长江口横沙通道近岸冲刷地貌形成机制

近岸河床的剧烈冲刷是引起岸坡失稳的主要原因之一。通过SeaBat 7125多波束测深系统对长江口近岸冲刷最为剧烈的横沙通道北口冲刷坑地貌进行测量,同时利用双频ADCP采集区域水流数据,结合历史海图资料,研究该冲刷坑演变过程及形成机制。结果表明：冲刷坑呈椭圆形,长约430 m,宽约150 m,最深处距床面约38 m;1984-2017年期间,冲刷坑附近河床经历了冲—淤—冲的演变模式,整体呈冲刷状态,净冲刷量3.45×10 ⁷ m ³,平均冲深4.68 m;2005年后冲刷坑快速发育,且持续向南延伸。这主要是因为长兴北沿圈围工程的建设减小该处弯道曲率半径,导致了侵蚀加剧。其次,青草沙水库的建设使得北港上中段深泓线发生偏移,横沙通道北口水动力增强,这也是快速冲刷的主要原因之一。另外,长江口深水航道工程、横沙东滩围垦、横沙通道两侧圈围工程及港口码头的建设也一定程度加剧了冲刷之势。由此可见,人类涉水工程活动是引起该处大型冲刷坑快速发育的主要驱动因素。

Formation mechanism of near-shore erosional topography in the Hengsha passage of the Yangtze Estuary

The intense erosion of a near-shore riverbed is one of the main factors for the slope failure of a bank. During a detailed investigation carried out in August 2017 and May 2018 of underwater topography of the Yangtze Estuary, a large scour pit was recognized near the shore of the Hengsha passage. The morphological and geometrical parameters of the scour pit were measured using the SeaBat7125 multi-beam system. Dual-frequency ADCP was used to collect hydrodynamic data near the scour pit. Further, a historic nautical chart was digitalized to analyze the evolution and formation mechanism of the scour pit. The results indicated that the scour pit is in the shape of an oval, with a length and width of around 430 m and 150 m, respectively; the deepest point is approximately 38 m below the surrounding riverbed. Since the formation of the -20 m isobaths line in 1992, the scour pit area has been expanding continuously, and the average depth has been increasing yearly; in particular, after 2005, the depth increased sharply. From 1984 to 2017, the riverbed around the scour pit underwent the process of erosion-silting-erosion. In the 33 years, the scouring amount is 3.45×10 ⁷ m ³, and the average scouring depth is 4.68 m. The ebb tide from the North Channel flows into the Hengsha passage, forming a circulation flow, thereby eroding the channel near the west bank of the Hengsha island and forming a scour pit. After 2005, the scour pit grew rapidly and expanded southward. This is mainly because the reclamation engineering performed in North Changxing reduced the curvature radius of the bend that resulted in intensified erosion. The reservoir construction engineering performed in Qingcaosha moved the thalweg in the upper and middle sections of the North Channel, and the northward entrance of the Hengsha passage expanded owing to the ebb current, which is also one of the main reasons for rapid erosion. In addition, the construction of reclamation and deep-water channel projects in the vicinity has contributed to the erosion of the channel bed, thereby accelerating the expansion of the scour pit. It can be seen that human engineering activities are the main driving factors for the rapid development of large scour pits in the Changjiang Estuary.