长江口第二、三代冲积岛浅滩演变特征分析

Evolution characteristics of the shoals of the second and third generation alluvial islands in the Changjiang Estuary

利用GIS对 1879～ 2 0 0 0年 7幅长江口地区海图数字化 ,建立各时期长江口水下数字高程模型 (DEM) ,在此基础上 ,对长江口第二、三代冲积岛———长兴岛、横沙岛和九段沙浅滩的面积、体积、冲淤量、冲淤厚度和等深线水平推移速率进行计算。结果表明 :近 15 0年来 ,长江口第二、三代冲积岛浅滩总体上呈淤涨趋势 ,并沿东南方向向海推展。 1879～ 2 0 0 0年间共淤积泥沙 19 4 8亿t,平均每年淤积 0 16 1亿t,约占大通泥沙通量的3 72 % ,年均淤厚 0 83cm。不同时期淤积速率不一样 ,有的时期甚至发生一定程度的冲刷。各个浅滩的冲淤状态不尽相同 ,长兴岛浅滩和横沙浅滩呈冲刷状态 ,横沙东滩和九段沙呈淤积状态。浅滩的自然演变是长江河口发育过程中重要的一环 ,受长江流域来沙量的制约。随着流域开发强度日益增大和河口工程日益增多 ,人类活动对冲积岛浅滩的影响越来越强烈 ,浅滩的自然演变过程甚至会发生改变。

Supported by GIS, 7 pieces of charts of the Changjiang Estuary from 1879 to 2000 are digitized and Digital elevation model (DEM) of the Changjiang Estuary in different periods is established with Kringing gridding method. Based on that the area, volume, the amount and thickness of erosion_accretion and bathymetric contour progradation rate are calculated. The results show that the trend of the shoals of the second and third generation alluvial islands in the Changjiang Estuary in the recent 150 years is that deposition as a whole and progradation seaward along southeast with deposition rates faster than that of previous time, but deposition rates in different terms were different and erosion occurred in a few terms. About 1.948 Gt of sediment was deposited between 1879 and 2000, with average annual deposition rate of 16.1 Mt, which amounts to 3.72% of sediment from the Changjiang River, and average annual thickness of 0.83cm respectively. Hengsha east beach and Jiuduansha are the main positions of siltation, while the shoals of Changxing Island and Hengshaqiantan are the main positions of erosion. As an important chain of the evolution of the Changjiang Estuary, the natural evolution of the shoals is controlled by the sediment supply from the Changjiang basin. With the increasing intensity of development in the basin and more and more projects constructed in the estuary, the impacts of human activities on the shoals in the study area is becoming more and more intense, the natural evolution of the shoals may reverse.