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<br />success of the models was how well each model reproduced scour and fill measured in <br /> <br /> <br />repeat field surveys. Both models were tested for applicability to management questions <br /> <br /> <br />of !lushing flows to minimize or eliminate the hazards of pool infilling from reservoir <br /> <br /> <br />sediment releases. In most cases, reservoir management tools would likely be used by <br /> <br /> <br />water resource specialists with minimal hydraulic and fluvial geomorphic background. <br /> <br /> <br />Thus, a truly useful reservoir management tool would be one with data requirements that <br /> <br /> <br />are not too labor or cost intensive, and that could be developed and employed in a timely <br /> <br /> <br />manner. Additional criteria for evaluating the models included data requirements, time <br /> <br /> <br />investment, level of expertise, and utility of the model output. <br /> <br />HEC-6 <br /> <br />HEC-6 is an one-dimensional model developed by the U.S. Army Corps of <br /> <br /> <br />Engineers to predict scour and deposition within rivers and reservoirs (Hydrologic <br /> <br /> <br />Engineering Center. 1993), It is the most widely used and economical commercially <br /> <br /> <br />avai lable sediment transport model. The major limitation of HEC-6 is that it is a purely <br /> <br /> <br />one-dimensional model, which leads, by necessity, to drastic simplification of the <br /> <br /> <br />complex. three-dimensional !low in rivers. HEC-6 cannot account for the following <br /> <br /> <br />hydraulic conditions which occur in pools along the North Fork: 1) flow separation, or <br /> <br /> <br />eddies. \'oihere a majority of sediment was deposited during the sediment release from <br /> <br /> <br />Halligan Reservoir. 2) bank erosion or lateral channel migralion, which occurred as a new <br /> <br /> <br />thalweg was scoured in the channel after the release, and 3) differential scour and <br /> <br /> <br />deposition over the width of a cross-section, which occurred as sediment was <br /> <br /> <br />simultaneously eroded out of the central. high velocity thalweg and deposited along <br /> <br /> <br />channel margins. <br /> <br /> <br />In spite of the limitations, HEC-6 is the model most likely to be selected by water <br /> <br /> <br />resource managers faced with understanding and mitigating the effects of a reservoir <br /> <br /> <br />sediment release, Therefore. we felt it would provide a useful, first order approximation <br /> <br /> <br />of channel changes over time, in a relatively quick and economical manner. <br /> <br />7 <br />