PROSPECTS OF SEDIMENT MANAGEMENT OF THREE GORGES PROJECT

摘要

Engineering and research efforts resulted in working out a unique mode of reservoir operation for the Three Gorges Project(TGP): Keeping reservoir at the lowest pool level, Flood Control Level (FCL) 145m, in June through September of most years and thus releasing heavily silt-laden flood flow can minimize the extent, amount, and growth of sediment deposition in the fluctuating backwater region. In such a way, most of flood control storage capacity over FCL may remain for permanent use; and the retrogressive extension of coarse sand deposition at the entrance of reservoir as well as the rise of flood stage by a manageable magnitude at Chongqing may be predicted from mathematical model computations. Possible sedimentation troubles in the fluctuating backwater region harbors and waterways and engineering counter measures are studied in physical models. The model experiments verified the general layout of TGP with spillway's 23 deep outlets at low elevation, navigation guide walls, and a series of silt flushing sluices underneath power plant intakes and lock approaches provides spatial sediment control in the dam area. Downstream river channel degradation is predicted through mathematical models and counter measure works are suggested by experiments. TGP reservoir configuration in plane likes a long narrow ribbon, and the relatively very large dry period sub-saturated flow discharge are two outstanding peculiarities limiting retrogressive extension of sediment deposition. Besides, the quite small ratio of TGP reservoir storage capacity to annual runoff leaves a wide flexible margin for adjusting either the right time of reservoir filling up.or a favorable spot as the entrance to the fluctuating backwater region. Localizing the uppermost point of TGP reservoir within a narrow gorge may result in none retrogressive extension of coarse sand deposition like in Danjiangkou reservoir or Gezhouba reservoir. Human activities as on going cascade development of the Jialingjiang river, and mining of construction materials, gravel-pebbles-sand, from Yangtze river upstream of TGP reservoir in the past decades, as well as the foreseeable impoundment of Xiluodu Project on the Jinshajiang river in next decade, remarkably reduce the incoming sediment load to TGP reservoir, their effects are roughly analyzed in this paper.