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• SCOPE <br />The mine plan prepared by CI:I has made two important assur„ptions with <br />respect to the drainage plan: <br />1. There is great risk of loss of life and property from <br />flash flooding in the present mine configuration, Flood <br />protection to minimize this risk in Westmoreland Canyon <br />is justified. <br />2. Assuming adequate flood protection, the drainage system <br />for the lower basin (mine site) should be desinned to <br />minimize structural and er,vironr,enta', gar„sge from an <br />extreme hydrologic event. <br /> Two levels of desicn have been used based on assuc•,ptions 1 . and 2. <br />• above. These include development of a "",aximun. Probable Flood" hydro- <br />graph for upper Iestmoreland Canyon and one percent probability (OPP) <br />hydrographs far key points of reference in the lower basin. <br />FLOOD HYDRCLOuY <br />Probable I~~axir..um Thunderstorm <br />The probable maxircum thunderstorm (PI^'S) is Lhe design storm com- <br />puled by engineers when the stakes are high in terms of life and property, <br />and the area of interest is small, It is defined as the simultaneous <br />occurrence of t:ie maxi^iur. of several elements or conditions v~hich con- <br />tribute to the "big" flood (USBf'., 1973). This concept is necessary, <br />since data from recorded floods in a given area are not adequate to <br />provide a reliable basis for estimating naximum probable flood flows. <br />• <br />