三峡大坝建成后长江输沙量的减少及其对长江三角洲的影响

三峡大坝建成之后,大量泥沙滞留于库区,出库泥沙量减少,坝下河床冲刷而提供相当数量的泥沙,支流湖泊供沙也发生变化,这将使进入河口地区的泥沙有所减少。三峡大坝以上长江干流和支流建设新的大坝,南水北调、封山育林、退耕还林以及减少水土流失都将进一步减少长江进入河口地区的泥沙。由此估计,三峡大坝建成后的百年内长江输入河口地区的泥沙约为2.0×10⁸～2.5×10⁸t/a;冰后期长江三角洲形成和发育期间的长江年均输沙量为1.84×10⁸～2.28×10⁸t。二者的数值相当接近,然而与近50年的观测(4.33×10⁸t/a)相差甚远,长江流域的气候变化和人类活动可能是造成这一现象的原因。文章着重说明中国和长江上游人口的增长、种植作物的改变可能是水土流失、长江泥沙量增长的主要原因。

THE CHANGE IN CHANGJIANG SUSPENDED LOAD AND ITS IMPACT ON THE DELTA AFTER COMPLETION OF THREE GORGES DAM

Based upon the data obtained from hundreds of investigation sections on middle and lower reaches of the Changjiang River and from mathematical and physical models, the sediment discharge at Datong in 100 years after the Three Gorges Dam (TGD) completion has been predicted, which is 3.84×10 ⁸ t/a on average. Considering the factors for reducing sediment discharge, such as South to North Water Diversion Project, construction of reservoirs on the river and its tributaries, afforestation of the hillsides and convertion of cultivated land back to forest in the basins in the Changjiang upper reaches, and decrease of the population in 21 th century, the suspended sediment load at Datong is estimated to be about 2.0×10 ⁸～2.5×10 ⁸ t/a after TGD completion. The sediment discharge at Datong, calculated from drillcores in the Changjiang Delta, was 1.84×10 ⁸～2.28×10 ⁸ t/a during the postglacial period, which is close to that predicted in 100 years after the dam completion, and is deviated from that measured at Datong during the last decades. Examination of the environmental change in the Changjiang basin that affected by population growth, deforestation, soil erosion etc. in Chinese historical time leads to conclude that the predicted suspended discharge in 100 years after damming may be more representative for the Changjiang during the period of delta formation and evolution. The suspended load at Datong, measured during the last five decades, is only a record of a short period with highest suspended load in the history of the Changjiang development, resulted from abrupt population growth, serious deforstation and severe soil erosion. The Changjiang Delta would not subject to erosion in 100 years after the completion of the Three Gorge Dam. However, accelerated tidal flat reclamation project in the Changjiang Delta might likely trigger a serious coastal erosion of adjacent provinces, where the sediments of coastal zone is transported from the Changjiang estuary.