Experimental Study On Composite Bank Failure Process And Interaction With Riverbed Deformation In Sharply Curved Channels

Bank failure is a natural phenomenon of fluvial processes in the alluvial river.After entering into channels,the sediments that derive from bank failure will be mixed with the bed material,which can bring huge effects on the bed topography and its composition,and then affect the river bed evolution.In this paper,a typical sharply curved channel downstream the Three Gorges Dam——the Shishou reach locating in the lower Jingjiang Reaches of the Yangtze River,was selected as the research region.A generalized model was built.A series of immovable experiments were conducted to investigate the hydrodynamics characteristic during the interaction between bank failure and river bed evolution.We also carried out mobile bed and bank experiments to reveal the processes and the mechanisms of the interactions.The main findings are as follows:(1)Based on the immovable experiments,the hydrodynamic characteristics of the sharply curved channel under the different water discharges were investigated.Under the initial geometry condition,the depth-averaged longitudinal velocity in channel bends presents a lateral distribution characteristic of “high velocity in the middle of the channel and low velocity in its banks”.In contrast,the depth-averaged longitudinal velocity tends to be uniform in the straight section.With an increase in the water discharges,the top scouring point moves toward the downstream and the distribution of turbulent kinetic energy in this section is basically remained stable with an increase in the values.The lateral distribution of sidewall shear stress increased.After the scouring,the river bed topography was changed and the longitudinal velocity distribution of the section tends to be uniform,the primary circulation shifts to the concave bank with the widening of the concave bank,the turbulent kinetic energy of each section increases,the distribution of the sidewall shear stress in this section is basically unchanged,but the location of the peak values was changed obviously.(2)The relationship between the three modes of the cantilever bank failure were analyzed theoretically.We found that when the cohesive soil layer in the upper part of the composite bank is homogeneous,the critical width for occurring of the beam failure is the smallest,following the critical width for occurring of the shear failure and the critical width for occurring of the tensile failure is largest.The theory demonstrates that the beam failure is the dominant mode of the cantilever bank failure.(3)By comparing the variation characteristics of the flow structures,the impacts of the slump blocks on the flow structure and the bank erosion processes were studied under the conditions with and without the existence of slump blocks.Compared with the case that slumps blocks do not exist,the slump block locating at the upstream of the jacking point makes the region with the maximum longitudinal flow velocity far away from the concave bank.The average wall shear stress between slumps block and the slope toe of the concave bank can be reduced to 0.94 times,and thus the erosion ability of the water flow on the bank foundation decreases;Compared with the case that slumps blocks do not exist,the slumps block locating at the downstream of the jacking point makes the region with the maximum longitudinal flow velocity closer to the concave bank.The average wall shear stress in the area between the concave bank and the slump blocks and the region at the top of the collapse body can be increased to 4.32 times,the average wall shear stress in the area between the slump blocks and the slope toe can be increased to 1.71 times,and thus the erosion ability of water flow on the bank foundation increases.(4)Based on the moving bed and the moving bank tests,the effects of the different water discharges,the different layer thickness ratios of the composite bank and the local revetment on the interactions between the bank failure and the riverbed evolution were studied.With the increase of discharge,the erosion rate of the concave bank increased while the deposition rate of the riverbed decreased;the increment of the erosion rate of the bank in the main stream adjacent to the bank area was obviously higher than that in the main stream away from the bank area with the values up to 2.1 times.The increment of the deposition rate of the river bed was opposite between them.With an increase in the clay layer thickness,the erosion rate of the river bank and its deposition rate both decreased.The increment of the erosion rate of river bank in the main stream adjacent to the bank area was smaller than that in the main stream away from the bank area.In contrast,the deposition rate of river bed presented a same variation characteristic.The local revetment reduced the sediment supply from the bank of the river bed surface,and the bed surface scour rate increased obviously.After bank failures,the collapses were mixed with the sediment of the river bed surface,and the mixed degree decreased from the concave bank to the convex bank.With the increases of water discharges and the thickness of clay layer,there had no obvious changes in the mixed thickness of the sediment at the foot of the concave bank slope near the curved top section,but the mixing degree near the top scour point decreased obviously.(5)With an increase in the water discharges,the erosion rate of the bank increased and the deposition rate of the riverbed decreased in the concave;With an increase in the thickness of clay layer,the erosion rate of the bank and the deposition rate of the riverbed both decreased;After carrying out the local revetment in the top scour point of the concave,the erosion rate of the bank in upstream without the protection increased,and the scour rate of the riverbed increased in protected area,and the deposition rate of riverbed decreased.The deposition ratio of the bank collapse was between 10% and 50%.