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. ; ' � . j <br /> . <br /> ��, �/1����tQ�t Q �O.QOC�Q�Cd. -/iLC. ENGINEERS • SURVEYORS • PLANNERS <br /> 32/ M�In Slreel • EAmonds, Weshlnplon 4A010 • Trlrphone (?Ofi) 775J<31 <br /> RECEIVED <br /> CITY OF EV[R[TT Ma rch 9, I 979 <br /> llmc Wor��o.ot F 1 1 e No. 5657A <br /> STORM WATER DETENTION SYSTEM CALCULATIONS <br /> 11AR �1 9 � Foa <br /> PROPOSED APARTMENT BLDG. COMPLEX <br /> on Ptn. Tract 19. Beverly Berry Tracts Div. No. 2 <br /> Everett, Washington <br /> Extsting Site Informatfon <br /> Area � 1 .50 ac. <br /> [xistfng tcpography - 3•5 [0 2$ northeasterly to SR 52F <br /> Exist(ng s(te �ondf[lon - Pasture and lawn area <br /> Runoff coefftcient = 0.25 <br /> 7c = 435/�1 .25) I60) + 10 = 15.8 min, <br /> In[ensity � i a 1 .46 in/hr @ 10 year storm frequency (Everett curve - Chart 18) <br /> Q = cIA = (0.25) (1 .46) (1 .50) = o.y5 c.t.s. <br /> Q allowable � 0.55 �• F•s. <br /> Future Si [e Conditions <br /> Using the Yrjanainen-Warren me[hod for simple retention/det�ntion basin design , <br /> the following calcu�ations a re to determine the rcquired volume of de[en[ion <br /> (or this development: <br /> Proratfng fu[ure runoff coefficient - C <br /> 6uilding area = 0. 43 ac. P 0.90 <br /> llsphalt area = 0.66 ac. P O.dS <br /> Planter 6 lawn arca = 0.41 ac. � 0.2 <br /> � � (0.41 ) (0.2)+(0.66) (0.85)+(0.43) (0.9) _ o.G9 <br /> I .5 <br /> �o s Q allowablc a .� 0.5� = 0.53 c. f.s./AcC <br /> utc c <br /> T � -25 + 2255/Qo = - 25 + 22y5/o.53 = 40.23 n�in. <br /> vs � 36077 _ 40 QoT = (3607) (40.23� _ 40 (0.53) (40.23) = 2224.5 - 852.5 = <br /> T+15 40.23 + ZS <br /> i37i .7 fc.3/nc(Fucc) <br /> vca�ai = vs x nc x v�i c = (i37i .7) ( � •5) (0.69) = i4i9.7 ri.3 <br /> To[al stora9e required = 1419.7 ft .3 <br /> 1.111111C11'AI INf,IIIIII+IN�I • A':/� Illi '.III'PIA • Al'Alll IP.I �I1,1[ NI • '�Uf ', IA ' n CAI��'� II:AI I �I[.IIlII1: IN�� <br /> nlf!PORT PL�NNING fr DCi�(�N • Id/�Ii111A5 • POHI lr INUU; tkIAL UfV11 �71'Mf.NT • fI�.A5111111i1 S111U1[S <br />