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Peak Discharge Rate 22.02 cfa 96.71 cfs 160.50 cfa <br />Input consists of precipitation amount Iinches 1, watershed area lacres), time of <br />concentration (hours 1, runoff curve number (LT1), representative particle-size <br />distributions of soils Sn Che area, sedimentology 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 [he drainage area to change during the life of the <br />sediment structure. When the pit moves into [he watershed, runoff is intercepted by the <br />pit and 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 fox the largest anticipated disturbed area and watershed area that will <br />contribute runoff [o 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 and "k" Eacto r. The curve numbers are based on information presented Sn Tab <br />13, Attachment 13-1. The "k" factors were determined with the aid of the USDA-ARS•s <br />revised USLE software package and information presented Sn Tab 13, Attachment 13-1. The <br />curve numbers and "k" factors were then "weighted" for each subwatershed as shown in <br />Addendum 3. <br />Precipitation and Storm Type. The precipitation amounts at the Seneca II-W Mine permit <br />area foc the 30-year, 25-year and 100-year storms were obtained from NOAA Precipitation <br />Frequency Atlas Precipitation - Frequency Atlas of the Western States - Volume II - <br />Colorado, 1973. A standard SCS Type II storm was utilized for each anal ysls. <br />Unit Hyd cograph and Surface Condition. The SEDCAD~ computer program provides three unit <br />h yd rograph shapes to choose f[om in determining the runoff characteristics of a watershed. <br />Time of Concentration. The time of concentration, tc, Ss defined as the time required for <br />water to flow from the hydraulically most remote point In the watershed to the watershed <br />outlet. <br />3 Revised 09/28/95 <br />