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December 9, 1980 <br />File No. 80-166 <br />STORi4 WATER DETEP�TION SYS <br />Existin9 Site Conditions <br />Area = 24,847.5 ft.Z = 0.57 Ac. <br />Slope = 1.4% Northwest direction <br />• <br />Reid, Middleton fi Associates. Inc. <br />Engineers • Surveyors • Plonners <br />IONS fOR P <br />0 <br />Lot is covered with mostly pastures. naturally sloping in the south- <br />west direction and an existing ravine on the south-east portion of <br />lot drops about 25 feet from the remaining area at about 30%. <br />Existing runoff coefficient = 0.20 <br />Length = 150 ft. <br />Time of concentration = 150 + 10 = 12 min. <br />. 5 0 <br />Area is identified as Area A and required to design for design frequency of <br />one in ten years. <br />Intensity = 1.73 in/hr <br />Qallowable = CIA <br />(from 10 year Everett frequency curve) <br />= 0.2 X 1.73 X 0.57 = 0.20 c.f.s. <br />Future Site Conditions <br />Using Yrajanainen-Warren method for simple retention/detention basin design. <br />The required volume of detention for this developr�ent is calculated as follows: <br />Cfuture = 0.9 <br />Qo = Qallowable = 0.2 = 0.39 c.f.s./Ac. C <br />Ac X Cfuture o.57 X 0.9 <br />Assume ori°ice conditions exist <br />T = -25 r 2255 <br />no <br />121 Sth Avenue No«h, Sui[e 200, Edmor,d;. `:.'�shiny�on ^v8v2u i2uoi 77�•J4J4 <br />