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 a nd temporal variations of mid-channel bars in the middle Hanjiang Rive r, a large sand-bed braided river in China. The river channel can be d ivided into alternating sedimentation zones and transport zones, the f ormation of which is basically controlled by local channel boundary co nditions such as natural and artificial bank controls (bedrock outcrop s, local hill spurs, terraces, artificial dikes and influence of tribu taries). A systematic comparison between sedimentation and transport z ones is made in this study, including the flow shear stress, stream po wer, mid-channel bar features and channel stability. The results obtai ned indicate that different channel boundary conditions in sedimentati on and transport zones control channel width and width-depth ratio, wh ich in turn control the reach's hydraulic and sediment transport chara cteristics. The storage and removal of sediment in sedimentation zones correspond to the increase and decrease of the degree to which mid-ch annel bars develop. When the sediment input of the channel system is a ltered by reservoir construction, the bed-load sediment historically a ccumulated is released by clear water scour, thus a macroscopic bed-lo ad 'wave' forms and moves gradually downstream. Associated with this i s the complex response phenomenon of mid-channel bar evolution. (C) 19 97 Elsevier Science B.V.