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<br />27 <br /> <br />. <br /> <br />Chapter III <br />PROFILES OF NEAR-CRITICAL FLOWS - PROGRAM PR~FILE <br /> <br />. <br /> <br />. <br /> <br />The objective of this study is to provide a computer oriented <br /> <br />solution to backwater curves for varying channel configurations when <br /> <br />flow is near critical. The desired computer program must predict the <br /> <br />reaction, insofar as possible, of the natural stream to imposed <br /> <br />changes. Hydraulic characteristics of primary interest include cross- <br /> <br />sectional area, discharge, water surface elevation and velocity <br /> <br />expected in a given set of cross-sectional geometries. <br /> <br />~ <br /> <br />. <br /> <br />Development of Method <br />Computation of a water surface profile based on energy principles <br />indicates that, if all energy losses are properly accounted for, the <br /> <br />resulting flow profile will be reasonably accurate. Such computation <br /> <br />appears to be valid, especially in areas of mild slope which contain <br /> <br />contractions and expansions. However, in configurations such as <br /> <br />chutes or drops, acceleration effects predominate in the flow and much <br /> <br />energy is dissipated in turbulence and energy changes cannot be <br /> <br />accounted for by means of available flow equations. Chutes and drops <br /> <br />are widely prevalent in mountain streams. Reaches containing chutes <br /> <br />and drops have relatively high roughness values and relatively large <br /> <br />changes in cross-section. Small changes in cross-section have a <br /> <br />greater effect on the rapid flow than on tranquil flow. Therefore, <br /> <br />uniform rapid flow is not maintained over very great distances, with <br /> <br />an average of one hundred meters. Consequently, backwater computations <br /> <br />for rapid flow in natural channels are applicable only for a limited <br /> <br />. <br /> <br />. <br /> <br />. <br />