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In some instances, mining will cause the d[ainaye area to change durlnq [he life of the • <br />sediment structure. When the pit moves into the watershed, runoff is intercepted by the <br />pi[ and diverted away from the siructu re. This pit area would reduce [he watershed area <br />by as much as 20 acres in si^=. This would decrease the runoff volume and sediment <br />loadiny tn[o [he structure. Ho~.+ew r, a ro ns=rvati'ae ptoced ore was used; the structure <br />was analyc ed fur tt~e la ryest anticipated disturbed area and watershed area [hat will <br />contribute ru nuff [o the str uc[ure, which is riyh[ now. <br />curve Number and "t" Ea c[or. The curve numbers are based on infurma tion presented in Tab <br /> <br />13, Attachment 13-1. The "k " factors were derermtned with the aid of the USDA-ARC'S <br />revised LISLE sef[ware package aroi information presented i n Tab 13, Attachment 13-L. The <br />curve numbers and "L;" facie c s were itien "w=r3hred" for =ach subwa[ershed as shown in <br />Addendum 3. <br />Frecipita[ion and St^rm T ype. Thr: Fir ecipira[um amuun*s a[ [he Seneca II-W Min= permit <br /> <br />area for [he 1G-year and `.5-year storms were obtained from NOAA Precipitation Er equency <br />Atlas Frecipi tat v.~n - Freq uency Atlas :•f rh= 'rles[ern Stares - '/olume II - Co Lr.r ado, 193. <br /> <br />A standard S. . Type II ste rn was uti lited fer each anal,'s is. <br />Unit 4ydfoy ra F.h and Surface Condition. The SEDCADr computer proyram provides three unit <br />hydrograph shapes to chew= from rn determining the rU ncff characteristics of a <br />watershed. <br />Time of ponce nr ration. The time of cuncentratior~, [c, is defined as the time required <br />for water to flew from the h,draulically inusi remote point in the watershed to [he <br />watershed outlet. The r.l me •?f rcncenL tar i~..n for Fond nJ6 and the Fre-Settling Basin 'were <br />calculated using [he SE GOAD{ rompu[er prop ra m. The program determines overland flow to a <br />pond structure by u[iliziny the seven SC, Upland Curves whi r_h yield velocity as a <br />fund ion of slr~pe and land use. Fleas= refer [o the detailed subwat eished -npu[/output <br />table fur the summary of the cline of cencentia[ion for each of the subwatersheds in <br />Addendum 1. <br />Sedimen[ulog~. The SEDCA.D~ computer program was also uu lined [o calculate the sediment <br />flowing into and through r.he ptir,c rpal spillway using [he DEPOSITS portion of [he <br />ping ram. Sediment flew ih r~ ~ugh the pond is within the Limits of [he I1 E'DES permit. <br />4 Fevised Vii 02/9N <br />