Abstract:

The Yangtze Estuary (YE) is a complicated dynamic system influenced by both huge river inflow and medium tides Large amounts of sediments input have built large subaerial and subaqueous deltasis The annual averaged discharge in Datong hydrological station (DT) is 28300m3/s (1950-2009). The YE receives 389 million tons sediment annually (1951-2009)，which is material filling the Holocene incised Yangtze valley and building the modern Yangtze Delta Previous studies usually consider the discharge of DT (tidal limit of the YE) as the discharge of the Yangtze River into the sea However，DT is still 540 km from the mouth of the river Xuliujing station (XLJ) is about 500 km downstream DT，and since 1960s，the channel in XLJ has been stable XLJ is also the starting point of threeorder bifurcations and four outlets of the YE So the runoff and sediment flux of XLJ are more representative for intosea flux than those of DT In addition，after the third construction phase (2004-2009)，all construction of the Three Gorges Reservoir (TGR) has nearly been finished and most of its units have been into operation，and then the normal reservoir storage water level will reach 175 m The flood control reservoir storage capacity will be 2215 billion m3 The impoundment of the Three Gorges project and [JP2]the constant drought situation (2006-2009) [JP]in the Yangtze River basin jointly result in that downstream water flow becomes very low，which will bring certainly corresponding impacts on the runoff and sediment flux into the YE So it is urgent to understand what the respondse of the YE to these change is However，in view of the complexity of the flow and sediment movement in the YE，the impacts of the change of water and sediment environment in the Yangtze River basin on estuarine flow and sediment processes have not been clearly studied In this paper，we try to examine the hydrodynamics and sediment dynamics at XLJ in the YE in the circumstances of low waters in the Yangtze River basin and the TGR impoundment in 2009 based on field data The analysis shows that the runoff reduction during the impounding period is caused by both regional dry climate (87%：the proportion of the upper reach of the TGR (its lower boundry is Cuntan station) and that of the basin below the TGR are 38%，49%，respectively) and TGR impoundment (13%). The flood tidal duration increases by 05 hour and mean tidal range increases by 04 m due to reduced discharge The mean flood tidal velocities increase by 35%-126% and ebb tidal velocities in the channel decrease by 22%. These results indicate that the tidal asymmetry is lessened during lower discharge period The suspended sediment concentrations (SSCs) decrease from 0129 kg/m3 in preimpoundment to 0052 3 kg/m3 in postimpoundment period Correspondently，the median suspended sediment grain size (D50) increase from 3 μm to 5 μm Overall，it is concluded that the hydrodynamics and sediment processes at Xuliujing are noticeably influenced by the upstream hydrologic regime which undergoes changes in both climatic aspects and TGR regulation.