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l I <br />.. <br />Prope:t hydraulic design of the outlet is critical to achieving good pc:rfonnaoce of the dctentioo basin. The two most <br />ccmmoo outlet ~!ems th.a! occur are: <br />l) Toe capacity of the outlet is too g:rea.t resulting in partial filling of tbe basin and less than designed for drawdown <br />time. <br />2) Toe outlet clogs because it is oot adequately jXOteeted against trash and debris. <br />To avoid 1:1:J.esc proble:m.s two altema.ti ve outlet tyix:s are rcx:ommc:nded for use: 1) Y -notdl wier, and 2) perl orated riser. <br />Toe V-DO(Cb wia-will not clog, but it is also difficult to maintain srruill rnough release rares at low be.ads. Toe perforated <br />riser, if pruperl y de&ignw and gra vcl pack.ed., gives m ucb better control and is recommended ovc:r tbe Y -DO{Cb wia-. <br />Two different approaches can be used to cootml the outflow. Ooe is to use a singk orifice outleq:ro{ccted by a perfo- <br />rated riser pipe. Toe ofuer is to use the perforated riscr itself for discharge cootroL Both approaches are pRSCnted below. <br />Bow Control Usine a SineJc Qdfia:: <br />Toe outlet cootrol orifice s.hould be sized using the following equation (GKY, 1989). <br />where: a = area of prifice (fi2) <br />A = avenge surface area of tbe ·pond (ft2) <br />c = orifice coe'rficien t <br />T = drawdown time of full pond (hrs.) <br />g = gravity <br />H = elevation when tbe pond is full (ft) <br />Ho ;= fin.al ekvatioo wben pond is empty (ft) <br />With a clra wdown time of 4D hours the eq ua tioo becomes: <br />a = 160A/c(H -Ho)0.5 <br />(1) <br />(2) <br />Assuming an average release rate at ooc half tbe pood dcptb, a common approa,ch in several design manuals, leads ID <br />cooside:rabie error. If the pood has a significant variation of surface area with deptb, do not use Equatioo (2); consult <br />GKY (1989). <br />Care. must be taken in tbc selectioo of ~ c": 0 .60 is roost often recom.rncnded and ~-Howeve.r, based oo actual t.es ts <br />GKY (1989) recommeods the following: <br />c = 0.66 for thin materials, tha.t is, I.be thickness is c.qual to cr .less th.an orifice d.iarneter <br />c = 0.80 when tbe material is thicker than tbe orifice diameter <br />Drilling the orifice in to an ouUd structure tha.t is made of coocrete can res ult in considerable impact oo tbe coefficient, as <br />does tbe beveling of the edge. The expaiments by GKY ( 1989) were with sharp edged orifices. <br />E{Juatioo (1) defines the orifice area where a single orifice outlet is used to regulate the detention basin outflow. How- <br />ever, a recent survey of extended detentioo facilities (Galli,~-comm.) found the drawdowo time of small storms that <br />do 00{ fill the facility to be too short IO ~vide effective trcatmeoL The fucilities surveyed were designed for a draw- <br />down time of 24 boors. A 4D hour drawdowo may provide su fficieot time for the small er stams. But it may be prudco t <br />TC5 <br />Municipal Handbook 5 -39 1 v1m2 . D R A F T