Bank retreat processes and characteristics in the Jingjiang Reach after the Three Gorges Project operation
摘要
The Jingjiang Reach is undergoing considerable channel degradation, and significant bank retreat processes have occurred at local sites due to the operation of the Three Gorges Project (TGP). By investigating the adjustments in planform and cross-sectional geometries based on the measured topographic and hydrological data after the TGP operation, it is discovered that the process of bankline migration was remarkable in the Jingjiang Reach, with the average annual bank-erosion rate of 15.0 m/a and the cumulative bank-erosion length of 42.3 km; 59.2% of the bank-erosion regions were located at the left bank, and 68.7% of them were located in the Lower Jingjiang Reach (LJR); and severe bank retreat processes also occurred at convex banks, with 35.5% of the bank-erosion regions in the LJR. In addition, the variation in cross-sectional profiles indicates that 21% of all the cross-sections underwent obvious bank retreat processes, of which 74% occurred in the LJR. In summary, the bank-erosion regions were mainly located at the left bank and the bank retreat intensity was much higher in the LJR. Various factors influencing bank retreat in the Jingjiang Reach have been presented, and the results indicate that the incoming flow-sediment regime is a dominant control factor. Finally, empirical relationships were developed between the section-scale bankfull widths at typical sections with severe bank retreat and the previous five-year average fluvial erosion intensity during flood seasons, with the correlation coefficients of being higher than 0.85. Therefore, the proposed empirical relations can better consider the effect of the altered flow and sediment regime on the variation in bankfull widths at typical sections.