Analysis On The Riverbed Morphodynamic Process In Front Of The Qingcaosha Reservoir And Precaution Of The Dyke Instability Risk,Changjiang Estuary

As the joint between land and sea,estuary is of vital importance for local and global development owning to its unique natural and human resources such as biodiversity and waterway.Therefore,estuarine morphodynamic process in response to intensive anthropogenic activities is one of the significant issues of the Land and Ocean Interactions in the Coastal Zone and Future Earth Coasts program.With the recent construction of hydraulic engineering,the upstream water and sediment inflow conditions have changed,which induce variations of energy distribution and sediment transport amount along the downstream,and further affect the natural pattern of estuarine landform,thus result in grim challenge to today’s science and society.As one of the largest water projects along the Changjiang River in recent years,Qingcaosha reservoir(QCSR)is not only the largest freshwater source to Shanghai,but also the largest reservoir for avoiding saltwater and storing freshwater in China’s tidal estuary.Located in the South and North Channel bifurcation(SNCB),QCSR connects to the South Branch and dominates the river regime of the South and North Channel.Area in front of QCSR,being filled with frequent changing underwater sandbar,indicates complicated river regime and unclear evolution tendency,which dramatically affect the channel embankment stability and drinking water supply to Shanghai.Thus,it is of great theoretical and practical value to research on the morphodynamic process of the area in front of QCSR in response to the reservoir regulation.Based on in-site hydrological and sediment measurements,including flow velocity,flow direction,sediment load and sediment gradation,this study analyzes the hydrodynamic and suspended sediment distribution features of QCSR region in the North Channel(NC).Meanwhile,bathymetric data in 2000,2002,2004,2007 and 2009-2013 around NSCB area are collected to study the channel geometry variation and stability characteristics as well as the erosion-deposition process of channel bed that in front of QCSR.The main results are summarized as follows:The tidal current in the North Channel is of typical alternating current with predominant ebb current.Both the duration and velocity of the ebb are larger than those of the flood.The flood and ebb tidal force in dry season is respectively stronger and weaker than flood season.Suspended sediment concentration(SSC)during ebb tide is higher than spring tide in the study area(area in front of QCSR).The flood and ebb SSC in dry season is larger than flood season.In general,the SSC decreases in vertical from the bottom to the surface.During the slack time,however,there is no obvious vertical distribution characteristic in SSC due to disordered vertical velocity.Meanwhile,high flow velocity,strong flow dynamic and sediment-carrying capacity are observed at urgent rising and falling tide period,but relative small flow velocity and sediment load are detected at slack time.SSC around the Xinqiao Passage is lower than the area around the lower part of the QCSR.Grain size of suspended sediment in the area around Xinqiao Passage is coarser while that around the lower part of the QCSR is finer.In view of the suspended sediment particle composition,the SNCB reach has relative coarser sediment(fine sand and sand)with good sorting while the main channel of the study area has relatively fine sediment(fine sand and silty sand)with relatively good sorting,indicating that flow dynamics in the main channel is weaker than SNCB.Channel bed of the study area indicates different evolution patterns:the upper part erode first and then deposit,while the middle and lower part deposit continually with a gradually weakened intensity.The main deposition area are the tidal flats in front of QCSR and the lower part of Baozhensha.Deposition in the main channel of NC is invisible.The erosion and accretion processes of the study area present significant difference before and after QCSR construction:in the pre-QCSR period,channel erosion and shoal deposition are observed with quasi-erosion-deposition oscillations;in the post-QCSR period,continuous erosion is detected with a decreasing intensity.Meanwhile,the developmental sand bodies in the study area originally exhibite accretion and subsquently eroded track,thereafter reach an equilibrium state.While the clocked sand bodies mainly locate around SNCB and the frontal area of the reservoir dyke,with some distributing near Baozhensha.Both of them move downstream along the ebb tide direction.Moreover,the channel configuration shifts from U-shape to compound shape by coupling U-shape and V-shape.Specifically,the upper area indicates tidal flat erosion;the middle area is under the balanced state while the lower area combines the U-shape and V-shape channel.It is revealed that the area approximately 600 m ahead of the study area is the key risk area for riverbed instability,especially the river bed along the upper and middle part of QCSR dyke.There is a possibility of further erosion in the river bed if the channel,where the frontier slope locates,gets closer to QCSR.