上游来水来沙与洞庭湖入汇顶托对荆江河段冲淤的影响

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

三峡工程运用后荆江河段处于持续冲刷状态,分析上游来水来沙与洞庭湖入汇顶托对荆江河段冲淤的影响,对掌握坝下游河道演变规律具有重要意义。本研究基于2003—2020年荆江河段实测水沙及冲淤量资料,建立河段累计冲淤量与河段上下游边界条件之间的函数关系,并重点分析顶托作用对荆江河段冲淤的影响。结果表明:(1)荆江河段累计冲淤量与河段进口水流冲刷强度呈正相关,与进出口水位落差呈负相关,河段冲淤过程是二者共同作用的结果;(2)建立进口水流冲刷强度、进出口水位落差与荆江河段累计冲淤量的函数关系,能较好反映上下游边界条件对河段冲淤过程的影响(R^(2)>0.92);(3) 2003—2020年荆江河段累计冲刷量为12.3亿m^(3),在假设无洞庭湖入汇顶托的情况下比实测值增加0.67亿m^(3),即入汇顶托能使荆江河段冲刷量减少约5%;同期上、下荆江累计冲刷量分别为7.3亿和5.0亿m^(3),入汇顶托能使冲刷量分别减少约1%和16%。洞庭湖入汇顶托会导致荆江河段的冲刷量减少,对下荆江冲淤影响更显著。

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:①The 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.②The 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(R^(2)>0.92).③The 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.