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<br />linear (straight-line) water-surface profile along the channel. A river <br />channel undergoing changes usually does not have a linear water-surface <br />profile or uniform sediment discharge, but river channel adjustments are <br />such that the non-uniformi ties in water-surface profile and. sediment dis- <br />charge are effectively reduced. 'rt1e rate of adjustment is limited by the <br />rate of sediment movement and subject to the rigid constraints such as <br />grade-control structures, bank protection, abutments, bedrock, etc. <br /> <br />The energy gradient at a river cross section varies wildly. This <br />variable is usually included in a hydraulic canputation such as that of a <br />HEC-2 study. The output of any HEC-2 study, even for a fairly uniform river <br />channel, usually exhibi ts non-uni formi ty in energy gradient along the <br />channel. This variation is 1IIUch more pronounced in disturbed rivers. A <br />mathematical modeler realizes that a river channel will change in order to <br />attain streamwise uniformity in sediment load. It is equally important to <br />perceive that it will also adjust toward equal energy gradient along the <br />channel. Because sediment discharge is a direct function of 'YOS, channel <br />adjustment in the direction of equal power expenditure also favors the <br />uniformity in sediment discharge. The sediment discharge in the reach will <br />match the inflow rate when the equilibrium is reached. <br /> <br />IV. """t"'~ WID'Df AOJtJS'iH!N'l'S IXl1WC SCOOR AND PILL <br /> <br />A stream channel's adjustment in the direction of equal power expendi- <br />ture, or straight water-surface prOfile, provides the physical basis for the <br />modeling of channel width changes. HOWE!ver, this adjustment does not neces- <br />sarily mean movement toward uniformi ty in channel width. For one thing, the <br />power eXpenditure is also affected by channel roughness and channel-bed <br />elevation, in addition to the width. BUt, more importantly, the adjustment <br />toward uniformity in power eXpenditure is frequently accomplished by signi- <br />ficant streamwise variation in width. Such spatial width variation <br />generally occurs concurrently with streambed scour or fill to be illustrated <br />in the following by several examples. <br /> <br />The transient behavior can be more clearly demonstrated by more <br />dramatic river channel changes in the short term. Field examples of this <br />5 <br />