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The time of concentration for Pond 006 was calculated using the SE DCAD+ computer program. <br />The program determines overland flow to a pond structure by utilizing the seven SCS Upland <br />Curves which yield velocity as a function of slope and land use. Please refer to the <br />detailed subwatershed input/output table for the summary of the time of concentration for <br />each of the subwatersheds 1n Addendum 1. <br />Sedimentology. The SEOCAD+ computer program was also utilized to calculate the sediment <br />flowing Snto and through the principal spillway using the DEPOSITS portion of the program. <br />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 the sediment storage. The amount of sediment storage is determined for a <br />defined length of time and Ss based on the design storm R, design storm sediment load, <br />average annual R, and the number of years of sediment storage desired. R is the rainfall <br />erosivity factor. The equation util ized for Required Sediment Storage Ss: <br />Required Storage = (Annual R) (M Years) (Storm Sediment Load) <br /> Storm R <br /> • <br />Annual R and the number of years are input. Storm R and Storm Sediment Load are <br />calculated for the design storm by SEDCAD+. <br />A value of 30 for [he average Annual R value and a period of one year was used. SEDCAD+ <br />determined the required Sediment Storage for the one year period to be 2.36 ac-f t. The <br />amount of available sediment storage Ss 12.9 ac-ft (elevation 7068.01. Therefore, when 60 <br />percent of the sediment storage capacity has been consumed (7.74 ac-ft), the pond will be <br />cleaned out. <br />Seneca Coal Company will monitor the sediment storage capacity to ensure enough capacity <br />is available. The estimated design storm sediment volume Ss based on a worst-case <br />disturbance in the watershed. This worst case will not occur until approximately 2000. <br />Therefore, the constructed sediment storage capacity should be adequate fo[ at least three <br />years. <br /> <br />4 Revised 09/2B/95 <br />