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I I ,I I I I I I I I I I I .. I I I I I routing effect. The unit hydrographs were developed for an incremental time of five minutes. The two basins were combined in the HEC-I analysis and were routed through the spillway. The probable maximum precipitation (pMP) for Reservoir No. 2 was obtained from NOAA Hydrometrological Report Number 55A. This report was released in June 1988 and supersedes Report 55. In Report 55A, general storms and local thunderstorms were analyzed over the study area. The local storm was selected for the PMP estimate due to its significantly higher rainfall amount. The I-hour, 1- mile-square rainfall is 10.30 inches based on an elevation of 5,000 feet. The rainfall was adjusted to 6.36 inches for the site elevation of 10,000 feet and for the basin area. Incremental PMP depths were determined from the point rainfall amount in HMR-55A and were input into HEC-I per procedures suggested by the State Engineers Office for short duration intervals. The revised Rules and Regulations for Dam Safety and Construction allow for a spillway design based on 75 percent of the PMP. However, because this dam replaces the existing dam, PMP reduction was not analyzed; and the 100% PMP was used for design purposes. The peak inflow into Peterson Reservoir is estimated to be 13,095 cfs. 100 - Year Analysis ror Service SpiIlway The service spillway section of the dam is designed to pass flows resulting from a storm equal to twice the 100-year event. This design criteria is based on a premise that would consider the probability of this event happening one or two times during the life of the structure. Procedures for determination of rainfall distribution for a lOO-year storm are defined in the NOAA Atlas. The incremental values were then doubled to simulate a storm event larger than the lOO-year event. The I-hour rainfall amount used was 3.94 inches. The resulting peak inflow into the reservoir was determined to be 2,529 cfs. Flood Routing Reservoir routing was performed for the new spillway design. The existing spillway capacities, or dam breach conditions, were not analyzed as a part of this study. The new dam will be constructed of Roller- Compacted Concrete (RCC). The upstream face will be vertical, and the downstream face will have a slope of 0.65 horizontal to I vertical. The center section of the dam contains a 75-foot-wide service spillway. The crest is at elevation 9491.5 (G.H. 32). The spillway crest consists of an ogee-shaped designed to pass the 2 x lOO-year storm. Storage volumes above the spiIIway were calculated from existing reservoir documentation. Routing the storm through the reservoir resulted in a peak spillway discharge of 1,854 cfs. The maximum water surface elevation was 9494.97 (G.H 35.47). At elevation 9495, a 75-foot section of the dam crest will overtop during the PMF event. This section will pass water against the left abutment which is founded in firm rock. This section, in combination with the service section, wiII pass a peak flow of 8,140 cfs. The maximum stage is 9499.08 (G.H.