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HYDROLOGIC ANALYSIS OF THE FLOOD 6000 5000 1982 dem failures Peak discharge 4000 '" z 8 3000 '" '" '" '" "- ~ 2000 '" ~ u iii :J U i!: ",' ~ 1000 .. 900 :l: ~ 800 C 700 600 500 400 2 10 25 50 100 500 RECURRENCE INTERVAL, IN YEARS FIGURE 13.-Flood.frequency curves station 06733000 Big Thompson River at Estes Park 1951-77. Annual flood peaks have been separated into rainiall and snowmelt floods, then recombined with conditional probability into a compoaire f100d- frequency curve based on Log-Pearson 'JYpe III flood-frequency analysis (Interagency Advisory Committee on Water Data, 1981), previous flood of record, from a rapidly melted snow- pack, was 1,660 fts/s in June 1965 (table 2); this flood was estimated from the composite flood-frequency curve in figure 13 to be about a 15-year flood. Upstream on the Fall River at its confluence with the Roaring River (elevation 8,550 ft), the flood peak (water only) was estimated to have been 12,000 ftals (see "Dam-Break Modeling" results). This represented a unit discharge of 1,000 fts/s/mi', an unprecedented value in historic times for the Colorado Front Range above an elevation of 7,500 ft. A regionalized flood-frequency curve for the mouth of the Roaring River is shown in figure 14, constructed from regional regression equa- tions for mountain areas in Colorado (McCain and Jar- rett' 1976). Based on historic flow data under the present climate regime, the estimated 500-year flood for this location on the Roaring River is 400 ftS/s. The 1982 dam-break flood was 30 times the 500-year flood for this stream; a flow of 12,000 ftals would be very rare. Along the Roaring River, deposits of Pleistocene ground moraine and outwash were extensively scoured and eroded. A late Pleistocene end moraine at an eleva- tion of 10,900 ft (Richmond, 1960) was overtopped and deeply eroded. In July 1983, extensive field work was con- ducted in Rocky Mountaln National Park to investigate 21 15,000 '" z 8 10,000 '" '" '" '" "- I- '" '" ~ u iii :J U i!: ui '" '" .. :l: U '" is 1982 dam failure Peak discharge 500 400 300 25 50 100 500 RECURRENCE INTERVAL, IN YEARS FIGURE 14.-RegionaI flood-frequency curve for Roaring River at Horseshoe Falls. based on regional regression equation of McCain and Jar- rett 11976). whether or not any stratigraphic or geomorphic evidence existed of comparable large postglacial floods in any of the streams draining into Lake Estes. The sediments and landforms produced by the Lawn Lake dam failure are so distinctive that the evidence of any comparable floodflow in the Holocene should be easy to recognize. No unequivocal evidence of a similar large flood was found in any stream valley that drains into Lake Estes. The dam-break flood of 1982 was very likely the largest flood that has occurred in the Roaring River, Fall River, and Big Thompson River aince at least the last glacial retreat, about 10,000 years ago. Along the Fall River between the Roaring River con- fluence and Cascade Lake dam, slopes are very gentle, and no extensive modifications occurred. The flood surge of 16,000 ft'/s from the Cascade Lake dam failure caus- ed a large amount of scour and erosion along the Fall River between the dam and just below Aspenglen Cam- pground. Extensive Pleistocene moraine deposits were scoured and reworked. The flood surge from the Cascade Lake dam failure may have been the largest flow in this stretch of the Fall River since the draining of the glacial lake in Horseshoe Park. This unquestionably was an ex- tremely rare discharge. These estimations of the flood fre- quencies from the two dam failures in Rocky Mountain National Park are not unprecedented; similar conclusions were reached following the failure of Hell Hole dam on the Rubicon River, Calif., in 1964 (Scott and Gravlee, 1968). Floods of such magnitude from dam failures would be expected to accomplish an enormous amount of