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I I I I I I I I I I I I I I I I I I I B-4 2. Rainfall storm Flows: The USBR, in calculating their PMF, developed a rainfall storm hydrograph using the April 14-16, 1921 rainstorm. This storm was calculated to have produced a 24hr rainfall of 6.6-inches over the Granby Reservoir basin which related to 17.170 cfs peak flow on the USBR Hydrograph. By comparison, SCS Technical Release No. 55 lists the following 24hr rainfalls for the Granby Reservoir basin: 100yr 25yr 10yr 2.8 inches (average for basin) 2.2 inches (average for basin) 1.8 inches (average for basin) For our calculations, we used the USBR Hydrograph, adjusted for flow volume based on the SCS projected 24hr rainfall values. 3. Dam Operating strategy: The Granby Reservoir is essentially an irrigation and recreation facility. As such, the yearly oper- ating plan is developed to fill the reservoir in June/July from snow melt runoff. Flood control was not a listed function in the dam proposal and design features at full reservoir do not provide a high amount of flood mitigation. Further, releases from Granby are typically coordinated with releases from the other major reservoirs above Gore Canyon to mit- igate flow variations affecting property along the river and to meet downstream obligations. The specific operating plan typically calls for the reservoir to be filled to a level of 8279.5 (top of gate is 8280.0) before considering flood releases. At this stage then the gates are opened as required to maintain the reservoir level below 8280.0. For most snow melt conditions, the USBR is able to an- ticipate reservoir inflows and in turn increase stream flow releases prior to reaching 8279.5 and early in times of peak inflow after reaching 8279.5 in order to reduce actual releases required to main- tain reservoir level below 8280.0. Based on rec- ords, it appears that releases prior to full stage are varied (sometimes quite substantially) as re- quired to maintain a relatively uniform reservoir filling rate while at the same time taking into account status of other reservoirs and streams in the area. After reaching the full stage, it appears that the reservoir level is maintained in the range of 8279.5 to 8279.8. The USBR is legally prohibited to allow reservoir levels in excess of 8280.3.