BED LEVEL VARIATION IN A RIVER BEND DURING FLOODS

摘要

Bank failure during flood along the middle and lower reaches of the Yangtze River forms a major treat for the most developed part of China. One of the causes of bank failure is river bed erosion near the toe of the bank. This kind of bed erosion can be considered as a local short-term fluctuation of the bed level, driven by variations of the hydrodynamic conditions. Earlier analysis (Li et al., 2002) has identified bend and constriction effects as important mechanisms causing such short-term fluctuations. The bed in the outer bend will erode during flood, because the equilibrium bed level difference between outer and inner bend increases with increasing discharge. It has been shown that the potential range of bed level variation for typical field conditions in the middle reach of the Yangtze River can be many metres. In this paper we present the results of a numerical model study to further underpin these conclusions. The numerical results are also used to address questions that could not be answered by the analytical approach. The first question is which characteristics of a flood (duration? total amount of water? peak discharge?) determine the depth of scouring. The second one is to what extent the equilibrium scour depth will be attained during one flood season. The numerical simulations using Delft3D, a software package developed by WL|Delft Hydraulics, have been carried out for a schematic river bend that is held representative of the bends in the middle reach of the Yangtze River. Simulations with constant as well as varying discharge have been carried out. The numerical simulations show that the magnitude of the peak discharge and the flood duration are two key parameters determining the scour depth and the extent to which equilibrium is reached.