2717 FEDERAL AVE 2018-01-02 MF Import

r0EodE[M STORMWA PER COLLECTION AND CONVEYANCE INTERIM REMEDIAL ACTION IMPLEMENTATION DECO 210 MOBIL ADGMH,LER PROPERTY 2101-27!7FEDERAL AVENUE CITY OF FVERETT EVEREn. , WASHINGTON permit Services Under an Agreed Order with the Washington Department of Ecology (copy attached), Mobil Business Resources Company (Mobil) and American Distributing Company (ADC) have initiated implementation of the Interim Remedial Action at the subject property. The Interim Remedial Action involves demolition of existing structures, installation of a product recovery trench along the western and northi., n boundaries of the site, and placing a low -permeability cap over the surface of the site. The attached design drawings show the existing grade (Figure 1) and the proposed final grade (Figure 2). The stormwater collection plan involves utilizing the two existing connections to the sanitary sewer that runs in Federal Avenue (see attached sewer map). The following information is provided to satisfy the substantive requirements for stormwater permitting in the City of Everett. The estimation of peak discharge was performed using the rational method as provided in Section 1-5 of the City of Everett Public Works Department Stormwater Management Manual, dated January 1997. The following assumptions were made: • Stormwater will be routed to the City of Everett sanitary sewer running along Federal Avenue. • The two existing connections will be utilized. Catch basins, gas traps anti any additional piping will be installed according to City of Everett requirements. • The existing connections are 4-inch cast iron pipe. To determine the peak runoff, a 25-year design storm was used. The corresponding intensity was conservatively taken as 3 inches per hour (the maximum value on the City of Everett Stormwater Manual, Fig::rr 1-5.1). The area encompassed by the proposed asphalt surface is slightly less than one acre in size (0.96 acres). The ; k discharge is calculated as follows: q = CIA where: q = peak discharge (cubic feet per second, cfs) C = runoff coefficient (City of Everett Stormwater Manual, Table 1-5.1) I = design rainfall intensity (in/hr, City of Everett Stormwater Manual, Figure 1-5.1) A = drainage area (acres) q = (0.9)'(3 in/hr)'(0.96 acres) q = 2.6 cfs This is the peak discharge for the entire site. Based on topographic survey information tied to City Datum, the planned grade for the catch basin will be approximately 12 to 13 feet. The invert grade for the sanitary sewer line is approximately 6.5 feet. This results in a slope of approximately 0.11 ft/ft. We assumed that this peak discharge would be divided equally between the two catch basins. Collected flow will be conveyed to the sanitary sewer by two 4-inch cast iron pipes, one from each catch basin. Even with the conservative 3 inchthour intensity assumption, the corresponding water velocity in each pipe is approximately 15 f /sec (extrapolating from City of Everett Stormwater Manual, Figure 1-5.13 or using Mannings Equation) . This is within the specified range of 2 to 30 ft/sec. Copyright 1998 Kleinfelder, Inc. 60-1914-01-008