1958—2015年长江口水下三角洲地形演变特征及趋势

Characteristics and trends of morphological evolution of the Yangtze subaqueous delta during 1958—2015

基于近期流域减沙背景下长江口水下三角洲地形的演变特征与趋势,选用1958—2015年覆盖面积超过7 000km^2的长江口实测水下地形数据,在Surfer软件支持下开展水下三角洲地形冲淤分析,探讨了水下地形演变影响因素。结果表明:1958—2015年,长江口水下三角洲经历了淤积—平衡—剧烈波动3个阶段;持续的流域减沙已导致10 m等深线以浅区域从2009年开始进入净侵蚀状态;流域来沙锐减与极端气候引发的口外及邻近海域对河口泥沙的补给是近年长江口水下三角洲冲淤变化剧烈的主要原因;长江口向外海年均输送泥沙量可能低于1.20亿t,与近年流域年均来沙量较为接近,未来长江口水下三角洲有望逐渐进入整体冲淤平衡状态。

How the topography of the Yangtze subaqueous delta has evolved in response to sediment decline has aroused wide attention. In order to examine the Yangtze subaqueous delta evolution and influencing factors during the past six decades,bathymetric data surveyed in eight different years during the period of 1958—2015 covering more than 7 000 km2 of the subaqueous delta in the Yangtze River Estuary were analyzed together with the river sediment dataset of the same period using Surfer software. The results show that the overall morphological evolution of subaqueous delta in the Yangtze River Estuary went through three phases： sedimentation, equilibrium, and intense fluctuation. The decline in sediment had led to net erosion in areas with depth less than 10 m since 2009. Sharp decline in sediment supply and sediments transported from the outer estuary induced by extreme whether are the two important reasons for the dramatic change in the erosion and deposition of the subaqueous delta in the Yangtze River Estuary in recent years. The annual sediment delivered from the Yangtze River Estuary to the open seas is likely to be less than 120 million tons in the past decades,which is almost equivalent to the sediment load in recent years,indicating that the entire subaqueous delta in the Yangtze River Estuary may gradually reach equilibrium state through internal adjustment in the future.