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<br />Hydrology <br /> <br />. <br /> <br />The hydrologic analysis is aimed at determining the peak <br />rate of flow for the 100-year frequency flood. <br /> <br />In August 1977, a study entitled "Major Drainageway <br />Planning, Phase B, Master plan" was prepared by Wright-McLaughlin <br />Engineers for Jefferson County, the Urban Drainage and Flood <br />Control District, and the City of Arvada. This report was <br />officially designated and approved by the Colorado Water <br />Conservation Board on January 19, 1978 and revised an earlier <br />report prepared by the Army Corps of Engineers. This document <br />is the most current and reliable source of floodplain informa- <br />tion on Ralston Creek in the Denver metro region. Pertinent <br />information relating to the hydrology of the upper Ralston <br />Creek Basin extracted from the Wright-McLaughlin report are <br />included in the Appendix. <br /> <br />From the Wright-McLaughlin report for Ralston Creek, the <br />100-year discharge above Ralston Reservoir, drainage area <br />of 48 square miles, was computed to be 7,200 cubic feet per <br />second. This is a runoff ratio of 150 cfs per square mile. <br />Considering that the drainage area at the Schwartzwalder Mine <br />is approximately 41 square miles, the 100-year discharge at <br />that point is reasonably estimated at: <br /> <br />. <br /> <br />150 cfs/sq. mi. x 41 sq. mi. = 6,125 cfs. <br /> <br />The runoff ratio above Ralston Reservoir for the la-year flood <br />was computed to be 83 cfs per mile. Similar calculations <br />result in a 10-year discharge at the mine of about 3,400 cfs. <br /> <br />. <br /> <br />-5- <br />