge calculated for lhe pre-development design storm results in increased storage requirements. The technical release from the Engineering Division of the Soil Conservation Service entitlad "Urban Hydrology for Small Watersheds" provides a table of corre::tion factors given the ratio of the total site drainage area to the area of ponding or swampy area. For this site, t:hat correction factor is 0.69, meaning that the peak disch�irge allowed from the site should be 69� of that calculated without taking into considerat•ion the storage effects of the isolated wetland. However, the peak discharge calculated for this site during the 2 year, pre-development design storm was a'_ready less than the minimum discharge that the City of Ev^rett wi.11 allow. The calculated flow was 0.015 cfs ac opposed to the City minimum of 0.13 cfs. (The City minimum is calculated using a 2" borda orifice, the smallest orifice diameter considered safe for these applications) . Therefcre, a peak flow of C.13 cfe was used in the detention calculations. Given that the peak discharge calculated from the site was Less than the minimum allowed by the City, the application of the correction factor does not increase the amount of detention required. Therefore, the amount of detention calculated does meet the conditions of the SEPA.