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<br />Peak Stages and Discharges
<br />
<br />Peak stages and discharges are important in assessing the magnitude of a
<br />flood and its associated damages. For the most part, this was true for this
<br />flood. damages were compounded because of the high flow velocities and
<br />because of the debris load transported by the flood. As the flood far exceed-
<br />ed the magnitude of previous flows at the two streamflow-gaging stations, and
<br />since it peaked very rapidly, direct-discharge measurements could not be made,
<br />Peak- fl ow rates can be computed i ndi rect ly us i ng exi st i ng methodo logy with
<br />reasonable accuracy following a flood.
<br />
<br />Flood discharges were computed at seven locations along the flood path
<br />(fig. 1), Several different methods were used, depending on the hydraul ic
<br />conditions at each site. These methods were the standard U.S. Geological
<br />Survey slope-area method (Dalrymple and Benson, 1967), flow over weirs method
<br />(Hulsing, 1968), and critical-depth method (Barnes and Davidian, 1978). These
<br />techniques, although different in type, are based on similar hydraulic princi-
<br />ples. These techniques generally give reasonable results when flow conditions
<br />are within the basic assumptions and limitations for which the methods were
<br />developed, Flow conditions for this flood did not meet all these limitations;
<br />hence, the peak discharges may be less accurate, and the magnitude of errors
<br />was diffi cult to determi ne. The mai n factors that affect the accuracy of
<br />measurements i ncl ude unsteady fl ow, Manni ng' s n-val ues, hi gh sediment con-
<br />centrations, and scour and fill that affect the cross-sectional flow area, To
<br />varyi ng degrees, these factors i nfl uenced peak-di scharge measurements,
<br />However, until further research is undertaken to improve indirect-discharge
<br />measuring techniques under extreme conditions, these methods provide the most
<br />accurate results available.
<br />
<br />The methods used to compute peak discharge assume steady flow; however,
<br />fl ow is unsteady for dam-fail ure fl oods (and fl ash floods), V, R. Schnei der
<br />(U. S, Geological Survey, written commun., 1982) indicated that, when the
<br />slope-area method was used to determine the peak flow of an unsteady flood
<br />wave in a channel, the true discharge was overestimated by as much as 21
<br />percent, Indirect flood measurements could not be made along the Roaring
<br />River or on Fall River in Horseshoe Park (fig,. 1) because of the highly
<br />unsteady fl ow (descri bed as a "wall of water" in the Roari ng Ri ver, or as a
<br />rapidly attenuati ng flood wave in Horseshoe Park). The near-i nstantaneous
<br />failure of Cascade Lake dam increased the unsteady nature of the flood wave;
<br />however, it rapi dly attenuated in a short di stance downstream, Indi rect
<br />discharge measurements were made at locations where unsteady flow was not
<br />considered to affect the computed discharges significantly, because the reach
<br />lengths generally were less than 200 ft,
<br />
<br />Available guidelines on Manning's roughness coefficient n or n-values
<br />(Barnes, 1967; Limerinos, 1970) have been made primarily on low-gradient
<br />streams. Data collected by Jarrett (1984) indicated that n-values were much
<br />greater on high-gradient cobble-and-boulder-bed streams than previously
<br />recognized, because of unaccounted turbulence and energy losses. Other
<br />factors, such as large amounts of debris, channel obstructions, and irregular
<br />banks caused more turbulence and roughness, part icul arly on hi gh-gradient
<br />streams, and therefore resulted in larger n-values. Methods of estimating
<br />n-values on high-gradient streams (Jarrett, 1984) were used in the computation
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