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Input consists of precipitation amount (inches), watershed area (acres), time of <br />concentration (hours), runoff curve number (CN), representative particle-size <br />distributions of soils in the area, sediment of ogy information, spillway information, etc. <br />The drainage area for this sediment pond was measured on a 1"=400' topographic map (see <br />Exhibit 13-2). <br />In some instances, mining will cause the drainage area to change during the life of the <br />sediment structure. When the pit moves into the watershed, runoff is intercepted by the <br />pit end diverted away from the structure. This pit area would reduce the watershed area <br />by as much as 20 acres in size. This would decrease the runoff volume and sediment <br />loading into the structure. However, a conservative procedure was used; the structure was <br />analyzed for the largest anticipated disturbed area end watershed area that will <br />contribute runoff to the structure. It was estimated that this condition would occur when <br />mining nears the final highwall area in the southern portion of the permit area. <br /> Curve Number end "k" Factor. The curve numbers area based an information presented in <br /> <br /> Tab 1i, Attachment 13.1. The "k" factors were determined with the aid of the USDA-ARS's <br /> revised USLE software package and information presented in Tab 13, Attachment 13-1. The <br />• curve numbers and "k" factors were then "weighted" for each subwatershed as shown on <br /> Tables 13-4-2 through 13-4-9. <br />• <br />3 <br />