Laserfiche WebLink
��ter t7e design stor�, gronr.d slope ard c:ound .cover have <br />�een dete�ined fa: the site, t::e allovable out_lov rate in <br />c:s pe: acre can be dete�ined �y inter�olati.on :=oa tables 2 <br />.:lrough 5. Z: a site contains t.o t•y^res of g_our.Z cover, .`or <br />exa�ple, then a separate interolation is �ade Eor each g.-ound <br />cover type and the results added to deter�ine the total <br />alloc�able out�loc+ Tate in c�s per acre. <br />After the alloc:able outPlov rate in c:s/acre is calcnlated, <br />the required detention in cubic feet per ac:e is deterained ry <br />inter�olation fron table 6 or 7. Again if tao around cover <br />types are pronosed aiter develooaent, a separate inte�olation <br />is aade for each c.-ound cave� type and the resalt added to <br />dete=-ai:�e the total detention recuirenent in csbic feet ner <br />acre. The detention reauire�ent is then deterained by <br />aultiplying by the site area in ac=es. <br />For residential suhdivisions ::�e ac�ual ext=nt of <br />types ctill not be ]cioF�n duriag plan reviea. <br />su=�ey of existinq ho�es in t`�e £�erett area, <br />aacunc of i�per�ious surface per sincle fanily <br />scuare feet. The apnlicant should, t`:ere£ore, <br />by t'�e nuabe� of lots and then zcd t4e result t <br />ianerrious sur�ace contained vi`�ia ar.y new roa <br />for the subaivisior. to est�at= the area of i�pe: <br />thzt vi?i uZt.iaa.ely be contained wit'�ia t'�e plz� <br />Examnle- <br />cround cove- <br />Based uzon a <br />�he average <br />lOt i5 2800 <br />aultiply 28ao <br />� t�e area of <br />d cons.rsc`ed <br />.-�ious su::zce <br />Given: A 3.0 ac=e parcel locate� wit`�in Nor`.� Cres�C <br />Drainage Bas:n. The slope of ove-land flow is 3.5k. The <br />, existing ground cover c�nsists o: 2•0 acr=s oE Poreste3 <br />I C�j land and 1.0 acre af li5nt grass and sparse tr==s. The <br />�i pr000sed land use is co�aercial vith 5� landscaping and <br />, , 95$ iapervious graund cove= condit�ons. <br />�' .� <br />1 --� <br />� � � <br />Solution: Froa table 1, the design stora c=iter�a is a 5 year <br />pre-developaeZt stora and 25 year pos: develootaent stor.s. <br />Fron table 2 the allocrable relezse rate for the foresLed <br />portion of the site is by inte=rolation .085 cfs/ac. The <br />allowable release rate for t:�e arzss ares is .23o c�s per <br />acre. The total allouable rel=ase rate is therePore <br />(2)(.085) +(1)(.230) or 0.40 c�s. T:�is is .1J3 c:s per <br />'ac�e. From table 7, the cetention reauireaent for the <br />landscaning area and imner�ious zrea is 885 c:/ac=e and <br />4009 cf/ac�e, respectively. The *otal detentian <br />reauirenent is (.95) (3) (4009) + (.05j (3) (585-) �cr 11,558 <br />cf. <br />n <br />