近30 年来长江口岸滩沉积物与地貌演变特征

通过对长江口潮间带宽度、表层和浅地层柱状样沉积物、典型水下岸坡剖面的现场测量,以及对潮间带宽度及水下岸坡剖面的多幅历史海图分析,研究近30 年长江口岸滩演变特征及岸滩演变的主要原因。结果表明:(1) 岸滩沉积物整体上呈“北细南粗”的变化特征,横沙东滩北岸、长江和东海大桥下等主要大型工程附近潮间带柱状样粒序向上显著变粗;(2) 长江口部分潮间带宽度减少90%以上;水下岸坡由1980 年的“S”形向凹形、斜坡形等侵蚀型形态转变,岸坡明显变陡,侵蚀型岸滩特征凸显;南汇嘴至芦潮港岸段水下岸坡闭合水深1978-2004 年为-8 m,近10 年来滩底发生侵蚀,刷深约1 m;(3) 流域来沙量锐减导致长江口侵蚀型水下岸滩格局的形成;河口大面积围垦、青草沙水库和长江与东海大桥等大型工程建设是造成潮间带大面积损失和局部沉积物粗化的主要原因;沙体变迁是狭长型河槽局部岸滩演变的主要影响因素。

Sedimentary and morphological evolution of nearshore coast of Yangtze Estuary in the last 30 years

By analyzing the status quo of tidal flats and nearshore areas, including the sedimentary and morphology with different historical charts and observations of the Yangtze Estuary, the paper discusses the evolutions of nearshore coast of the Yangtze Estuary and its influencing factors in the last 30 years. The results show that: (1) the sediments collected on the tidal flats in the northern part of the Yangtze Estuary changed from coarse to fine, while sediments of the southern part became coarse, especially those near the large construction sites, including the north bank of Eastern Hengsha Shoal, Yangtze River Bridge and East China Sea Bridge; (2) the width of tidal flats in certain parts of the Yangtze Estuary decreased by more than 90%; the morphological characteristics of nearshore profiles were characterized by erosion, including concave and steep slope; the depth of closure (D℃) of Nanhuizui nearshore area from 1978 to 2004 kept at -8 m, while it was scoured by about 1 m because of the bottom erosion in recent 10 years; (3) the erosion characteristic of nearshore area in the Yangtze Estuary resulted mainly from the reduction of basin sediment loads; intensive human activities, including reclamations, Qingcaosha reservoir and bridge constructions resulted in the reduction of tidal flats and the coarsening of sediments; sand ridge migration had a great impact on the nearshore coast of the channels in the Yangtze Estuary.