Study On The Influence Of Water Level Falling Rate On River Bank Collapse Under Scour Condition

For a long time,river bank collapse has occurred frequently in the middle and lower reaches of the Yangtze River,which has seriously threatened the safety of river embankment,river stability,shipping development and coastal infrastructure,and caused a large amount of land loss on both sides of the river.Since 2003,influenced by many factors such as the upstream reservoir construction and soil and water conservation,the sand flow in the middle and lower reaches of the Yangtze River has been greatly reduced,and the riverbed erosion has been intensified.At the same time,influenced by the reservoir operation,the process of dam drainage has changed greatly in the year,and the speed of water recession has been significantly accelerated.The middle and lower reaches of the Yangtze River are facing a new situation of bank collapse.At the same time,river bank collapse has always been a key and difficult problem in river bed evolution and river control engineering.It is of great significance to study the influence of water level falling rate on river bank collapse under scour condition for bank collapse control,disaster prevention and mitigation.In this paper,with the help of the methods of the generalized model test and numerical simulation,the influence of water level falling rate on river bank collapse under the condition of scour is studied.In the generalized model test,the process of bank collapse under different water level falling rate,water flow action and water level difference inside and outside the bank slope is analyzed by observing the change of coastal topography,distribution of flow velocity,pore water pressure and bank collapse process.By using numerical simulation method,the stability of bank slope under the condition of water level change is calculated and analyzed,and the calculation results of whether considering the effect of water erosion and different falling conditions of water level fall are compared and analyzed.The specific conclusions are as follows:(1)River water level drop and seepage promote the occurrence of bank collapse.The faster the water level falling rate is,the more obvious the bank collapse is;when the water level falling rate is small,the bank is relatively stable and the bank collapse is less.With the river water level falling,the water level difference inside and outside the bank slope becomes larger,the seepage slope becomes larger,and the stability of the bankdecreases.(2)The hydrodynamic condition is the main factor of bank collapse.The results show that the strength and scale of bank collapse increase with the increase of discharge.In the low water period,the inflow of the river is small and the bank is relatively stable;in the rising water period,with the increase of the flow,the flow velocity near the bank increases significantly,the bank becomes steep by erosion until the bank collapse occurs,and the strength of the bank collapse reaches the maximum in the flood period;in the falling water period,the strength of the bank collapse decreases due to the weakening of the flow erosion.(3)River boundary conditions,such as river bank plane shape,affect the occurrence of bank collapse.In the bend section,due to the water flow close to the bank top,under the same flow and water level conditions,the strength of bank collapse is greater than that of the straight channel before entering the bend,and the collapse mainly occurs in the concave bank top and downstream section.(4)In terms of the generalized model test and numerical simulation calculation conditions,the river water level falling rate will promote the occurrence and development of bank collapse,but its impact is to a certain extent covered by the erosion of water flow on the nearshore riverbed,which is more likely to cause bank slope instability.Therefore,in the river regime control engineering,it is very important to protect the bank slope,especially the slope toe.