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• construction of the disposal pile over a 21-year period in a prac- <br />tical scheme of surface drainage control. <br />Design storm <br />In 1974, the consulting engineering firm of Cornell, Howland, Hayes <br />and Merryfield (CH2M Hill Inc.) prepared a zeport, "Reconnaissance <br />Study Middle Fork and Davis Gulch Dams Colony Shale Oil Plant". <br />That study reviewed the hydrologic data available for the area <br />in detail, and concluded the design value of a PMF (probable <br />maximum flood) together with peak flow runoff values for design <br />storms of various recurrence frequencies. In addition to the <br />estimates of a PMF, an "Assumption B" storm was also estimated <br />through the application of reduction factors outlined in the <br />United States Department of the Interior, Bureau of Reclamation <br />publication "Design of Small Dams," where an "Assumption B" <br />storm is defined as: <br />• <br />"Failure would cause only loss of structure with little <br />additional damage to property and project operation." <br />The report states, "The PMF method is probably conservative. <br />This can be illustrated by applying the PMF of 12,400 cfs to an <br />extension of the recurrence curve shown in Figure G-4. The <br />recurrence interval for 12,400 cfs would be about one million <br />years..." The report nonetheless concludes, for various reasons, <br />that the PMF method is a reasonable basis for design of major <br />hydraulic structures in Parachute Creek basins. Thus the dams <br />to be constructed on Davis Gulch and Middle Fork will be designed <br />to protect against the PMF storm and possible loss of human <br />life. <br />The PMF method is not applicable to the design of the surface <br />runoff collection system. However, the storm producing such <br />L.I <br />G-37 <br />