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I I I I I I I I I I I I I I I I I I I I -17- with respect to time. The parameters required for application of Snyder's Unit Hydrograph include: 1) the time to peak (TR) of the basin, which is based upon geometric characteristics of the basin; and 2) a peaking coefficient referred to as Cpo We derived values for these parameters by interpretation of aerial photographs and topographic mapping, field inspection, previous experience, and interviews with the COE and the U.S. Bureau of Reclamation (USBR) . Probable maximum precipitation data (including both temporal and spatial variation) were derived from the National Weather Service document HMR-49. Using the computational procedure delineated in HMR-49, an applicable 6-hour local storm PMP value of 6.87 inches was determined for the Sheriff Reservoir basin. For HEC-l modeling, the rainfall was distributed on an hourly basis as follows: Rai nfa 11 Duri ng Hour Hour (inches) 1 0.17 2 0.35 3 3.95 4 1.95 5 0.31 6 0.14 A IS-minute rainfall time increment was selected for computing the total runoff hydrograph, with a distribution procedure based on guidance from HMR-49. For the Kinematic Wave model, an aggregate eN value of 69 was selected. For the Snyder's unit hydrograph procedure, initial losses were assumed to be 1.0 inch, while the uniform loss rate throughout the duration of the storm was assumed to be approximately 0.2 inch per hour. The low uniform loss rate and relatively high CN value are due to the assumption of saturated soil con- ditions prior to the occurrence of the PMP. Past PMF studies for high moun- tain basins indicate that peak snowmelt runoff events are small when contrasted with intense local rainfalls; consequently, snowmelt hydrology was not evaluated. The PMF hydrograph for Sheriff Reservoir, as defined by both the Kinematic wave method of analysis and by Snyder'S hydrograph procedure, is provided on Figure 3. The reasonableness of this runoff hydrograph was con- fi rmed by compari son to 1) PMF "envelope curve" developed from SED-approved projects similar to this project; and 2) Approximately one dozen preliminary and final design studies conducted by Wright Water Engineers for projects similar to this project, several of which have been approved by the SED. Information from these sources corroborated the estimated peak runoff of 20,000 cfs; therefore, the PMF runoff hydrograph determined using these methods of analysis was selected for use in the remaining evaluations.