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• <br />© MONTGOMERY WATSON <br />September 21, 1993 <br />Mr. Luke Russell <br />Corporate Manager, Environmental Affairs <br />Homestake Mining Company <br />650 California Street <br />San Francisco, CA 94108 <br />Subject: Hydrologic/Hydraulic Evaluation for Bulldog Mine <br />Deaz Luke: <br /> <br />iI_~ ;~, t~19:: <br />~ We have prepared this hydrologic hydraulic evaluation for the Bulldog Mine near Creede, <br />Colorado. This evaluation includes evaluating the capacity and integrity of the existing <br />' Windy Gulch channel at the base of the 9360 Waste Dump and the diversion channels at <br />the Upper and Lower Tailing Ponds assuming probable maximum precipitation (PMP) and <br />~ 100-year runoff events. To this end we have computed the estimated peak runoff from a <br />I 24-hour, probable maximum precipitation (PMP) event that would occur in open channels <br />located at the toe of the waste dump and in the diversion channels above the upper and <br />i lower tailing ponds at the Bulldog Mine (Figure 1). We have also computed the runoff <br />from a 100-year, 24-hour event for these locations. The methodologies, assumptions and <br />II conclusions of this evaluation are described below. <br />Hydrologic Evaluation of PMP and 100-year Events <br />A hydrologic evaluation of the 24-hour, PMP event for general storms was performed <br />using the methodologies found in HMR SSA (NOAA, 1988). This methodology utilizes <br />recorded precipitation data from actual severe rainfall events to estimate a PMP rainfall <br />amount. Site specific factors such areal location, orographic occurrences, and geographic <br />' subregion are used as part of the evaluation. Based on HMR SSA, a PMP rainfall amount <br />of 16.0 inches was used for design purposes. A 100-year, 24-hour rainfall amount of 2.8 <br />inches (NOAA, 1973) was also used for channel design. <br />Methodologies described in SCS Field Manual, Chapter 2 (SCS, 1984) were used to <br />estimate the peak runoff amounts from 24-hour rainfall events using the PMP rainfall of <br />j 16.0 inches and the 100-yeaz rainfall of 2.8 inches. This method employs factors such as <br />subbasin area, average subbasin slope, subbasin length, soil type, condition of vegetation, <br />soil Curve Number, rainfall distribution and rainfall amount. The runoff evaluation was <br />performed for three subbasins: Upper Tailing Pond, Lower Tailing Pond, and Windy Gulch <br />above Waste Dump. These three subbasins drain into the open channels for which design <br />criteria aze being developed. Figure 2 shows the location of these subbasins. A summary <br />C7 <br />I <br />I6I Mallard Orive Te1:2063655665 Serving the Warld"s Environmental Needs <br />i Boise, Idaho Fax: 208345 5697 <br />63706-3974 <br />