A method to predict sediment transport processes in drainage basins for sediment management

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A method which is composed of sub-models such as rainfall runoff, water flow, sediment transport and bed variation is proposed for evaluating sediment transport process in river basins and is tested for its validity and applicability. Especially, it is applied to investigate the sediment runoff from mountain areas and the associated bed evolution of alluvial reach in Takatoki river basin where a dam is planning to construct for flood control, supposing an occurrence of a plan size rainfall event with numerous landslides. Numerical computations were conducted for three cases: no impact of the dam on sediment transport process (Case 1), performances of flood control to reduce flow discharge with the dam (Case 2) and no performances of flood control but with the dam (Case 3). In cases 2 and 3, it is supposed that all sediment inflowing into the reservoir is caught in the storage area. Predicted results suggest that, in Case 1, a fine part of sediment produced by landslides in mountain area can be transported during a period of the flood to the alluvial reach and causes the bed variation although most of the coarse sediment remains in mountain reaches. After the flood, a very active sediment runoff from the mountain area to the flat region continues for long years, and causes river bed aggradation. The dam, which drains about 45 percent of the total basin area, catches a lot of sediment and suppresses the bed variation of alluvial reach during the flood as well as for long years after the event. In addition, sediment transport activities in mountain regions are impacted by reservoir sedimentation and decreases to suppress remarkably river bed aggradation of downstream flat area.