Integrated impacts of multi-factors on the channel evolution of a shoaling reach downstream of the Three Georges Dam

To investigate the cause of an abrupt change in the morpho-dynamic processes of a shoal reach downstream of the Three Georges Dam, the mechanisms of fluvial processes pre and post the dam closure were compared for the recurrent shoaling in the Lujiahe reach. The impacts of multi-factors including the incoming flow and sediment regime and the channel adjustment were identified using the hydrological and topography data measured after dam construction. These results indicate that the degree of bed erosion caused by sediment deficit is more intensive in the flood flow path in a multi-branched channel, which leads to a reduction in the critical discharge for the bed scour in the main flow path during dry seasons. Furthermore, with the operation of the Three Georges Reservoir at a pool level of 175m, the recession rate of the dam released flow is more rapid and the suspended sediment load is mostly discharged during flood periods. All these changes tend to accelerate the deposition in a shoal reach during a flood season, instead of causing the channel erosion during a post-flood period. The integrated effects of all these factors lead to an abrupt change in the channel evolution of the shoaling reach since 2008. Therefore, it is concluded that recurrent shoaling may exist for a long period in the Lujiahe reach, and more attention should be paid to the similar shoals in multi-branched reaches in the middle and lower Yangtze River.