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7/14/2009 5:01:44 PM
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UCREFRP
UCREFRP Catalog Number
7050
Author
Andrews, E. D.
Title
Effective And Bankfull Discharges of Streams In THe Yampa River Basin, Colorado and Wyoming
USFW Year
1980
USFW - Doc Type
Journal of Hydrology
Copyright Material
YES
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<br />325 <br /> <br />I <br /> <br />only uniform size of bed material to develop his relation. For bed material <br />with a range of grain sizes, the smaller grains tend to be "hidden" behind the <br />larger grains. Thus, for a given flow, the shear stress on the smaller particles <br />of a poorly sorted mixture is less than it would be on a bed of uniform size <br />grains. Consequently, a larger shear stress is necessary to initiate motion of <br />the smaller grains in a mixture of bed-material than would be required in a bed <br />of uniform grain size. Einstein (1950) developed a technique for estimating <br />the reduction in forces on a particle that is part of a mixture of grain sizes. It <br />was found that the reduction of force on a particle that is part of a mixture <br />of grain sizes was approximately proportional to (DID6S)2 for D less than D6S' <br />For grain sizes larger than D6S' there is no apparent reduction of force due to <br />the mixture. The relation between discharge and grain size of particles at the <br />threshold of motion shown in Fig. 9 was constructed by modifying the Shields <br />critical-shear-1ltress relation in accordance with the reduction of force. <br />No more than 35% of the bed material of the Little Snake River near Dixon <br />is transported by even the largest recorded flood (Fig. 9). The maximum ob- <br />served discharge at this site is 270 m3/s in 43 years of record, and has a recur- <br />rence interval of approximately 75 years. At the effective discharge, 113 m3/s, <br />only the smallest 25% of the bed material is transported. The most significant <br />feature, however, is that a small fraction of the bed material is transported by <br />even moderate discharges. Thus, although the median-size bed material is very <br />coarse, the threshold of bed-material transport is small, because some relative- <br />ly fine bed material is available. To varying degrees, this circumstance is true <br />for the eight stream channels studied which had predominantly coarse bed <br />material. The threshold of bed-material transport occurs at relatively moderate <br />discharges, even though the median diameter of bed material is very coarse <br />and, in some instances, not even transported by extreme flood flows. As a <br />result, the threshold of transport for some fraction of the bed material (the <br />intercept of the sediment-transport curves, or curve A in Fig. 1) does not vary <br />greatly between gaging stations even though there is considerable variation in <br />the median bed-material size. This condition may explain the lack of correla- <br />tion between the frequency of the effective discharge and the median bed- <br />material size. <br /> <br />COMPARISON OF THE EFFECTIVE AND BANKFULL DISCHARGES <br /> <br />\(~~' The probable significance of the effective discharge in forming channel fea- <br />tures was recognized by Wolman and Leopold (1957), and Wolman and Miller <br />(1960). They suggested that channel size and meander pattern were closely <br />correlated to, and, hence, were the result of the effective discharge. In parti- <br />\ cular, they noted that the effective discharge might approximate the bankfull <br />\ discharge - that is, the discharge that filled the channel to the level of the <br />\floodplain. Although this hypothesis has been considered by several studies, <br />I <br />~he results have generally been inconclusive, Harvey (1969) and Pickup and <br />!Farner (1976). Frequently, these investigations have been limited by the avail- <br />
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