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I I I I I I I I I I I I I I I I I I I Chapter III - Background Information E. Hydrology and Water Rights E.I Flood Hydrology The flood hydrology was approved by the SEO on January 26, 2000. A copy ofthe SEO approvallelter is presented in Appendix A. MorlUment Lake Dam has been classified as a small, Class II dam by the SEO. The SEO requires that the spillway for a Class II dam safely convey 50 percent of the probable maximum precipitation (PMP) storm event, unless an incremental damage analysis (IDA) is used to justify a lesser capacity. The flood hydrology was performed in accordance with the SEO guidelines. Figure III-I shows the contributing watershed to the reservoir. The flood hydrographs from the local PMP, general PMP and I DO-year events were developed using Bureau of Reclamation (USBR) methods outlined in their Flood Hydrology Mamral. The flood hydro graphs were in turn used to study their hydrologic affects on Monument Lak<'l Dam. The results listed in Table III-I are presented with respect to flood peaks and dam overtopping depths and durations. The results of the hydrologic analysis were used as input into We hazard classification analysis and the IDA. Table III-I Flood Hydrology Results Storm Event Percentage of Inflow Maximum Maximum PMP Reservoir Dam Elevation Overtop (ds) (ft) (ft) General 100 145 080 6,943.76 12.26 , PMP 50 71,576 6,937.86 6.36 Local 100 65,172 6,925.98 5.58 PMP 50 31)16 6,937.08 1.55 100-Year N/A 24,878 6,931.42 0.00 The SEO requires that a hazard classification be performed to evaluate the potential consequences of the subject dam failure on existing residents and property below the dam. Based on the hazard classification analysis and the SEO Rules and Regularions, Monument Lake Dam could be classified as a Class III dam. Even though the dam meets State guidelines for a Class III Monument Lake Dam Feasibility Study I3 BOYLE