Numerical Study For The Impacts Of The North Branch Narrowing On Sediment And Salinity Distributions In Yangtze River Estuary

The hydrodynamic environments of Yangtze River estuary are complex and changeable. Due to the effects of the reclamation on shoal and the evolution of erosion and deposition of riverbed, the phenomenon that saltwater intruded into the south branch from the north branch has occurred since 1950s. This problem has become more severe since 1980s and affects the social activities seriously. In order to solve this problem, a reclamation project named "narrowing of the middle of north branch" was proposed. However, the construction of reclamation will surely change the hydrodynamic environment and distributions of sediment concentration and salinity in this region.This dissertation established the numerical model of flow field, sediment concentration and salinity in Yangtze River estuary using ECOMSED numerical model. And simulate and analyze the variation characteristics of flow field and distributions of sediment concentration and salinity in Yangtze River estuary before and after the reclamation. It concludes that:(1) The simulation results of flow field, distributions of sediment concentration and salinity match the factual observation data well. It indicates that the numerical model established in this essay has a good capability to simulate the flow field, distributions of sediment concentration and salinity in Yangtze River estuary.(2) After the reclamation, the channel of the north branch is narrowed on a large scale. Due to the narrowing of the channel, the tidal dynamic is weakened a lot which lead to a reduction of water level. This change is helpful to prevention of waterlogging. The narrowing of the north branch causes a slight increase of the current velocity in the north branch during spring tide and notable reductions of the current velocities during moderate and neap tide. Except the spring tide period, the current velocity in the north branch has a overall reduction.(3) After the reclamation, the sediment concentration in the north branch has some reductions, meanwhile, the increase of current velocity during spring tide also benefits the desilting in this area.(4) After the reclamation, the decrease of tide prism in north branch cause the reduction of salinity in north branch water area. And it reduces the salinity in the water coming from north branch and will invade into the south branch which will contribute to solve the problem of flowing backward of saltwater. (5) After the reclamation, the hydrodynamic environments, the sediment concentration and salinity of north branch water area have significant changes. The hydrodynamic environments and sediment concentration of the south branch don’t changes, but the salinity in this area has a slight reduction. The water level, current velocity, sediment concentration and salinity of the entrance area do not change obviously.