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6. Sediment Ponds <br />~ 6.1 Hydrology <br />Regulations impose several constraints on the hydrologic design of sedimentation ponds. <br />First of all the pond must be capable of storing the l0yr-24hr event volume. In addition to <br />this volume it must also be constructed and maintained to account for watershed sediment <br />yield. In time, sediment inflow to the pond will decrease the effective storage capacity. <br />Estimation of sediment yield is discussed in section 6.2. <br />The next hydrologic requirement stipulates that the pond must be capable of either <br />storing or safely passing the 25yr-24hr event immediately following the l0yr-24hr event. <br />One foot of freeboard from the maximum water surface elevation to the top of the pond <br />embankment must be maintained at all times. These criteria are satisfied through the <br />combined effect of storage volume, a low level outlet and a primary spillway. The two outlet <br />works must be separate structures. The final hydrologic regulation specifies that the entire <br />l0yr-24hr volume, stored in the pond, must drain through the low level outlet in 48 hrs. <br />Flowrates and volumes were computed using HEC-1, as previously discussed. They <br />were routed through the ponds using the Modified Puls method assuming level pool <br />conditions. The modeling scenario consisted of the pond, initially empty, with subsequent <br />inflow of the l0yr and 25yr events, back to back. <br />~ West Pond: The volume of the l0yr-24hr event for the West pond is 0.98 AF and the <br />annual sediment load is estimated at 0.05 AF, resulting in a total required pond volume of <br />1.03 AF. The volume in the West Pond at the time of the as-built surveys was 1.86 AF <br />below the spillway crest and 0.87 AF between the spillway crest and the low level outlet. <br />During the modelled IOyr event the initial water surface elevation in the pond is equal to the <br />elevation of the low level outlet, 7497.5 ft. In practice the initial water surface elevation will <br />most likely be below this level. As runoff enters the pond most of the water is stored but a <br />portion is dischazged through the low level outlet. The outlet consists of a 3" orifice plate <br />connected to a culvert with a dischazge rate that varies with the elevation of the water surface <br />in the pond. The peak discharge through the outlet during the l0yr event is 0.37 cfs. The <br />peaks stage in the pond is 7499.32 ft which is below the crest elevation of 7500.0 ft. As <br />seen, the combination of storage between the low level outlet and the spillway crest and the <br />volume of water discharged during the event adequately handle the l0yr-24hr volume. <br />In modelling the 25yr-24hr event the initial water surface elevation was assumed to be at <br />the spillway crest, 7500.0 ft. The inflow storm hydrograph was routed through the pond <br />resulting in a peak inflow of 2.5 cfs and a peak combined outflow over the spillway and <br />through the low flow outlet of 2.35 cfs. The slight attenuation of the peak is due to storage <br />in the pond above the crest elevation. The peak stage during the event was 7500.24 ft. The <br />spillway is modelled as a sharp crested weir with a length of 5 ft. <br />U <br />9 <br />June 1995 <br />