Grain size and sediment concentration effects on channel patterns of silt-laden rivers

摘要

The Yellow River in China is characterized by channel erosion and a meandering channel pattern at both low sediment concentrations and at hyperconcentrations, whereas deposition and braiding tends to dominate at intermediate sediment concentrations. A 3D numerical model (Delft3D) is used to analyze the effect of sediment-induced density effects on the formation of channel patterns using a highly simplified river geometry. Results show that the effect of vertical sediment density stratification and hindered settling significantly influence river channel patterns. At some critical concentration (the saturation concentration), vertical mixing is totally damped by the density gradients at the lutocline (a strong vertical gradient in sediment concentration). This increases the secondary circulation in the river channel significantly, which in turn promotes meander development. When the flow is not close to saturation, the main channel-forming process seems to be channel avulsion through blocking of old channels and subsequent channel cut-off. When the concentrations are so high that sediment is primarily held in suspension by hindered settling, sedimentation is so much reduced that channels are no longer blocked and therefore the river course becomes confined to a single meandering channel.