Author(s): Yu Li; Z. B. Wang; H. J. De Vriend
Linked Author(s): Huib J. de Vriend, Zheng Bing Wang
Keywords: Numerical experiment; River bed scour; Yangtze River
Abstract: Bank failure during flood along the middle and lower reaches of the Yangtze River forms a major threat for the most developed part of China. One of the causes of bank failure is flood-induced riverbed erosion near 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. This paper focuses on the mechanisms underlying these short-term bed level fluctuations. Two important mechanisms had been identified, one due to river width variation, the other one due to bend effects. Both mechanisms had been studied by analysing the equilibrium morphological configuration as a function of the river discharge conditions. It is found that the difference between the equilibrium depth in a narrower and in a wider section is not constant, but depends on the river discharge: the higher the discharge, the larger the difference. A river in flood will therefore erode in its narrower sections. Similarly, the bed in an outer bend will erode during flood, because the equilibrium bed level difference between outer and inner bend increases with increasing discharge. It is shown that the potential erosion depth for typical field conditions in the middle reach of the Yangtze River can be many metres. However, a natural river is never in morphological equilibrium, but ever adjusting to the changing river discharge. The analysis of the morphological equilibrium as a function of the discharge therefore does not reveal whether or not the potential erosion depth will be attained. A key parameter determining this is the ratio between the relevant morphological time scale and the duration of the flood. In the case of river width variation the relevant morphological time scale can also be well estimated analytically. It is shown that this time scale is relatively short so the potential erosion depth can probably be attained within a single flood. For the case of river bend this is more difficult. Moreover, the axis symmetrical approach for the morphological equilibrium does not apply for most river bends. Therefore numerical experiments have been carried out for the river bend case, in order to verify the analytical results and to determine the relevant morphological time scale.
Year: 2002