黄河洪水的非恒定性对输沙及河床冲淤的影响

Influence of unsteady floods on sediment transport and riverbed evolution in Yellow River

本文根据黄河主要干支流冲积河段实测高含沙洪水的输沙资料,从黄河下游河道比降、河宽、流速沿程调整、洪水非恒定性等方面分析研究了冲积河道输沙特性与河床冲淤特性。分析了动床阻力与输沙特性间的关系。当水流流速大于1 8～2 0m s时,床面进入高输沙动平整状态,河道输沙特性呈现“多来多排”状态,是形成河床沿程冲刷的水流动力条件。从洪水期作用在床面上的剪力或功率的变化,引起底沙输移强度的变化,论证了涨水期河床强烈冲刷与落水时迅速淤积的必然性。指出底沙运动速度比洪水波传播慢是造成河道长距离冲刷的根本原因。

The variations of slope and width, adjustments of flow velocity along the river course as well as the variability of flood in the Yellow River are analyzed based on the observation data. The characteristics of sedimentation transport, evolution of riverbed and the relationship between riverbed resistance and sedimentation transport are studied. It is found that the riverbed surface will be in the regime of upper-stage plane bed when the flow velocity is higher than 1.8-2.0m/s, this is the critical flow condition for occurrence of streamwise erosion in the period of flood season with high concentration sandy flow. The cause of strong erosion happened to the flood rising stage and the cause of rapid silting during flood recession are explained according to the variation of shear force and shear power in the whole process of flood. It is concluded that the movement velocity of bed load slower than the propagation velocity of flood wave is the root cause for happening of long distance streamwise erosion.