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•eIi• e a a .:. -------- =771 . Proper hydraulic design of the outlet is critical to achieving good performance of the detention basin. The two most common outlet problems that occur are: 1) The capacity of the outlet is too gnat resulting in partial filling of the basin and less than designed for drawdown time. 2) The outlet clogs because it is not adequately protected against trash and debris. To avoid these problems two alternative outlet types are rccommeaded for use: 1) V -notch wier, and 2) perforated riser. The V-notcb Wier will not clog, but it is also difficult to maintain small enough release rates at low beads. The perforated riser, if properly designed and gravel packed, gives much better control and is recommended over the V -notch Wier. Two different approaches can be used to control the outflow. One is to use a single orifice outlet protected by a perfo- rated riser pipe. The other is to use the perforated riser itself for discharge control. Both approaches art prtsented below. Flow ( Coil UVOP a SingleQd5 The outlet control orifice should be sized using the following equation (GKY, 1989). a = A/cT(H - H,)OS(2g)0-5 (1) where: a = area of ,Orifice (ft2) A = average surface arta of the pond (ft2) c = orifice coefficient T = drawdown time of full pond (hrs.) 8 = gravity H = elevation wben the pond is full (ft) Ho = final elevation when pond is empty (ft) With a drawdown time of 40 boors the equation becomes: a = 160A/" - HO)0-5 (2) Assuming an average release rate at one half the pond depth, a common approach in several design manuals, leads to considerable error. If the pond has a significant variation of surface area witb depth, do not use Equation (2); consult GKY (1989). Care must be taken in the selection of "c": 0.60 is most often recommended and used. However, based on acetal tests GKY (1989) recommends the following: c = 0.66 for thin materials, that is, the thickness is equal to or less than orifice diameter c = 0.80 wben the material is thicker than the orifice diameter Drilling the odfkc into an outlet structurc that is made of concrete can result in considerable impact on the coefficient, as does the beveling of the edge. The experiments by GKY (1989) were with sharp edged oditcts• Equation (1) defines the orifice area where a single orifice outlet is used to regulate the detention basin outflow. How Equation a rtcent survey of extended daention facilities (Galli, pens. comm-) found the drawdown time of small storms that do not fill the facility to be too short a provide effective trcatutent. The facilities surveyed were designed for a draw- down time of 24 hours. A 40 hour drawdown may provide sufficient time for the smaller storms. But it may be prudent TC5 Municipal Handbook 5-39 11/13/92 - A