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t t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vrn - EFFECT OF WATER CONTROL ~LAN I I I " ~ I . 8-01, Gene",!. Ridgway Dam and Reservoir regulates floodflows in the Uncompahgre River by controlling releases, insofar as possible, to obtain the maximum practical reduction in flood damages. The flood control storage in Ridgway Reservoir is adequate to control and <liven the floods of record to existing aon~ging safe channel capacities below the dam. Seasonal storage is provided for irrigation, recreation. and fisheries to balance the benefits for farmers and the public. 8-02. Flood Conrrol. The principle objective of the flood contra! plan is prevention of immdating areas in the lower Uncompahgre River Basin. a. Reservoir Desi I!l1 Flood. The size of Ridgway Reservoir was based on the initial design flood smdy completed by the Upper Colorado Regional Office in February 1973. b. Spillway Desi~ Flood. Tile USBR's Spillway Design Flood (SDF) for Ridgway Reserv{)ir was routed through the spillway (in conjunction with the outlet works). resulting in a maximum pool elevation of 6.880 feet (2,097 m), with the staning pool at the spillway crest c:Ievation of 6,871.13 feet (2,094.3 m). The Spillway Design Flood is shown on Plare 8-1. The rain on snowmelt flood had a peak of 16,300 cis (461.6 m'/s) and a 15-<lay volwoe of 110,000 acre-rect (136 hm'). \ , I c. Probable Maximum Flood rPMFl. The Probable Maximum Flood (PMF) smdy was prepared in 1985 to update the 1973 Ridgway Inflow Design Flood Smdy. In a memorandum to the Chief, Concrete Dams Branch, dated January 23, 1985, the E&R Center Hydrology Branch of the USER, recommended that this earlier study be modified by using Hydrometeorological Report No. 49 (HMR 49) in the preparation of the design storms. There were three separate scenarios smdied to detennine the PMF, These three eventS are a spring ram-{)n-snow, a local thunderstorm, and a fall general stonn. The routings had a starting elevation at the spillway crest. These scenarios a.re presenred in Table 8-1. All three of these PMF's overtopped the dam; therefore, an Early Warning System was developed instead of raising the dam. For more information on the Early Warning System, see Exhibit A-05. Refer to Section 1-03, Related Manuals and Repons, for further information. d. Other Floods. Between Colona and the juncmre with the Gunnison River, there are [IQ eye wimess reports of extensive or devastating floods along the UllCompahgre River near Delta, Colorado. However, in 22 of the 73 years on record. shallow lIUisance flooding bas occurred in this area. Channel capacity is estimated to be 2,000 cfs (57 m'/s) at, and below Colona. Table 8-2 tabulates the floods of record ex;ceeding 3,000 cfs (85 m'/s) downstream at Colona gage. Ridgway reservoir would have been able to regulate these flood flows below the dam downstream to Colona gage at or below the safe channel capacity of 2.000 cfs (57 m'/s). 8-\