Response of riverbed evolution of Yichang-Jianli reach of the Yangtze River to water and sediment conditions of Yichang Station

Since the first impoundment of the Three Gorges Reservoir (TGR), water and sediment conditions of Yichang station have undergone a significant change, which has triggered a new round of riverbed adjustment of downstream reaches. In this study, regression analysis, mechanism analysis and other methods were used to investigate the response of the riverbed evolution of Yichang-Shashi reach (YSR) and Shashi-Jianli reach (SJR) to Yichang water and sediment conditions before and after the TGR. Results show that the correlation between scouring and silting amount (SSA) of YSR and Yichang water and sediment indexes is stronger than that of the SJR and the correlation of the former reach becomes stronger with time. On a monthly basis, the delayed response of riverbed evolution to Yichang water and sediment conditions cannot be observed for both YSR and SJR. Based on monthly average Yichang discharge, the univariate regression model for SSA can well reflect the riverbed evolution of both river reaches. According to the developed models, YSR riverbed before the TGR shows the pattern of “weak scouring under small discharge and strong silting under large discharge” and the critical Yichang discharge corresponding to the change of state is about 23,500 m³/s. After the TGR, YSR riverbed shows the characteristics of “weak scouring under small discharge and strong scouring under large discharge”. The scouring amount of YSR gradually decreases due to riverbed coarsening. SJR riverbed has the same evolution pattern with YSR riverbed before the TGR. However, its critical Yichang discharge of state change is slightly smaller (about 22,500 m³/s) and its silting amount is larger than that of YSR under the same Yichang discharge. After the TGR, SJR silting amount is slightly larger than that of YSR when Yichang discharge is small, while the former becomes much smaller than the latter when Yichang discharge is large.