Scouring effect of the middle and lower reaches of the Yangtze River and its impact after the impoundment of the Three Gorges Project

Abstract

三峡及其上游干支流梯级水库建成运用后,大坝下游“清水下泄”引起了长江中下游河道发生长时间、长距离的冲刷。2001-2021年期间宜昌至长江口已累计冲刷了50.3亿m³,需厘清河道冲刷对河势、防洪、航运等方面产生影响。分析表明:三峡工程运用后,长江中下游河势总体稳定,弯道段出现切滩撇弯、汊道段出现塞支强干等现象;河道槽蓄量的持续增加,进一步增大了河道调蓄和行洪能力,但水流顶冲点的变化和近岸河床的冲刷下切,使得河道崩岸频发,并影响河势稳定和防洪安全;在河道冲刷、河势控制与航道工程综合作用下,长江中下游航道条件总体得到改善,但河道不均衡冲刷使得坝下游砂卵石河段出现“坡陡流急”,沙质河段出现洲滩散乱、航槽移位等现象,影响航道条件;同流量下枯水位呈下降态势,逢极枯年份影响长江中下游用水安全;江湖关系发生新变化,“三口”分流道由原淤积转为冲刷,两湖湖区水位不同程度的降低,枯水位出现时间有所提前,不利于湖区水资源和生态环境的安全与可持续发展;三峡工程实施枯水期补水调度和“压咸潮”调度以及长江口北支淤积减缓,对于遏制长江口咸潮入侵有利,入海泥沙显著减少引起长江口近岸河床冲刷,影响沿岸码头的安全运行。;After the construction of cascade reservoirs in the Three Gorges and its upper reaches, the downstream clear water of the dam was discharged, and a long-term and long-distance erosion occurred in the middle and lower reaches of the Yangtze River. From Yichang to the Yangtze Estuary, a total of 5030 million m³ was scoured. Under the background of large-scale erosion in the middle and lower reaches of the Yangtze River, it has a profound impact on river regime, flood control and shipping. The analysis shows that the river regime in the middle and lower reaches of the Yangtze River is generally stable, and the river regime in some parts of the river changes greatly, resulting in the phenomenon of beach cutting and bending, and the phenomenon of strong trunk. The increase of channel storage capacity increases the capacity of river discharge and flood storage, but the change of flow top scour point and the erosion of nearshore riverbed also make the river bank collapse frequently, affecting the stability of river regime and flood control safety. Under the combined action of channel scour, river regime control and channel engineering, channel conditions in the middle and lower reaches of the Yangtze River improved overall. However, due to the unbalanced scouring of the river channel, the sandy pebble river section in the downstream of the dam has a 'steep slope and rapid flow’, and the sandy river section has phenomena such as scattered beaches and channel displacement, affecting the channel conditions; the continuous erosion of river bed results in the decrease of low water level, which affects the water supply safety of life, production and ecology along the middle and lower reaches of the Yangtze River in extremely dry years. The relationship between rivers and lakes has undergone new changes. The three diversion channels have changed from original siltation to erosion. The water levels in the two lakes have decreased to varying degrees, and the low water level has appeared earlier, which has adversely affected the water resources and ecological environment in the lake area. The regulation of water replenishment and saltwater intrusion in dry season of the Three Gorges Project and the slowing down of siltation in the north branch are beneficial to the control of saltwater intrusion in the Yangtze Estuary. The sediment discharged into the sea from the Yangtze Estuary is significantly reduced and the riverbed near the Yangtze Estuary is scoured strongly. It seriously threatens the safety of the main structure of coastal enterprise wharf.