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<br />on the water surface. This pattern of significant spatial variation in <br />width actually represents the stream's adjustment toward equal energy grad- <br />ient as explained in the fOllowing. <br /> <br />'l'he streambed area undergoing scour had a steeper energy gradient (or <br />water-surface slope) than its adjacent areas. Fomation of a narrower and <br />deeper channel was effective to reduce the energy gradient due to decreased <br />boundary resistance and lowered streambed elevation. In addition, the cross <br />section developed a sanewhat circular shape which conserved power for being <br />closer to the best hydraulic section. On the other hand, the streambed area <br />undergoing fill had a lower streambed elevation and a flatter energy <br />gradient. Chann:l widening at this area was effective to steepen its energy <br />gradient due to the increasing boundary resistance and rising streambed <br />elevation. In stmnary, these adjustments in channel width effectively <br />reduced the spatial variation in power expenditure or non-linearity in <br />water-surface prOfile. Because sedUuent discharge is a direct function of <br />stream power 'YQS, channel adjustment in the direction of equal power expen- <br />diture also favors the equilibrium, or unifomity, in sedUuent diSCharge. <br /> <br />The significant spatial width variation shown in Fig. 1 was temporary. <br />The small width lasted while streambed scour continued and the large width <br />persisted with sustained fill. At a later stage when scour and fill ceased, <br />the energy gradient or water-surface slope associated with the small width <br />became flatter than that for the large width. The new profile of energy <br />gradient or water surface became a reversal of the ini tial profile. Then, <br />the small width started to grow wider while the large width began to slide <br />back into the channel, resulting in a more unifom width along the channel. <br /> <br />The above example illustrates that a regime relationship for channel <br />width may not be used in simulating transient river channel changes. under <br />the regime relationship, the width is a direct function of the discharge, <br />i.e., B 00 00.5; but under transient changes, the channel can have very <br />different widths even though the discharge is essentially unifom along the <br />channel. <br /> <br />--;"~J~<"_ 'W:r:::>;!': <br />The characteristic changes in channel width during channel-bed scour <br /> <br />7 <br />