Hydromechanical Characteristics Of The Radial Sandbanks In The Southern Yellow Sea

First.upon the former research work and field measurements in the radial sandbanks area, the characteristics of geomorphology, hydromechanics and the relationship between them are preliminarily analysed. Though adopting the σ-coordinate transformation in the control equations and mode-splitting technique, the external mode (surface gravity waves) is solved by the improved double-sweep-implicit finite difference method; the internal mode (internal gravity waves) untilizes the Eulerian-Lagrangian method, which improves the implicit-difference scheme for the vertical direction. The newly developed 3D numerical tidal model with implicit scheme has been successfully applied in the radial sandbanks area. Through studing the plane and vertical hydromechanical characteristics in the radial sandbanks area, the mechanisms of the formation, maintenance and asymmetrical distribution of the radial sandbanks are understood.It's well known that the directional to-and-fro movement and strong tidal current are the necessary dynamic conditions for the formation of tidal current sandbanks. The tidal current in the radial sandbanks area apparently has directional to-and-fro movement, which can be observed not only from tidal ellipses, but also from water particle traces. The calculated distribution of tidal velocity in this area shows that the average velocity is around 0.6 to 1.0 m/s. It has been proven that, from the propagation process of the tidal waves and wave energy rates, the whole radial sandbanks area is controlled by the progressive tidal wave in the south and the rotational standing tidal wave in the north. These two waves merge at Qianggang and induce a large tidal range. The distributions of the residual flow and the residual discharge from the numerical model show that the residual circulation flows around the sandbanks contribute significantly to the deposition at the peak of sandbank, and closely related to the sandbank development.The existence of the asymmetrical residual transverse circulation flows in pair is discovered in the residual vorticity field along the passage and sandbank cross section. The transverse residual circulation flow is favorable to sediment's moving to the peak of sandbank and the development and maintenance of sandbank. Using the Hulscher model of the formation of sand waves and sandbanks, under the ancient bathymetry, or under the modern bathymetry, the Stokes number of the tidal flow and resistance parameter in the radial sandbanks satisfy the conditions for the formation of sandbanks. The research demonstrates that the dynamic flow conditions result in the development of tidal current sandbank. The radial sandbanks and it's asymmetrical distributions will still maintain in the future.It has been proven that the differences of magnitude and distribution of tidal currents, wave energy rate, suspended sediment concentration and the Stokes number result in aasymmetrical distribution of sandbanks between the north and south. The sandbank surface area and length in the north are lager than those in the south, but its cross section area is less than that in the north.