Process And Mechanism Of Bank Collapse Of Meandering River Under Different Bank Composites

Bank erosion and collapse in meandering river play an important role on lateral migration of meanders,sediment transport,and channel meandering.The process and mechanism of bank collapse vary with the meanders of different bank materials.The combination method of field measurement,theoretical analysis and numerical simulation(BSTEM),was applied to develope the stability modle of cantilever collapse of composite bank,which includes the peat-type and meadow-type bank,in the Black River within the Zoige basin in the Yellow River source region.In addition,the concept of efficiently average discharge based on hydrograph was put forward,and the BSTEM was applied to simulate the process of bank erosion in the lower reach of the Black River in order to reavel the chiefly controlling effect.Finally,we reveal the impact of riparian vegetation on the silt-bank stability in the middle and lower reach of the Tarim river.The main results are as follows:(1)A theoretical model of cantilever river bank stability was developed for this peat-type meandering river with composite bank layers of different soils.The static equilibrium equations were derived to describe cantilever tension failure and shear failure of the upper and lower layers.Moreover,the critical failure width of peat layer and critical scour depth of bank toe were concluded.BSTEM was used to simulate the bank failure process of the silt layer at the five sections.We calculated the bank stability coefficient at a threshold collapse condition of the bank,Ks=0.17,and predicte time changes in bank profiles at the five sections caused by three straight occurrences of bank failure?(2)The bank erosion rate and retreat width are increased with the increase of the discharge,and the bank collapse time are decrease with the increase of the discharge under the constant hydrograph.While for the three runs in the same average discharge,the bank erosion rate and retreat width are similar and increased with the increase of the average discharge,which is independent of the shape,peak discharge,and dispersion of hydrographs under the variable discharge condition indicating that the bank erosion of Black River in the Yellow River source region is controlled by the average discharge rather than the magnitude of peak discharge and the shape of hydrograph.(3)BSTEM was used to calculate the additional cohesion provided by three kinds of riparian vegetation roots(reed,Tamarix and Phragmites Australis)for the river bank in the middle and lower reach of Tarim rive.The changes in the amount of the bank erosion and the stability coefficient of the bank within a hydrological year were calculated.Furthermore,the mechanical effect of vegetation roots on the improvement of slope stability was quantified.The result indicates that riparian vegetation roots can decrease the strength and rates of bank erosion.The root-soil bonds which play an important role in process of bank collapse in the meandering channel of Tarim river provide a mass of additional cohesion for the bank slope,also increase the bank stability coefficient,and reduce the number and amount of bank collapse.