2002-06-26 CC Packet - Laserfiche WebLink

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etention_Svstems - werponcts RlfiI.'litlAlJSflts Sediment Storage Design Sediment volume should be at least the MPCA permit requirement of 250 cubic feet (ft3) per acre of impervious surface in the tributary watershed. Alternatively, the wet pond can be built with capacity for about 25 years of storage. A detailed analysis of pond sediment storage volume may be helpful to determine cost-effective sediment control plans. Methods such as the NRCS use equations that address many of the sediment storage factors, but they should be evaluated by professionals on a site-specific basis. The basic equation and design considerations are; Vol ~ AxY x DR x TE x E I (2l7,800xG) where: Vol = design sediment storage capacity, E == average rate of erosion in the watershed in tons/acre/year, A = area of the watershed in acres, DR = sediment delivery ratio in percent, G = estimated sediment density in the basin in pounds per cubic foot, TE = trap efficiency, in percent, and Y = design storage period in years. It is important to remember that the Walker design (using a 25", 24-hour water quality event) already incorporates approximately 25 years of sediment storage in its design assumptions. Pond Inlet/Outlet Structures and Pipes: The outlet area should be a deeper micropool to provide final settling and prevent resuspension of sediments. The outlet device should be carefully designed, since it is important to the operation of the entire pond system. Inflow points should be designed with energy dissipaters to reduce inflow velocity. Several options that are available for pond outlets are discussed in the "Alternative Outlets" fact sheet. Two different outlet designs- one without and one with a riser, are shown in Figures 2 and 3, respectively. In most cases, emergency spillways should be included in the pond design. Emergency spillways should be sized to safely pass flows that exceed the design storm flows. The spillway prevents pond water levels from overtopping the embankment, which could cause structural damage to the embankment. The emergency spillway should be located so that downstream buildings and structures will not be negatively impacted by spillway discharges. The pond design should include an emergency drain to assist in pond maintenance. The drain pipe should be designed for gravity discharge and should be equipped with an adjustable gate valve. Embankments and spillways should be designed in conformance with the state Dam Safety regulations and criteria. The design of the wet pond embankment is another key factor to be considered. Proper design and construction of the embankments will prolong the integrity of the pond structure. Subsidence and settling will likely occur after an embankment is constructed. Therefore during construction, the embankment should be overfilled by at least 5 3-260 Minnesota Urban Small Sites BMP Manual