1991-05-16_REVISION - C1981035 (2)

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MLRB regulations require that the sum of the embankment inslope and outslope be no greater that 5: 1 at any location. In addition a minimum topwidth of 8 ft is required for the East Pond. To comply with these standards the East Pond is to be constructed with a 3 : 1 outslope and a 2: 1 inslope around the entire perimeter of the pond. Using these values and an outslope height of 4 ft from the top to the bottom of the embankment, and an inslope height of 12.5 ft, an enlarged volume of 2. 3 AF was computed. Volumetric calculations were initially made longhand using prismatic volume formulas and then verified in the HEC-1 model. See appendix 5. A volume of 0. 1 AF is needed annually for sediment storage. Even though sediment inflow does not distribute itself evenly throughout a reservoir, for hydrologic modeling purposes it is assumed that this is the case. The effective bottom used in the HEC-1 model is thus 87. 17 ft instead of the 86. 5 value. Outlet Works Outlet works for the East Pond include a low level orifice and a secondary emergency spillway. The spillway consist of a sharp- crested, rectangular weir. The discharge capacity of the weir is computed from the well known weir formula; Q = CLH1•5, where Q is the discharge in cubic feet per second (cfs) , C is the discharge coefficient, L = L' - 0. 1NH, L' is the length of the weir, N is the number of contractions, and H is the head measured above the crest. A typical value of C for a rectangular weir is 3.27 (Chow, 1959) . As previously stated, the crest elevation for the East Pond spillway is 97.7. The maximum water surface elevation during the 25yr event is 98 and the top of the embankment is 99. See figure 1. The effective length L is 7.94 ft and is computed with L' = 8, N = 2 and H = 0. 3 ft. Low-Level Outlet Type: Circular Orifice Co = 0.7 Area = 0.055 ft2 Diameter = 3 .2 in Invert Elev. = 87.2 ft Max. Head = 10.8 ft The low level outlet is a circular orifice sized to drain a full pond in 48 hours. The discharge equation is Q = C,Al2—g. Ho•5, 5