山区河流宽窄相间河段山洪水沙输移2维数值试验

山区河流山洪过程常挟带大量泥沙，并引发河床剧烈变形，尤其是宽窄相间河段河床急剧调整、水位陡涨极易增加展宽河段的洪水淹没风险，深入研究山洪演进的水沙输移及河床变形对于防洪减灾具有重要意义。本文采用二维水沙动力学数值试验探究了上游来沙变化条件下宽窄相间河段水沙输移及河床变形过程。结果表明：当上游来沙充足时，洪水携带泥沙在展宽段淤积、缩窄段冲刷，河床在泥沙输移过程中整体上抬；而当上游没有来沙时，河床整体呈现下切趋势。究其原因，主要在于河道饱和输沙导致河床整体淤抬，而河床淤抬导致上游水位陡涨增加水面比降，床面切应力变大，从而提高河道输沙率，而宽窄河段床面起伏则增大床面形态阻力，促使上游水位上涨；泥沙来量较少时，河床冲刷为主，水位下降、水面比降趋于变缓。

Numerical Experiment of Flash Flood Related to Sediment Transport at Local Diverging-converging Sections in Mountainous Rivers

Flash floods propagating in mountain rivers deliver massive sediment, resulting in significant change of riverbed topography. Particularly in the diverging-converging sections, the significant topographical change and associated flow stage rise can easily lead to high risk in flood inundation. Deepening the understanding of the characteristics of flow, sediment transport and riverbed morphological evolution is significant to flood disaster prevention and mitigation. This study employed a two-dimensional hydro-morphological model to simulate the processes of flow, sediment transport and riverbed morphological evolution at diverging-converging sections in a mountain river under the change of sediment supply. The modeling results show that when the sediment supply is sufficient, sediment tends to deposit in the widening section and be scoured in the narrowing section. This overall leads to the aggradation of riverbed. Without sediment supply, the riverbed, however, shows a degradation trend. As a result of sediment supply, the water stage grows in response to bed aggradation, leading to the increase in surface gradient. This mechanism in turn maintains the capacity of sediment transport of the river by increasing the bed shear stress because of the upstream sediment supply. The present study offers significant insights that upstream sufficient sediment supply can cause the water level to rise through reshaping the bed topography of the mountain river with width variation, thus inducing the flood inundation risk in the upstream reach.