Effects of the incoming flow-sediment regime and the backwater effect caused by the Dongting Lake confluence on channel degradation in the Jingjiang reach

The Jingjiang reach (JR) has experienced continuous and significant channel degradation after the Three Gorges Project operation. It is urgent to quantify the backwater effect caused by the confluence of the Dongting Lake, and the consequent impact of incoming flow-sediment regime on the channel degradation in the JR. Based on the measured hydrological data and data for channel degradation from 2003 to 2020, an empirical function for clarifying the relationship between the cumulative channel evolution amount and boundary conditions for the upstream and downstream of the studied JR reach has been established. In addition, the contribution of the backwater effect to the cumulative channel evolution amount was investigated. The results indicate that : ① The cumulative channel evolution amount in JR was positively correlated with the fluvial erosion intensity at the inlet, and it was inversely with the corresponding difference between the average water stages at the inlet and outlet. Therefore, the channel degradation in JR is the result of these two influencing factors. ② The empirical function derived could effectively represent the impact of the fluvial erosion intensity and differences between the average water stages of channel inlet and outlet on the cumulative channel evolution amount in JR (R²>0.92). ③ The cumulative channel evolution amount of the JR section from 2003 to 2020 was 1.23 billion cubic meters. Under hypothetical conditions for the absence of the backwater effect caused by the Dongting Lake confluence, the cumulative channel evolution volume would increase by 67 million cubic meters. This indicated that a potential reduction in the cumulative channel evolution amount of nearly 5% was attributable to the backwater effect. At the same circumstance, the cumulative channel evolution volume in the upstream and downstream JR was 0.73 and 0.5 billion cubic meters, respectively. The backwater effect caused a reduction of the two values by about 1% and 16%, respectively. Therefore, the backwater effect decreased the channel evolution volume in JR and its influence on the channel degradation was more obvious in the downstream than that in the upstream.