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� <br />May 29, 1980 <br />File No. 80-075 <br />• <br />Reid, Middleton � Associotes, Inc. <br />Engineers • Surveyors • Planners <br />STORM WATER DETENTION SYSTEM LALLULATIONS <br />Id�7: <br />PROPOSED 9-UNIT APARTMENT <br />by RhM Enterprfses <br />Existing Stte Conditlons <br />Area = 0.32 ac., ercludfng 10' dedica[ton on 75th St. . <br />Slope - 4� northeasterly Coward 75th St. <br />Site conditton - Old resfdenttal lot with single-famtly residence, detached <br />garage. <br />Exlsting runoff coefficient = 0.35 <br />Tc = L/(V)60 + 10 min. = 180'/(1.4)60 + 10 = 12.14 min. <br />Intensi[y = i= 1.6A fn/hr � 10 year storm frequency (Everet[) <br />Q existtna = ciA = (0.35)�1.69)(0.3z� = 0.19 c.f.s. <br />Q allowable = 0.19 c.f.s. <br />Us�ng [he Yrjanatnen-Warren method for sinple retentlon/detentlon system deslgn <br />the follawing calculattons are to determine the reauired volume of detentlon for <br />this development. <br />Future Site Conditfon <br />Proreted future runoff cae(f(clent <br />Buildfng, conr_rete 6 parking arca <br />Plan;::rs G landscaped areas <br />(0.23)(0.9)+(0.09)(0.2) = 0.70 <br />0.32 — <br />= 0.23 ac. f� 0.90 <br />= 0.09 ac. P 0.20 <br />= fut C <br />Q� _ Q allowable � 0.19 � 0.85 c f.s./Ac <br />Area futC O.j2 0.70 <br />Assume orifice conditlons exist (10 year design storm) <br />T = -25 + ZQoS = -25 + �p�� = 26.51 min. <br />vs = 3607T _ 402oT =�3607)26.51 _ 4o(O.IIS)Z6.5I = 1856.37 - 3��•34 = 955.03 ft.3/Act <br />7+25 2 .51 + 25 <br />V total = Vs(Ac)fut C= 955.03 (0.32) 0.70 = 213.93 �t,3 <br />Sizing thc Jrifice <br />Q allowable = 0.1� c.f.s. <br />Utflfzing [he orifice equation 0. = cdA �gh <br />�ssume h = 2' <br />121 S�h Avenue North. Sulte 200. Edmonds. Woshington 06020 (?06) 775 J4�4 <br />