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. • - • III IIIIIIIIIIIII III <br />g~ <br />LUKE J. RUSSELL <br />CORPORATE MANAGER -ENVIRONMENTAL AFFAIRS <br />March 23, 1994 <br />Mr Allen Sorenson <br />State of Colorado <br />Division of Minerals and Geology <br />1313 Sherman St. Room 215 <br />Denver, CO 80203 <br />Re: Bulldog Mine Hyflrologio Evaluation <br />Dear Allen, <br />• <br />+~ECEIVEn <br />MAR 2 8 1994 <br />Or•islon C; PA~ilel8r: a L:co °f, ~• <br />The hydrologic evaluation presented in my letter of November <br />15, 1993 indicated that the constructed channel in the Windy Gulch <br />drainage would safely pass the 100 year, 24 hour storm event using <br />a Curve Number (CN) of 58. The channel sideslopes across the toe of <br />the waste dump would require armoring with 6-9 inch maximum <br />diameter riprap where none is present. We have reevaluated the <br />channel design and riprap requirements based on the new information <br />you sent on February 23, 1994. <br />The SCS maps and accompanying soil unit descriptions support <br />your classification of essentially all soils in the Windy Gulch <br />drainage as being in the Hydrologic Soil Group C, with minor <br />occurrences in Groups D and B. Based on Table 2-3c (SCS,1984), the <br />CN for a C type soil having a Pinyon-juniper and grass understory <br />cover type, average runoff condition, and good hydrologic <br />conditions is 61. Adjusting for the small areas of fair and poor <br />hydrologic conditions in the drainage, a CN of 68 seems reasonable. <br />Your recommended CN of 75 would be appropriate if the entire <br />drainage was in a fair to poor condition, which we do not believe <br />to be the case in Windy Gulch. <br />Using the SCS methodology for estimating runoff and a CN of 68 <br />the computed runoff from a 100-year, 24-hour precipitation event <br />(2.8 inches) is approximately 140 cfs. The toe of the waste dump <br />itself should not be directly impacted from this runoff event given <br />the average rock size present, however, portions of the Windy Gulch <br />channel may require upgrading to minimize headcutting and potential <br />undercutting of the dump during this theoretical event. The channel <br />sidslopes across the toe of the waste dump would require armoring <br />with 7-12 inch maximum diameter riprap, depending on channel slope <br />and width, where none is present. Table 1 in Mr Benowitz's letter <br />1 <br />HOMESTAKE MINING COMPANY <br />650 CALIFORNIA STREET • SAN FRANCISCO • CA 9406-2]66 1415) 96t~6 X50 <br />