Long-distance recovery of nonuniform suspended load in the Middle Yangtze River owing to upstream damming

Abstract

The amount of suspended sediment entering the Middle Yangtze River (MYR) was greatly reduced owing to the operation of the Three Gorges Dam (TGD) and other upstream dams, which has caused the process of sediment transport in the MYR at a severe non-equilibrium state. A detailed investigation was conducted into the transport of nonuniform suspended load in the MYR using about 30-year observed data: (i) firstly, the spatiotemporal variations in fractional sediment fluxes were investigated during the pre- and post-TGD periods. The total sediment flux decreased after the TGD operation, and the magnitude for the coarse fraction (d > 0.125 mm) was smaller than those for the fine (d < 0.031 mm) and medium (d = 0.031–0.125 mm) fractions; moreover, the sediment flux for each fraction continuously recovered along the 955-km-long reach, with the recovery distance for the coarse fraction being much shorter than the other two fractions; it was mainly attributed to the fact that the riverbed could supply abundant coarse sediment to the flow, and this fraction experienced a considerably higher recovery degree than the other two fractions; (ii) secondly, the sediment budgets for the fine, medium and coarse fractions were calculated to assess the contributions of different sediment sources to the recovery of graded sediments in the MYR. About 50 %, 29 % and 14 % of the increased fractional sediment amounts for fine, medium and coarse fractions were originated from the tributary confluence, while the remaining sediment was supplied from the riverbed during the process of channel erosion. Therefore, the recovery of suspended load was largely attributed to the supply of sediment from the riverbed, especially for the coarse fraction; (iii) finally, the impacts of the altered flow-sediment regime and the bed-material composition on sediment recovery efficiency were analysed. The sediment recovery efficiency greatly increased after the TGD operation, which was caused by the reduced sediment concentration; and the sediment recovery efficiency for the coarse fraction was much larger than those for the fine and medium sediment fractions. It is attributed to the fact that the channel can provide abundant coarse sediment to the flows in the MYR, while the sediment recovery efficiencies of fine and medium fractions were restricted by the low degree of sediment supply from the riverbed.