Study of sedimentation zones in a large sand-bed braided river: an example from the Hanjiang River of China

The concept of sedimentation zone is applied to the study of spatial and temporal variations of mid-channel bars in the middle Hanjiang River, a large sand-bed braided river in China. The river channel can be divided into alternating sedimentation zones and transport zones, the formation of which is basically controlled by local channel boundary conditions such as natural and artificial bank controls (bedrock outcrops, local hill spurs, terraces, artificial dikes and influence of tributaries). A systematic comparison between sedimentation and transport zones is made in this study, including the flow shear stress, stream power, mid-channel bar features and channel stability. The results obtained indicate that different channel boundary conditions in sedimentation and transport zones control channel width and width-depth ratio, which in turn control the reachʹs hydraulic and sediment transport characteristics. The storage and removal of sediment in sedimentation zones correspond to the increase and decrease of the degree to which mid-channel bars develop. When the sediment input of the channel system is altered by reservoir construction, the bed-load sediment historically accumulated is released by clear water scour, thus a macroscopic bed-load `waveʹ forms and moves gradually downstream. Associated with this is the complex response phenomenon of mid-channel bar evolution.