Bifurcations and Lateral Shift of Stream Channels Following to Heavy Sediment Deposit

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In relation to stream channel bifurcations and shifts caused by sediment deposit near the center of alluvial fans, changes in multiple sand bars were tried to predict from an equation for bar height estimation and an existing linear stability theory. Experiments of these stream channel process were carried out in an apparatus with both steep and mild slope reaches. In case that no stream channel was cut in the mild slope part, sediment transport capacity in the upstream reach and large amount of sediment deposit on the downstream part, but only such simple sediment deposit did not cause channel shifts and bifurcations.Data and document of bank erosion and sand bar change of the Brahmaputra and the Meghna rivers in Bangladesh were examined on difference between stream channel change in braided and non-braided large rivers conveying enormous amount of sediment. Channel changes are very active in the Brahmaputra river and severe bank erosion is accompanied with prominent bar deformations. Detailed inspection of bank slope shapes and two dimensional flood flow analysis in the Meghna river indicated that active bank erosion and bed scour parts correspond to those with high flow velocities. Bank erosion process observed in the Uji river, a main tributary of the Yodo river, was elucidated from both hydraulic and soil mechanic viewpoints. Erosion of bank slopes higher than 7 m are accelerated by slip failure which carries bank materials directly to the main stream part.Experiments on stream channel process with heavy sediment deposit and its control on alluvial delta were conducted. Flows in original channels were obstructed by sediment depositions and they change their course to new channels. The process were studied by one and two dimensional models of river bed variations. Results of the latter model showed a good agreement with experimental observations.