2007-08-17_PERMIT FILE - C1981035A (14)

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• • • weirs. SUMMARY OF POND HYDROLOGIC CHARACTERISTICS ITEM WEST POND EAST POND SPILLWAY CREST ELEV. 7500 ft. 7497 ft. LOW LEVEL OUTLET ELEV. 7494.5 ft. 7492.0 ft. 10YR-24HR PEAK STAGE 7499.97 ft. 7496.91 ft. 10YR-24HR PEAK INFLOW 3.5 cfs 0.6 cfs 25YR-24HR PEAK OUTFLOW 4.8 cfs 5.0 cfs 10YR-24HR VOLUME 1.96 AF 0.19 AF LOW LEVEL ORIFICE DIAMETER 3.1 inches 2.0 inches SPILLWAY LENGTH 5 feet 5 feet SEDIMENT VOLUME 0.1 AF 0.01 AF 6.2 Watershed Sediment Yield Sizing criteria for the sedimentation ponds must include the effect of the annual sediment yield from the watershed as well as the volume of the 10yr-24hr event. Numerous empirical equations have been developed for estimating annua! sediment yield. Probably the most popular are the variations of the Universal Soil Loss Equation (LISLE). This method was originally developed for agricultural lands and is dependent on at feast 6 parameters which describe the physical, land use, and management characteristics of the basin. For watersheds wi#h no observed data on these parameters, such as those at the National King Coal Mine site, the use of this type of equation requires guesstimating most of the parameters. The errors in this could be excessive. Furthermore, most of the sediment yield in the National King Coal Mine watersheds is most likely derived from channel erosion which is not considered in the LISLE formulation. As an alternative to empirical estimates of sediment yield, this study uses measured values from around the country to bracket the most likely value for this site. This technique will give at least an order of magnitude estimate of the true sediment yield to be expected. The design sedimentation volume for the ponds will be equal to the estimated .annual yield. Sediment levels in the ponds will be monitored and adjustments to the sediment storage will be made after 1 year i# necessary. 12 January 1998