长江口南支河床容积对来沙量减少的响应规律研究

Study on the response law of the riverbed volume of the South Branch of the Yangtze Estuary to the change of discharge and sediment load

近年来,长江中下游来沙量持续性减小,其对河床冲刷的效应已逐渐在长江口显现。然而,长江口河床容积的变化特征缺少定量研究,与来沙量减小的时空响应规律不明确。通过分析长江大通站、徐六泾站的泥沙资料,并统计不同高程下的河床容积随时间变化特征,建立不同区段、不同高程河床容积与上游不同时段平均来沙量的关系。结果表明,大通站2003—2020年的年输沙量较三峡工程建成前有大幅减少,其对徐六泾站的当年洪季含沙量有着直接且显著的影响。长江口南支0 m以下河床处于冲刷态势,2003年后河床冲刷速率有明显增加,其中-10 m以下深槽部分的冲刷量贡献最大。南支主要落潮通道河床容积与前1年大通平均来沙量具有显著的相关性,因此来沙量减小主要引起长江口落潮通道河床容积增加,其效应具有约1 a的滞后性。

In recent years,the sediment load in the middle and lower reaches of the Yangtze River decrease continuously.Its effect on river bed erosion has gradually been reflected in the Yangtze Estuary.However,there is a lack of quantitative research on the variation characteristics of riverbed volume in the Yangtze Estuary.Furthermore,the spatiotemporal response to the decrease in sediment load is not clear.The discharge and sediment concentration of Datong Station and Xuliujing Station of the Yangtze River are analyzed.The time-dependent characteristics of river bed volume under different elevations were analyzed.The relationship between river bed volume at different elevations and upstream sediment discharge was established.The results show that the annual sediment transport volume of Datong Station from 2003 to 2020 is significantly reduced compared to the period before the completion of the Three Gorges Project,which has a direct and significant impact on the sediment concentration of the Xuliujing Station during the flood season.The riverbed below 0 m in the South Branch of the Yangtze Estuary is in a state of scouring and the riverbed scour rate has increased significantly after 2003.Among them,the erosion of the deep groove below-10 m contributes the most.The riverbed volume of the main ebb channel in the South Branch has a significant correlation with the average incoming sediment volume of Datong in the previous year.Therefore,the decrease in sediment flux mainly caused the increase in the volume of the ebb channel bed in the Yangtze River Estuary.Furthermore,the effect has a hysteresis of about one year.