近百余年来长江口南支水下地形演变特征分析

长江口是我国最大的河口,监测数据有限且水沙变化和水下地形演变复杂,开展长江口各分区演变规律、演化趋势和影响因素研究至关重要。利用长江口南支1861～2013年间7个年份地形数据,建立了近百余年来长江口南支的水下地形数字高程模型(DEM),探究长江口南支整体冲淤演变规律、不同子研究区冲淤变化时空差异、典型断面发展演变特点,并从自然演化和人为活动干预角度分析了特大洪水、径流输沙、人类活动等因素对研究区水下地形演变的影响。结果表明:①长江口南支在1861～2013年间整体处于冲刷状态,呈现淤积-冲刷-淤积-冲刷的变化过程,各子研究区的冲淤变化存在明显的时空差异特征,只有崇明浅滩淤涨不断。②1861～2013年间,南支上段沙洲、槽沟摆动频繁,冲淤变化较为复杂;南、北港经历了一系列主次更替的变化过程;横沙东滩呈现上段冲淤变化不定、中段先冲后淤、下段相对稳定的演变特点;九段沙表现为不断淤高扩宽且下延的趋势。③长江口南支的演变是自然演化和人类活动共同作用的结果,1861～1958年,南支水道的冲淤变化主要以自然演化过程为主;1958～2013年,流域人类活动引起的长江来沙减少及口内工程建设对南支冲淤演变的影响显著。研究成果可为长江口的治理规划提供技术支撑。

Morphological evolution analysis of South Branch in Yangtze Estuary over past hundred years

Based on 7 years (from 1861 to 2013) bathymetric data of the Yangtze Estuary, a time series of digital elevation model of the submerged South Branch spanning more than one hundred years were established. This research analyzes the general erosion-accretion characteristics of the South Branch, the spatio-temporal variations in different sub-regions, and temporal changes of elevation along the typical cross sections. After that, the research focuses on the impact of extra-large flood, runoff and sediment load in the perspective of natural evolution and human activities on the evolution of submerged morphological evolution. The results show that: (1) In general, the study area was eroded during 1861~2013 in circulations of erosion-deposition. The spatial-temporal erosion and deposition characteristics of sub-regions varied significantly, except that accretion in Chongming shoal remained constant in this period. (2) In the past more than one hundred years, the sandbars and gullies in the upper South Branch were frequently modified by the complicated erosion-accretion process. The South and North Channels in the South Branch underwent a series of changes of primary and secondary exchange. The evolution of the three sections of the Hengshadongtan shoal was controlled by different mechanisms: the upper section has a noisy pattern of erosion and accretion alternation; the middle section were eroded first and then silted; the lower section was relative stable. The Jiuduansha shoal region showed a silting, widening and downward extending trend. (3) The evolution of the South Branch of the Yangtze Estuary was a product of the coupled nature-human system. The South Branch evolved naturally from 1861 to 1958, however after 1958, human activities took the leading role of changing the erosion and accretion pattern by drastically reducing the sediment load. The study results will provide technical support for the governance and control of the Yangtze River estuary.