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

提出了一种由降雨径流、水流、输沙、河床演变等子模型组成的流域输沙过程评价方法，并对该方法的有效性和适用性进行了检验。特别是，它适用于调查从山区的泥沙径流和相关的河床演变的冲积河段在Takatoki河流域，大坝计划建设防洪，假设发生一个计划规模的降雨事件与众多的滑坡。对大坝不影响泥沙输移过程（情况1）、大坝发挥防洪减流量作用（情况2）和大坝不发挥防洪减流量作用（情况3）3种情况进行了数值计算。在情况2和3中，假定所有流入水库的泥沙都被截留在库区。预测结果表明，在情况1中，虽然大部分粗泥沙留在山区，但山区滑坡产生的细泥沙在洪水期间可被搬运到冲积河段，引起河床变化。洪水过后，山区向平原地区的泥沙径流长期活跃，造成河床淤积。大坝排水量约占流域总面积的45%，在洪水期间以及洪水发生后的很长一段时间里，它捕获了大量的泥沙，抑制了冲积河段的河床变化。此外，山区输沙活动受水库淤积影响而减少，对下游平原河床淤积有明显抑制作用。

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.