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� <br />May 29, 1980 <br />File No. 80-075 <br />Reid, Middleton & <br />-�, <br />, <br />� '. ,' j . ��I � � <br />�..:s,+..:s..: -::.i+isl�i `vua`:':�l <br />Associo�es, Inc. <br />Engineers • Surveyors • Plonners <br />STORM WATER DETENTION SYSTEM CALCUTATIONS <br />FOR <br />PROPOSED 9-UNIT APARTMENT <br />by RAH Enterprises <br />Exts[Ing Sfte Conditions <br />Area � 0.32 ac., exclud(ng 10' dedica[ion on 75th St. <br />Slope - 4$ northeasterly toward 75th St. <br />51[e condi[ion - Old residen[ial lo[ with single-famfly residence, detached <br />garage. <br />Existfng runoff coefftctent = 0.35 <br />Tc � L/(V)60 + 10 min. = lA,'/(�.4)60 + �p = �Z,14 min. <br />Intens(ty = i= �,(,R tn/hr �� 10 year s[orm frequency (Everetl) <br />0 existin9 = cIA =(0.35)(i.6A)(0.32) = 0.19 c.f.s. <br />Q allowable = 0.19 c.f.s. <br />Usfng the Yrjanalnen-Warren method (or siriple re[entlon/detention system desi9n <br />the (oll�ing calculations are to Jetermine the reaufred volume nl detenfton for <br />tlifs development. <br />Puture Slte Conditfon <br />Prorated fut�re runoff coefficfent <br />Buflding, concrete 6 parkfng area <br />Planters L landscaped areas <br />(0.23)(0.9)+(0.09)(0.2) = 0.70 <br />0.32 — <br />_ �.?3 ac. � 0.90 <br />= 0.09 ac. (� 0.20 <br />= fut C <br />Co _ Q allowable s 0,19 <br />Area futC 0.32 0.70 -�•g5 c•f.s./Ac <br />Assume orff(ce cond(tions exist (25 year design storin) <br />T = -25 + 2106 � 2706 <br />Qo -25 + �--g; = 31.42 m(n. <br />Vs = 43?9 - 40QoT = (4329)31.47 - ��o(o.DS) 31.4z = 2��Io.Ro - IOG(i.2A = I?G2.� <br />r � 25 3i.�z + 25 �2ri3/ncc <br />V tocai = Vs(Ac)fut [= 1342.52 (0.32) 0.10 =?On.72 ft.3 <br />Sizfng [he Oriflce <br />Q allowable = 0.19 c.f.s. <br />U[flizing the orifice equa[ion 0. = cclA �qh <br />llssume h = 2' <br />121 5ih Avenue Nonh, Suice 20Q Edmonds, Woshington 9t)020 (206) 775-�.194 <br />