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FUTURE FLOODS Although floods with magnitudes and fre- quencies similar to those of past floods could recur, discussion of future floods in this report is limited primarily to one designated as the 100- year flood. A 1oo-year flood has a peak flow magnitude with a 1 percent chance of being equalled or exceeded in any given year, or a frequency of occurrence of about once in 100 years on the long-term average. It must be understood that the term "100-year flood" relates to flood magnitude and does not mean that the flood will occur one time in a 100-year period. Essentially, "probability of occurrence" is implied. Thus, during the term of an average mortgage (30 years), the chance of a 100-year flood occurring is approximately 1 in 4; during an average lifetime (70 years), the chance is approximately 1 in 2. I n a 100-year period, the chance of a 100-year flood is approximately 3 in 5. To determine flows of the 100-year and other frequency floods for this report, a flow- frequency analysis was made for five index points. (See Drainage Basin Map.) Three of the index points (Nos. 1, 2, and 3) were on the Gunnison River and two (Nos. 4 and S) were on the Uncompahgre River. Those on the Gunni- son were at locations reflecting significant tributary inflow, and those on the Uncompah- gre were at the locations of the Delta and Colona gaging stations. In general, the flow-frequency analysis for the Gunnison River involved developing peak flow curves reflecting natural (unregulated) flow prior to construction of Blue Mesa Reservoir and adjusting these curves to account for the regulatory effect of the reservoir. The unregulated peak flow curves were established from an analysis of unregulated flow at the "below Gunnison Tunnel" and "near Grand Junction" stream gages (periods of record 1906- 1965 and 1897-196S, respectively). Regulated peak flow-frequency curves for present condi- tions were developed from analysis of flows recorded after construction of Blue Mesa Dam (period of record 1966-1976), from reservoir routing studies previously made by the Corps of Engineers and the Water and Power Resources Service, and consideration of the Water and Power Resources Service objective flow of 1S,000 cubic feet per second for the Gunnison River upstream from Delta. Peak flow-frequency curves for the Uncom- pahgre River were based on records of flow at the Delta and Colona gages.' As noted earlier, floodflows on the Uncompahgre River usually result from snowmelt, but may also result from general rain, particularly at Delta. Therefore, annual peak flows for rain and snowmelt were analyzed independently, and frequency curves developed for each. All-event peak f1ow- frequency curves were developed by combin- ing the rainfall and snowmelt peak f1ow- frequency curves for each station. Since the all-event curves are essentially identical for the two stations, it was concluded that the com- bined peak flow-frequencies between them do not change. Approximate Drainage Peak Flow (Cubic Feet per Second) Index Area 10-Year 50-Year 100-Year 500- Year Point Location (Sq. Mi.) Flood Flood Flood Flood GUNNISON RIVER: 1 At Highway 92 bridge near Austin S,271 12,SOO 17,700 20,000 2S,400 2 Above mouth of the Uncompahgre River S,630 13,500 18,700 21,200 26,700 3 Below mouth of the Uncompahgre River 6,759 16,200 22,000 24,700 30,900 UNCOMPAHGRE RIVER: 4 At the mouth 1,129 3,000 4,400 5,000 6,600 S At Colona Gage 443 3,100 4,400 S,OOO 6,600 Peak flow-frequency values found for the 10-, SO-, 100-, and SOO-year floods at selected locations on the Gunnison and Uncompahgre Rivers are shown in the tabulation on page 6. Peak flow values for 10-, SO-, 100-, and 500- year floods (together with corresponding water surface elevations) at each cross section used in the hydraulic analysis made for this report are shown in Appendix A. Plates 2-S9 show the areas that would be inundated by the 100-year flood. In general, the 10- and SO-year floods would closely parallel the pattern of the 100-year flood. Both would be largely confined to the channel and low-lying overbank areas. Delineating the SOO-year flood was beyond the scope of the study authoriza- tion. As shown on Plates 22, 23, and 43, some areas are subject to sheet flow. That is, broad overland flooding generally less than 2 feet deep on the average and characterized by unpredictable flow paths. Water surface eleva- tions in sheet flow areas are essentially inde- pendent of those along adjacent streamways and are affected principally by obstructions and local topography in the area flooded. Water surface elevations of the 10-, SO-, 100-, and SOO-year floods along the stream reaches studied are shown on Plates 60-72. Flood elevations shown were computed through use of the Corps of Engineers step-backwater program HEC-2. Basic cross section input was from surveyed cross section data furnished by the Colorado Water Conservation Board. Depth of water in the channel and adjacent flooded areas can be determined from the profiles. Velocity of flow during a loo-year flood on the Gunnison and Uncompahgre Rivers would average about 7-8 feet per second in the channel and 4 feet per second in overbank areas. At most stream crossings in the study reaches, velocity of flow during a 100-year flood 'Periods of record: Delta gage, 1902-1931 and 1938-present; Colon. gage, 1903-1905 and 1921-present. " S would be about 10 feet per second. Water flowing at 10 feet per second will cause severe erosion of channels, destroy low water crossings, and transport large boulders. Streambanks and the fill around bridge abut- ments may be eroded, and large volumes of sediment transported by water flowing at a rate of S-7 feet per second. Water flowing at about 2 feet per second will deposit sand, silt, and flood borne debris. 6