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<br /> <br />001394 <br /> <br />- 9 - <br /> <br />Channel Erosion and Sediment Transport <br /> <br />If a stream is ephemeral, runoff in traversing the dry alluvial bed may <br />be drastically reduced by transmission losses (absorption by channel <br />alluvium). This decrease in the volume of flow results in a decreased <br />potential to move sediment. Sediment may be deposited in the streambed <br />from one or a series of relatively small flows only to be picked up and <br />moved on in a subsequent larger flow. Sediment concentrations, determined <br />from field measurements at consecutive stations, have generally been shown <br />to increase many fold for instances of no tributary inflow. Thus, although <br />water yield per unit area will decrease with increasing drainage area, the <br />sediment yield per unit area may remain nearly constant or may even increase <br />with increasing drainage area. <br /> <br />In instances of convective precipitation in a watershed with perennial flow, <br />the role of transmission losses is not as significant as in watersheds with <br />ephemeral flow, but other channel factors, such as the shape of the channel, <br />may be important. <br /> <br />For frontal storm runoff the flow durations are generally much longer than <br />for convective storms, and runoff is often generated from the entire basin. <br />In such instances, sediment removed from the land surfaces is generally <br />carried out of the area by the ru~off. Stream channel degradation and/or <br />aggradation must be considered in such cases, as well as bank scour. Because <br />many of the stream beds in the Pacific Southwest are composed of fine-grained <br />alluvium in well defined channels, the potential for sediment transport is <br />limited only by the amount and duration of runoff. Large volumes of sediment <br />may thus be moved by these frontal storms because of the longer flow durations. <br /> <br />The combination of frontal storms of long duration with high intensity and <br />limited areal-extent convective activity will generally be in the highest <br />class for sediment movement in the channels. Storms of this type generally <br />produce both the high peak flows and the long durations necessary for max- <br />imum sediment transport. <br /> <br />Sediment yield may be substantially affected by the degree of channel develop- <br />ment in a watershed. This development can be described by the channel cross <br />sections, as well as by geomorphic parameters such as drainage density, <br />channel gradients and width-depth ratio. The effect of these geomorphic <br />parameters is difficult to evaluate primarily because of the scarcity of sedi- <br />ment transport data in the Pacific Southwest. <br /> <br />If the cross section of a stream is such as to keep the flow within defined <br />banks, then the sediment from an upstream point is generally transported to <br />a downstream point without significant losses. Confinement of the flow <br />within alluvial banks can result in a high erosional capability of a flood <br />flow, especially the flows with long return periods. In most channels with <br />wide floodplains, deposition on the floodplain during floods is often sig- <br />nificant, and the transport is thus less than that for a within bank flow. <br />The effect of this transport capability can be explained in terms of tractive <br /> <br />