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HYDROLOGIC ANALYSIS OF THE FLOOD were used, depending on the hydraulic conditions at each site. These methods were the standard U.S. Geo- logical Survey slope-area method (Dalrymple and Ben- son, 1967), flow over weirs method (Hulsing, 1968), and critical-depth method (Barnes and Davidian, 1978). These techniques, although different in type, are based on similar hydraulic principles. These techniques generally give reasonable results when flow conditions are within the basic assumptions and limitations for which the methods were developed. Flow conditions for this flood did not meet all these limitations; hence, the peak discharges may be less accurate, and the magni- tude of errors was difficult to determine. The main fac- tors that affect the accuracy of measurements include unsteady flow, Manning's n-values, high sediment con- centrations, and scour and fill that affect the cross- sectional flow area. To varying degrees, these factors influenced peak-discharge measurements. However, un- til further research is undertaken to improve indirect- discharge measuring techniques under extreme condi- tions, these methods provide the most accurate results available. The methods used to compute peak discharge assume steady flow; however, flow is unsteady for dam-failure floods (and flash floods). V. R. Schneider (U.S. Geolog- ical Survey, written commun., 1982) indicated that, when the slope-area method was used to determine the peak flow of an unsteady flood wave in a channel, the true discharge was overestimated by as much as 21 per- cent. Indirect flood measurements could not be made along the Roaring River or on Fall River in Horseshoe Park (fig. 1) because of the highly unsteady flow (described as a "wall of water" in the Roaring River, or as a rapidly attenuating flood wave in Horseshoe Park). The near-instantaneous failure of C.ascade Lake dam increased the unsteady nature of the flood wave; however, it rapidly attenuated in a short distance down- stream. Indirect discharge measurements were made at locations where unsteady flow was not considered to af- fect the computed discharges significantly, because the reach lengths generally were less than 200 ft. Available guidelines on Manning's rougimess coeffi- cient n or n-values (Barnes, 1967; Limerinos, 1970) have been made primarily on low-gradient streams. Data col- lected by Jarrett (1984) indicated that n-values are much greater on high-gradient cobble-and-boulder.bed streams than previously recognized, because of Unac- counted turbulence and energy losses. Other factors, such as large amounts of debris, channel obstructions, and irregular banks caused more turbulence and rough- ness, particularly on high-gradient streams, and there- fore resulted in larger n-values. Methods of estimating n-values on high-gradient streams (Jarrett, 1984) were used in the computation of peak discharges; these 15 methods are believed to best represent the energy losses associated with turbulence and roughness in high- gradient streams. Sediment can alter the fluid characteristics of flow- ing water by increasing its viscosity and density. Pres- ently, flood-measurement techniques are made by assuming clear-water flow and no sediment in transport. Although sediment data for this flood were not avail- able, eyewitness, photographic, and postflood evidence indicated relatively low sediment loads in all locations where indirect flow measurements were made. Sediment load is assumed not to have affected the computed results significantly, although in local situations down- stream from large sediment sources (deeply scoured reaches), geomorphic and sedimentologic evidence in- dicates sediment loads were temporarily very large. Scour and fill during the passage of a flood can substantially affect the cross-sectional flow area along a stream and, consequently, peak-discharge computa- tions. The entire length of the Roaring River (fig. I) was either deeply scoured or filled with sediment (fig. 10; fig. 26), as discussed in the section "Geomorphic Effects of the Flood." Because of scour and fill, high sediment loads, and the unsteady nature of the flood wave, in- direct peak-discharge measurements could not be made along the Roaring River. Along the Fall River and the Big Thompson River, scour and fill generally were minor, and sites could be located where scour and fill did not significantly affect the computed peak discharges. Considering the factors discussed previously, the computed peak discharges shown in table 2 were the best available. Estimated error of the peak discharges, in view of these factors, is about 25 percent. In light of the hydraulic complexities of the flow and channel conditions where these methods were applied to com- pute peak discharge, results were much better than ex- pected, based on comparisons of peak discharges at the different sites and comparisons with the dam-break modeling results. Peak discharge at Site 6 was sup- ported by flow records (Site 7) of the U.S. Bureau of Reclamation (C. W. Huntley, U.S. Bureau of Reclama- tion, written commun., 1982) in which the flood hydro- graph entering Lake Estes was computed as shown in "Gaging-Station and Miscellaneous-Site Data." Com- putations of the Big Thompson River inflow to Lake Estes indicated that inflow to the lake peaked during the 5-minute interval from 0910 MDT to 0915 MDT, averaged 5,364 ft3/S, and had an estimated error of :tl0 percent (C. W. Huntley, U.S. Bureau of Reclama- tion, written commun., 1983). This value compared well with the indirectly determined instantaneous peak discharge of 5,500 fts/s at Site 6. Because sediment plugged the intakes to the gage at Site 6, stage and streamflow hydrographs were not available.