Studying On The Dynamic Formation And Variation Mechanism Of Freshwater Zone Near Meimaosha In The Changjiang Estuary

Based on ECOM-si, a three-dimensional hydrodynamic and salinity numerical model is developed in Changjiang estuary. A high resolution grid of Bohai Sea, Yellow Sea and East China Sea is designed by considering real coasts and all kinds of dynamical genes synthetically, to simulate and analyze the impacts on MeiMaoSha freshwater zone of Changjiang River estuary in winter season by runoff, tidal current, wind and coriolis force. This paper includes three parts:(1) The observed data shows that a coastal and discontinuous fresh water zone is found in MeiMaoSha area of south passage in Changjiang estuary in dry season. It occurred more frequently during spring ebb tide.(2) By considering high resolution grids a three-dimensional hydrodynamic and salinity numerical model of Changjiang River estuary has been set up. The calculate area of model include Bohai Sea, Yellow Sea and part of East China Sea. The velocity, direction, salinity, and tidal elevation results of validation are well coincide with the measurement of Feb 2003 and Aug 2005. It can correctly simulate the hydrodynamic process and salt water intrusion of Changjiang River estuary.(3) Many dynamical factors such as tide, runoff, Coriolis force, wind force, topography and shelf circulation is considered. The numerical simulation has verified the appearance of the coastal and discontinuous fresh water zone once again. The results of the numerical experiments show that the most direct and important dynamic factor is runoff volume, the fresh water zone is significant as large runoff flow, and disappear during small runoff volume. The upper stream fresh water is carried to MeiMaoSha area by advection of ebb tidal current, and it is most remarkable during spring tide. The strong ebb tidal current can carry farther upper stream water to MeiMaoSha area.it leads to more fresh water during spring ebb tide than neap ebb tide. Simultaneously, the lower high-salinity water is carried to here by advection of flood tidal current. Compare to neap flood tide, it causes the salinity serious increase during spring flood tide. This is the action mechanism of tidal pumping. In dry season, longshore EKMAN flow is formed by north wind in furcated estuary, it leads the current inflow in north channel and outflow in south channel, the salt water intrusion from out sea in south passage is blocked off by this wind-driven circulation. The wind takes greater influence in neap tide compared with spring tide. Furthermore, Coriolis force is favorable for the formation of the fresh water zone, but the effect is smaller than tidal pumping and wind force.