Abstract:

Estuaries, the junctions of rivers and seas, are complex and special areas where exchanges of fluvial and marine sediments take place. Yangtze River is the largest river on the Eurasia continent. The South Branch is the main channel of Yangtze Estuary, by which more than 98% of discharge getting disembogued into the sea. In the context of dramatic decrease of sediment load from the river basin, the aim of this paper is to gain better understanding of temporal change of suspended sediments due to the construction of the Three Gorges Project (TGP) and seabed sediment resuspension processes in the South Branch of Yangtze Estuary and to enhance our knowledge of geomorphic evolution in the South Branch. By analyzing tidal velocity, suspended sediment concentration (SSC), suspended sediment particle (SSP) and bed load observed at the lower reach of the South Branch of Yangtze Estuary from June 26th to July 3rd in 2011, the characteristics of temporal change of suspended sediment and seabed sediment resuspension processes were studied. The results indicated that in the lower reach of the South Branch hydrodynamics during the ebb tide was stronger than that during the flood tide, due to the tidal asymmetry caused by runoff and tide wave deformation. SSC and SSP rose from flood tide to ebb tide. From neap tide to spring tide, remarkable increases of tidal velocity, SSC and SSP were observed. When it was closer to the seabed vertically, SSC and SSP increased vertically and reached the maximum value in the nearbottom layer. The SSC and SSP fluctuated in a tidal cycle drastically. There were two SSC peaks near the seabed sometimes during a single flood and ebb period. The bed load was mainly composed of sand, whose content was over 90%. The resuspension of the bed load in the lower reach of the South Branch was strong, but the sediment supply from the bed load to the suspended sediment was limited. Excellent correlations were found among the tidal velocity, SSC, SSP and bed shear stress. Along with the alternating between the flood tide and the ebb tide, time series curves of the tidal velocity, SSC, SSP and bed shear stress all showed similar patterns of tidal change. Peak and valley corresponded to each other closely, meanwhile, the existence of hysteresis effect was evident at most of time. The decrease of SSC and change of relation between water and sediment in the past few years were associated with the recent sharp drop of sediment load from the river basin. The results suggest that the seabed sediment in the south branch of Yangtze Estuary had entered a more frequent activity period