30多年来人类活动对长江三角洲前缘滩涂冲淤演变的影响

Influence of human activities on the evolution of the tidal flat of Yangtze River delta front during the last 30 years

人类活动正愈来愈强烈地影响三角洲滩涂演变。近几十年长江三角洲前缘滩涂先后受到流域建坝等导致的入海泥沙锐减和河口一系列大型工程的影响。揭示三角洲前缘滩涂演变对这些重大人类活动的响应不仅可以为该区域的海岸管理提供支持,而且有助于加深人类活动驱动下海岸演变规律的理论认识。为了揭示30多年来长江三角洲前缘滩涂的阶段性变化,本文根据1977年以来多个年份的长江口口门区地形测图,利用GIS方法计算四大滩涂(崇明东滩、横沙东滩、九段沙和南汇东滩)面积的变化,分析阶段性变化的主要原因。结果表明:四大滩涂0 m等深线以上总面积的增长速率从1977-1983年时段的14.5 km2/a下降到1983-1994年时段的4.9 km2/a,继而回升至1994-2000年的12.2 km2/a和2000-2011年的26.4 km2/a;四大滩涂5 m等深线以上总面积的增长速率从1977-1983年的22.8 km2/a下降到1983-1994年的-1.4 km2/a,1994-2000年回升到5.2 km2/a,2000-2011年又回落至0.3 km2/a。前一阶段滩涂淤涨速率总体下降反映流域建坝等人类活动引起的入海泥沙减少在滩涂演变中起控制作用,且水下滩涂(5 m线)的响应较之潮间带滩涂(0 m线以上)更加敏感。后一阶段0 m以上滩涂淤涨速率的总体回升反映三角洲的一系列大型工程(特别是促淤工程)对潮间带滩涂起到了明显的促淤效果,且这种效果抵消并超过了流域入海泥沙通量下降的影响而居主导地位。总之,近三十年长江三角洲前缘淤涨速率的显著变化主要归因于人类活动的影响。另一方面,四大滩涂间的冲淤变化呈现较明显的差异,反映人类活动对三角洲滩涂演变的影响是一个复杂的问题,需要进一步研究。

Impact of human activities on the tidal flat of estuary delta has been getting more and more intense. During the last three decades, the tidal flat of Yangtze River delta front was influenced by two types of engineering projects. One was the building of the dams at the Yangtze valley, which led to the sharp cutoff of the sediments into the sea. The other was the series of large-scale coastal and estuarine engineering projects. To discover the evolution of the tidal flat responsive to these significant human activities can not only support to manage the estuary and the coast of the areas, but also help to bring about a better theoretic understanding of the evolution law of the coast driven by human activities （in the Yangtze Estuary）. In order to reveal the phasic variation of the tidal flat in the late 30 years, this paper calculated the areas of the four tidal flats （Eastern Chongming coast, Eastern Hengsha Coast, Jiuduansha and Eastern Nanhui coast） and analyzed the main reasons for the phasic variation of the tidal flat based on the topographic maps surveyed in the years from 1977 to 2011 by national professional department and other data. The results showed that the area growth rate of the four tidal flats above 0 m isobaths declined from 14.5 km2/a in the period of 1977-1983 to 4.9 km2/a in the period of 1983-1994, and then it increased to12.2 km2/a in 1994-2000 and to 26.4 km2 in 2000-2011. Similarly, the growth rate above 5 m isobaths decreased from 22.8 kmZ/a in the period of 1977-1983 to -1.4 km2/a in the period of 1983-1994, and it increased to 5.2 km2/a in 1994-2000 and then fell to 0.3 km2/a in 2000-2011. The general deerease ot lhe tidal flat growth rate in the preceding stage reflected a control role of deereasing sediments from the drainage basin into the sea, and the response of tidal flat underwater （5 m isobath） to it was more sensitive than that of mudflat of intertidal zone （0 m isobath）.The increase of its growth rate in the following stage indicateds that a series of large-scale coastal and estuarine engineering projeets had a clear effect of accelerative silting to the tidal flat, and the effect overtook the influence of the declining sediments and took the lead in the tidal flat evolution. In a word, the obvious change of the tidal flat growth rate in Yangtze River delta front attributes to human activities during the last 30 years. On the other hand, the changes of the four tidal flats as well as those in all phases show obvious differences, which reflects that the influence of human activities on the evolution of delta tidal flat is a complicated issue. Consequently, it is necessary to do further research.