近30年来长江口水下三角洲地形演变与受控因素分析

Erosion-accretion changes and controlled factors of the submerged delta in the Yangtze Estuary in 1982-2010

利用长江口1982-2010年间4个年份的高精度实测地形数据、大通站输沙量数据以及河口工程建设信息等,分3个阶段研究了长江口整体冲淤规律、不同分区冲淤变化时空差异、重点断面垂向冲淤特征、水下三角洲重要区域(崇明东滩、横沙浅滩、九段沙以及南汇东滩)的演变趋势与受控因素。结果表明:(1)近30年长江口整体上表现为略微冲刷,冲刷面积占51.83%,在3个阶段中呈现出"冲刷—淤积—再冲刷"的冲淤演化态势,且冲淤变化时空差异显著,仅长江北港区为持续冲刷。(2)崇明东滩和横沙浅滩东侧和北侧、九段沙北侧以及南汇东滩的5 m以浅区域以垂向淤积为主,而崇明东滩和横沙浅滩南侧、九段沙东侧和南侧的5 m以浅区域则以垂向冲刷为主。(3)长江口重要区域2 m和5 m等深线包络面积整体上是不断增加的,1982-2010年时段内年均增加量分别为10.42 km2/a和7.99 km2/a,但各分区则表现不一致;10 m等深线包络面积在整体上及各个分区内均表现为明显减小;15 m等深线包络面积变幅较小,基本保持稳定状态;(4)受河口工程建设影响,仅崇明东滩水下三角洲等深线包络面积减小的趋势与大通站输沙量减少趋势相同。主要结论:从长江口水下三角洲等深线包络面积和分布形态的变化看,基于目前上游输沙量持续减少的宏观自然背景下,大规模的河口工程建设对近年来长江口水下三角洲演变的控制作用日益明显。

This research analyzed the general erosion-deposition characteristics of submerged delta front, the spatio- temporal changing pattern in different sub- regions, the geomorphological changes in the typical cross-sections, and the geomorphological evolution and corresponding controlled factors of the four important sub-regions （i.e., the eastern tidal wetland of Chongming-CM, the Hengsha shoal-HS, the Jiuduansha wetland-JDS, and the eastern tidal wetland of Nanhui-NH）. Data sources include high-resolution DEM, sediment load data at Datong Station, and corresponding estuary construction information. Major findings are： （1） in general, the study area was slightly eroded （51.83% vs. 48.17%） in the past 30 years; specifically, it experienced an ＂erosion-deposition-erosion＂ pattern in the three periods; the geomorphological change was significant both spatially and temporally in the eight sub-regions, with an exception of the North Channel staying eroded; （2） the eastern and northern parts of CM and HS, the northern part of JDS, and the region within the 5m-isobath of NH were mainly dominated by the vertical deposition, while an opposite trend in southern CM and HS, and the region within the 5m-isobath of eastern and southern JDS; （3） on the whole, the envelop areas of 2 m and 5 m isobaths of the four important sub-regions kept increasing, and the average annual growth rates reached 10.42 km2/a and 7.99 km2/a respectively in the period of 1982-2010 （however, four sub-regions exhibited disagreements）; the envelop area of 10m-isobath decreased in the whole region as well as sub-regions; the envelop area of 15m-isobath changed slightly during the period and remained stable; （4） influenced by the estuarine engineering, CM was the only station whose isobaths envelop area decreased, in accordance with the decline of the sediment load at Datong Station. Major conclusions are that estuarine engineering plays an increasingly important role in affecting the geomorphological change in the submerged delta under the macro-background of the reduction of sediment load, in terms of change in the envelop areas and the distribution of isobaths in the Submerged Delta. Along with the accelerating construction of Shanghai International Shipping Centre and reclamation engineering, the geomorphological evolution of submerged delta will become more complex and thus deserves frequent monitoring in the future.