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<br />of peak discharges; these methods are believed to best represent the energy
<br />losses associ ated wi th turbul ence and roughness in hi gh-gradi ent streams,
<br />
<br />Sediment can alter the fluid characteristics of flowing water by increas-
<br />ing its viscosity and density. Presently, flood-measurement techniques are
<br />made by assuming clear-water flow and no sediment in transporL Although
<br />sediment data for this flood were not available, eyewitness, photographic, and
<br />postflood evidence indicated relatively low sediment loads in all locations
<br />where indirect flow measurements were made. Sediment load is assumed not to
<br />have affected the computed results significantly, although in local situations
<br />downstream from large sediment sources (deeply scoured reaches), geomorphic
<br />and sedimentologic evidence indicates sediment loads were temporarily very
<br />large,
<br />
<br />Scour and fill during the passage of a flood can substantially affect the
<br />cross-sectional flow area along a stream and, consequently, peak-discharge
<br />computations. The entire length of the Roaring River (fig. 1) was either
<br />deeply scoured or filled with sediment (fig. 10; fig, 26), as discussed in the
<br />sect ion "Geomorphi c Effects of the Flood." Because of scour and fi 11, hi gh
<br />sediment loads, and the unsteady nature of the flood wave, indirect peak-
<br />discharge measurements could not be made along the Roaring River, Along the
<br />Fall River and the Big Thompson River, scour and fi 11 generally were minor,
<br />and sites could be located, where scour and fill did not significantly affect
<br />the computed peak discharges,
<br />
<br />Considering the factors discussed previously, the computed peak discharg-
<br />es shown in table 2 were the best available. Estimated error of the peak
<br />discharges, in view of these factors, is about 25 percent, In light of the.
<br />hydraulic complexities of the flow and channel conditions where these methods
<br />were applied to compute peak discharge, results were much better than expect-
<br />ed, based on comparisons of peak discharges at the different sites and compar-
<br />i sons with the dam-break model i ng results. Peak di scharge at Site 6 was
<br />supported by flow records (Site 7) of U.S, Bureau of Reclamation (C. W.
<br />Huntley, U,S. Bureau of Reclamation, written commun, , 1982) in which the flood
<br />hydrograph enteri ng Lake Estes was computed as shown in "Gagi ng- Stat i on and
<br />Miscellaneous-Site Data." Computations of the Big Thompson River inflow to
<br />Lake Estes i ndi cated that i nfl ow to the 1 ake peaked duri ng the 5-mi nute
<br />interval from 0910 MDT to 0915 MDT, averaged 5,364 ft3/s, and had an estimated
<br />error of no percent (C W. Huntley, U.S, Bureau of Reclamation, written
<br />commun" 1983). This value compared well with the indirectly determined
<br />instantaneous peak discharge of 5,500 ft3/S at Site 6. Because sediment
<br />plugged the intakes to the gage at Site 6, stage and streamflow hydrographs
<br />were not available,
<br />
<br />Indirect measurements of peak discharges could not be made upstream from
<br />Si te 1 (fi g, 1), but peak di scharges estimated from the dam-break mode 1 are
<br />discussed in the section "Dam-Break Modeling." Channel and hydraulic condi-
<br />tions were such that a peak-discharge measurement could not be made immedi-
<br />ately downstream from Cascade Lake dam, but was made 0.9 mi downstream
<br />(Site 3), Because of rapid attenuation, the measurement at Site 3 did not
<br />reflect the dam-break peak discharge at the dam (see "Dam-Break Modeling" for
<br />an estimate of peak di scharge). However, the di scharge over the top of the
<br />
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