长江流域重大工程对滩涂冲淤变化影响及潜在土地资源的可持续利用

上海陆域面积现约7 000 km2,其中约65％是近6 000年由长江人海泥沙堆积而形成,约17.5％为1949年以来通过围涂新增的土地,后者极大缓解了上海市尖锐的人地矛盾关系.近几十年来,尤其是三峡大坝建成后长江年均人海泥沙已大幅减少,由1951-1968年间的4.97亿t降低至2003-2008年间的1.54亿t,今后随着流域内更多大型水利工程的实施完成将进一步降低.长江人海泥沙是建造滩涂的物质基础,随着人海泥沙大幅减少,河口滩涂自然淤涨速率趋于减缓,未来整个三角洲的冲淤演化趋势备受关注.以2005年为基准年,Delft3d模型预测结果认为长江口主要浅滩今后20年将继续向海淤涨,但速度减缓.综合分析认为,上海市今后20年可通过工程促淤770 km2,圈围485 km2,但滩涂资源利用一定要兼顾河口生态环境保护,努力实现人与自然相和谐的可持续发展.

Impact of Mega-Engineers in the Changjiang Drainage Basin on the Deposition and Erosion of Coastal Wetland, and Sustanability of Land Reclamation

Shanghai land area is about 7 000 km2, of which 65% has been built up by accumulation of Changjiang riverine sediment in last 6 000 years, and 17.5% is newly reclaimed land from coastal wetland. Land reclamation has greatly eased the tense relationship between socioeconomic development and available land resource in Shanghai, effectively slowing the reducing magnitude and speed of farmland area. In the recent decades, Changjiang sediment discharge has dramatically decreased especially after the completion of Three Gorges Dam, from 497 mt?yr-1 in 19511968 to 154 mt?yr-1 in 20032008. The decreasing trend will continue due to more major hydraulic engineerings under construction or planning in the drainage basin. Because of sharp decrease in Changjiang sediment discharge, coastal wetland has been observed to slow aggradation and erosion area undergoes expanding in the subaqueous delta. It is still questionable whether and when the Changjiang Delta will turn from net accretion into net erosion. Delft3d numerical model has been employed to simulate trend development of major mudbanks and river mouth bars in the next 20 years with a reference year in 2005. The result shows that coastal wetland above 5 m isobaths will continue its accretional trend whereas the rate will decrease. It is proposed that an area of 485 km2 and 770 km2 can be newly reclaimed or promoted sediment accretion by engineering. However, coastal wetland should be evaluated in more objective and synthetical ways, typically considering the coastal biodiversity conservation, for the sustainability of both human and natural societies.