| Bank erosion is an important part of channel evolution in alluvial rivers,and frequently occurred in the Jingjiang Reach,which is one of the key reaches in the Middle Yangtze River,in terms of flood control and navigation.Sever bank erosion would greatly influence channel stability,and cause damages to riparian hydraulic structures and serious problems to flood control management in the JR.However,early-warning system of bank erosion was not successfully established in the JR,with the accurate modeling of bank erosion remaining difficult.Therefore,proposing multi-scale advanced numerical models and early-warning technique for bank erosion,along with the investigation on its mechanism,are primary tasks for channel management and disaster alleviation engineering in the JR,which would also greatly benefit the understanding of channel evolution.Majors conclusions include:(1)Characteristics of bank erosion process in the JR were analyzed,in terms of the differences before and after the operation of the Three Gorges Project(TGP),and the temporal and spatial distributions of bank erosion events.Results show that:(1)as compared with1992～2002,the extent of bank erosion had increased in the JR after the TGP,with the length of eroded bank-line during 2003-2017 increasing by 1.2～3.6 folds in the Shashi and Gongan subreaches of the Upper Jingjiang Reach(UJR).Obvious increase in the length of eroded bank-line can also be noticed in the Jianli subreach of Lower Jingiiang Reach(LJR),based on the statistics during 1998～2012;(2)Bank erosion events primarily occurred in July and November of 2002～2012,which accounts of 24%and 28%of the total bank events;(3)the left bank was more prone to be eroded than the right bank,with the length of eroded left bank during2002～2012 accounting for 59%of the total length of eroded bank.Besides,bank erosion events usually occurred in the region where the main flow approached or impinged on.(2)Bank erosion mechanism was investigated,with multiple influencing factors being quantitatively analyzed.The erosion of river bank with a two-layer structure(a clay upper layer and a sandy lower layer)usually started from the fluvial erosion to the bank-toe/lower layer,followed by the failure of upper soil block under the actions of gravity and pore water pressure,etc.Based on the measurements and numerical tests,the analysis of influencing factors shows that:(1)an empirical relationship between bank retreat width and 5-year average fluvial erosion intensity was established at several typical sections in the JR,with the correlation coefficient exceeding 0.85,indicating the dominant role played by fluvial erosion in bank erosion process;(2)the delayed development of ground water level inside the bank has led to a higher pore water pressure during the recession period in the UJR,and thus decreased the bank stability.Its delayed development caused a 34%increase in the calculated bank erosion volume at section of Jing35 in the UJR;(3)An increase in the drawdown rate of river stage would lead to a more delayed change in the ground water level,with one fold increase in the drawdown rate at Jing35 corresponding to a 15%decline in the ratio of the decrease rate of groundwater level to the drawdown rate of river stage;(4)the groundwater flow also induced changes in the physical and mechanical properties of bank soil,with the calculated volumetric water content of bank soil at section of Jing 98 in the LJR increased by about 50%during the flood period of 2012,while the tensile strength decreased by about 28%;(5)the effect of critical shear stress on bank erosion was significant in the JR,and its decreasing by 50%caused a 49%and 44%increase in the calculated bank erosion volume at Jing 35 and Jing 98,respectively.(3)Multi-scale bank erosion models were proposed for banks with a two-layer structure,with the effects of different interacting processes being considered..A sectional bank erosion model was first proposed,through integrating fluvial erosion calculation with the calculations of groundwater level change calculation and bank stability degree.The sectional model was further incorporated with a one-dimensional and a two-dimensional hydro-and sediment dynamic module,in order to simulate bank erosion process at along-reach and local-reach scale,respectively.These models were then applied to simulate bank erosion process in the JR in different years,and the results show that:(1)the bank erosion volume(above the bank-toe)and bank profile,calculated by the sectional model,generally agreed with the measurements at typical sections in the JR,but due to the insufficient characterization of near-bank flow condition,there were large discrepancies at a few sections;(2)the 1D and 2D models reproduced well the flow and sediment transport process in the JR.For the 1D model,the calculation agreed well with the measurement,with the root-mean-square errors of daily discharge,suspended sediment concentration(SSC),and river stage being respectively less than1036 m~3/s,0.08 kg/m~3and 0.45 m.For the 2D model,in addition to the daily hydrographs,there were generally good agreements between the calculated and measured transverse distributions of depth,depth-averaged velocity and SSC;(3)the 1D model reproduce the major bank erosion events in the JR during 2005,2007 and 2010.The calculated maximum bank retreat width in the region near Lalingzhou of the UJR ranged between 15～22 m,close to the measured value of 18～37 m.The calculated width in the region near Beimenkou of the LJR was 28～44 m,compared with the measured value was 42～75 m.According to the numerical tests of 1D model,the cut-down on incoming high floods(discharge>30000m~3/s)and the rise of outlet river stage would lead a decrease in bank retreat width in the JR,especially in the LJR;(3)the 2D model was applied to simulate bank erosion processes in the Shashi subreach of UJR and in the Shishou subreach of LJR during the flood seasons of different years.The calculated maximum width of bank retreat were 11 m and 39 m in the flood seasons(July-October)of2004 and 2007 in the UJR,close to the measured values of 14 m and 42 m.Due to the complexities in flow and topographic conditions,there was a larger discrepancy between the calculation and measurement in the LJR.The calculated maximum width of bank retreat in Shishou subreach were 46 m and 27 mduring the flood seasons of 2004 and 2007,less than the measured values of 52 m and 75 m.(4)Early-warning technique for bank erosion process was proposed,through integrating then prediction with the evaluations ofthe ntensity level and early-warning level of bank erosion.Two methods were used to predict bank erosion process.The first method was based on the proposed multiscale bank erosion models,and was able to produce the location of bank erosion event,the retreat width and the length of eroded bank-line,which were then adopted to evaluate the bank erosion intensity under this method.The second was an empirical method relying on the analysis of channel evolution.The obtained representative parameters included the possibility of bank erosion event at all sections and the bank retreat width at some typical sectionsin the JR.These two parameters were also used to determine the bank erosion intensity under the second method.The predicted bank erosion intensities were integrated and categorized into three different levels.Early-warning level of bank erosion was then determined,based on the level of bank erosion intensity and importance of bank erosion region.This technique was preliminarily applied to JR,with the early-warning infographic being designed,in which the warning levels were represented by different colors.According to the results,the early-warning sites mostly located in the local reaches near the outlet of JR. |
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