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' �f�er t�e design stora, q=ound slope and qround .cover have <br /> . been dete�ine� Eor t,he site, the allowable out�laW rate in <br /> c:s per acre can �e dete�ined �y fnter�olation f:ou tables 2 <br /> throuqh 5. If a site contains t�o types aE ground cover, for <br /> examnle, then a se�arate inte rolation is made for each ground <br /> cover type and }he results added to dete�ine t�e total <br /> allowa�le outflaw rate in cfs per acre. . <br /> After the a1loWable out�low rate in c:s/acre is calcslated, <br /> the rea�uired deteation in cubic feet per ac:e is detei�ined by <br /> ir.terpolation Prom table 6 or 7. Again if t:�o ground cover <br /> types are pronosed a=:er develonment, a separate inte rolaticn <br /> is made for each c;ound cover twe and the result adde <br /> e � d �o I <br /> d te:-a_ae the total detentio <br /> ac�e. The dete. t' nr reeuire�ent in cubic Peet per <br /> Z ian requl_ement is t o <br /> hen det_raine3 bv <br /> multiplyir_g by the site area in ac=es. ' <br /> For residential :;ubdivi�ions the actual extent of around cover <br /> types will not be 7mown duriaq plan review. Based unon a � <br /> survey of existing hoaes in the Everett arca, the average I <br /> amount oE i�ne.rvious surpace per single Pamily lot is 2gpp <br /> sauara feet. T�e znnlicant should, t':ere:ore, multinly 2gpp <br /> by ��e number a= lots and then zdd the result to the area oi <br /> imnerrious sur:ac� contained within any new road cons sscted <br /> for the subd'ivisiorr to estimate t'�e area of imperrious surface <br /> that will u�,timately be contained withia t'�e p1at.. . . <br /> Exzmole• <br /> Given: A 3 . 0 ac�e parcel locate3 wit'�in Nor,.h C�-ee.k <br /> Drainage Basia. The slope of overland flow is 3.5�. The <br /> existing ground caver cansists of 2. 0 acres of forested <br /> land and 1. 0 acre of light grass znd s�arse tre�s. The <br /> pr000sed land use is commercial wit'i 5& landscaning and <br /> 95� imperviaus ,c-raund cover conditions. <br /> Solution: From tanle 1, t.he design stor� criteria is a 5 year <br /> pre—development stor.m and 25 year post develonment storn. . <br /> crca table 2 the allowable r=lease rate for the forested <br /> portion of the site is by inLe�olation . 035 cfs/ac. The <br /> allowable release rate for the grzss area is .23o cfs per <br /> acre. The total allowable release rate is therefore <br /> (2) ( . O85j + (1) ( .230) or 0.40 c:s. This is .133 c�s pe_ <br /> 'acre. From table •7, the detention requirement for the <br /> landscaoinq area and imoerviaus area is 885 c�/acre and <br /> 4�09 cP/acre, respectively. The t_otal detention <br /> requirement is ( .95} (3) (4009) + ( .0.5) (3) (8SS) 'or- 11,558 <br /> ct. <br /> � <br /> � <br />