长江供沙量减少对水下三角洲发育影响的初步研究——近期证据分析和未来趋势估计

A preliminary study on the influence of Changjiang River sediment supply on subaqueous delta——Evidences in late 20th century and an expectation for the coming decades

根据1951～2000年大通站实测水文资料和长江口地形图,分析了长江入海泥沙量的变化趋势及其对水下三角洲冲淤演变的影响.从20世纪60年代末开始,长江入海泥沙量出现减少趋势,90年代输沙量相对于60年代下降了1/3.流域大量修建水库是导致河流入海泥沙减少的根本原因.在此背景下,长江口门外的水下三角洲淤积速率从1958～1978年时段的55mm/a下降为1978～1998年时段的11mm/a.考虑三峡工程等多种人类活动的可能影响,估计本世纪上半叶和下半叶的河流入海泥沙量将分别减少约60%和40%.尝试建立了三角洲冲淤对河流供沙量响应的概念公式,并据此对本世纪长江三角洲的演变趋势做了初步预测:三角洲的总体淤涨速率将急剧下降,口门外水下三角洲将出现严重侵蚀.

Data of sediment discharge from 1951 to 2000 and bathymetric maps of a major portion to the subaqueous delta surveyed in 1958, 1978 and 1998 are utilized to study variation in river sediment supply and morphological response to the subaqueous delta. Since the end of 1960s, sediment discharge at Sta. Datong has shown a significant decreasing trend, one third lower in the 1990s than in the 1960s. The reduction in sediment supply is found to have resulted from dam construction in the drainage. As a response to this reduction, vertical accretion rate of the subaqueous delta front has been decreased from (55 mm/a) during 1958～1978 to (11 mm/a) during 1978～1998. Meanwhile, severe erosion has occurred on the outer margin of the subaqueous delta. It is predicted that river sediment discharge will probably be reduced to 40% and 60% of the present (the average of the past (50 a)) respectively in the first half and the second half of this century due to human activities especially the Three Gorges Project. As a result, the total progradation rate of the delta will be dramatically reduced and local severe erosion may be inevitable.