Coupled two-dimensional modeling of bed evolution and bank erosion in the Upper JingJiang Reach of Middle Yangtze River

Abstract

At the initial stage of dam operation in an alluvial river, significant vertical bed incision and transverse bank erosion processes always occur in the river reach downstream of the dam, with the latter being usually an integrated product of several interacting processes. A coupled two-dimensional (2D) model for the processes of bed evolution and bank erosion has been proposed in this study, which integrates a 2D morphodynamic module with a bank-erosion module and a groundwater flow module. In addition to the action of fluvial erosion, the effect of groundwater flow on bank erosion was considered, with the changes in pore water pressure and matrix suction being calculated. The proposed model was calibrated and validated through simulating channel evolution processes in a 30.54 km sub-reach of the Upper Jingjiang Reach of the Middle Yangtze River, with the effects of infiltration recharge on bank erosion and incoming flow on the central bar evolution being investigated. The results show that: (i) the proposed model had a good performance in reproducing flow and sediment factors, in terms of water depth, depth-averaged velocity and suspended sediment concentration, daily averaged discharge and river stage, and flow and sediment diversion ratios in local anabranching subreaches; (ii) the proposed model correctly calculated the bank erosion processes in the study reach, with the calculated bank retreat width being close to the measured value, however, it was not enough to fully reproduce the evolution of central bars, indicating that a specialized and improved module may be necessary for central bar evolution and its interaction with bank erosion; (iii) the groundwater level change lagged behind the river stage variation, and it showed a more obvious phase lag and a higher value at the same river stage (corresponding to a higher pore water pressure), when encountering with a more rapidly changing river stage; and (iv) the rate of infiltration recharge caused by rainfall process, would generally increase the groundwater level and thus the bank erosion degree in the study reach. In addition, the shifting of incoming main flow might take an obvious impact on the calculation of nearby central bar evolution.