长江口南北槽分流口动力地貌过程研究

Research on the morphodynamics of distributary mouth between the South and North Passage of Changjiang Estuary

分汊口的地貌演化格局对于三角洲建造和下游河槽的稳定具有重要作用.作为长江河口的第三级分汊口(南、北槽分流口),形成历史较短,但又经历了长江口最大的水利工程(北槽深水航道工程)的影响,故该分流口的变化是近年来长江河口研究的核心内容之一.本文基于近50 a的地形、流域入海水沙和南北槽落潮分水分沙比等数据,探讨了南北槽分流口的动力地貌变化过程.结果表明:(1)在1998年前,长江口南北槽分流口经历了由不稳定—相对稳定—动态平衡的阶段,其中分流口沙洲洲头出现下挫—上提—下挫、主泓线则经历分流口南侧—北侧—南侧的周期变化,河槽断面由U型发展为W型.(2)长江大洪水是导致分流口地貌格局出现变化的动力因素.在洪水发生的间歇期,分流口地貌则处于由不稳定向相对稳定发展的调整阶段.(3)1998年修建沙洲洲头潜堤工程后,沙洲洲头出现上提—下挫—上提的微弱变化,但总体上沙洲头向北偏移,这导致进入北槽的分水分沙比有所减小.(4)自2002年以来,沙洲洲头因分流口的淤积而缓慢上提,潜堤北侧北槽上段淤积加重,潜堤南侧尤其紧贴潜堤部位侵蚀加剧,南槽分水分沙增大,分流口落潮冲刷槽进一步向南槽发展.

Evolutionary formation of distributary mouth morphodynamic processes is important for deltas building and downward river channel stability. Distributary mouth between the South and North Passage（DMSN）,which is the third distributary mouth of Changjiang Estuary,had a comparatively short evolutionary time,which had been experienced serious impacts from the largest hydraulic engineering of the Deepwater Channel Regulation Project. Thereafter,the changes of distributary mouth become the key research topic of Changjiang estuary in recent years. In this paper,data including topography with the time interval of 1950s ～2012,water and sediment load into the estuary,and ebb tidal water and sediment load into the North and South Passage were collected to analysis the morphodynamic processes of the DMSN. Meaningful results are thus summarized as the following.（1） DMSN changes had been experienced 3 stages,i. e. instability, relative stability and dynamic equilibrium before 1998. During these stages,shoal head located at DMSN was appeared as downward decrease,upward shift,and downward decrease again. The thalweg depth was experienced similarly changes shifted from location situated at the south of the DMSN,the north,and the south of the DMSN again. The appearance of river section was from the originated‘U＇ to the present‘W＇ formation.（2） The largest floods occurred in the Changjiang River is the determinative hydraulic factor impacting the DMSN morphodynamic processes. However,during the interval of flood occurrences of Changjiang River,DMSN geomorphology is under adjustment from unstable to stable stage.（3） Due to construction of submarine dike at the shoal head in 1998,there were minor shifts of the shoal head from the upward to downward change. However,the shoal head was in general leaned to the north which caused decrease in water and sediment load into the North Passage.（4） The shoal head located at the DMSN has experienced slowly upward development since 2002 due to DMSN accretion. Meanwhile,the northward part of the submarine dike located in the upstream of the North Passage had been serious accreted. But the southward part of the submarine dike located in the upstream of the South Passage had been serious eroded which induced and increased in the water and sediment load into the South Passage and a further extension of the ebb channel located at the DMSN to the South Passage.