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• Sedimentology The SEDCAD+ computer program was utilized to calculate the sediment <br />flowing info and through the principal spillway using [he DEPOSITS portion of the <br />program. Sediment flow through the pond is within the limits of the NPDES permit. <br />To determine the sediment loading to the pond, the Average Annual "R" Method was utilized <br />to quantify [he sediment storage. The amount of sediment storage Ss determined for a <br />defined length of time and is based on the design storm R, design storm sediment load, <br />average Annual F, and the number of years of sediment storage desi ced. R is the rainfall <br />erosivity factor. The equation utilized for Required Sediment Storage is: <br />Required Storage = Annual R) ANY ea rs) IStorm Sediment Load <br />Storm R <br />Annual R and the number of years are input. Storm R and Storm Sediment Load are <br />calculaCed for the design storm by Sedcad+. <br /> A value of 30 for the average Annual R value and a period of one year was used, Setlcad+ <br /> determined the required Sediment Sto rage for the one year period to be 0.29 ac-ft. The <br />• amount of available sediment storage is 3.17 ac-f c (elevation 7004.401. Therefore, when <br /> 60 percent of the sediment storage capacity has been consummed 11.90 ac-ft), the pond <br /> will be cleaned out. <br />Seneca Coal Company will periodically monitor the sediment storage capacity to ensure <br />enough capacity is available. The estimated design storm sediment volume is based on a <br />worst-case disturbance in the watershed. This worst case will not occur until <br />approximately 1959. Therefore the constructed sediment storage capacity should be <br />adequate fct at least three years. <br />• <br />13-4 P.-5 Revised 12/12/97 <br />