Analysis Of Hydrodynamic Characteristics Based On Random Walk Model In Tonkin Gulf

Based on Random Walk Particle Tracking method, Random Walk Model (RWM) is established to study the hydrodynamics in Tonkin Gulf. Considering the diffusion effect, the RWM is able to avoid the possible deviation of water exchange induced by numerical oscillation in a convection-diffusion model and calculate the water exchange time such as residence time quickly. The RWM is validated through comparison of model output and analytical resolution of two-dimensional diffusion of instantaneous point source.First of all, the RWM is applied to study the water exchange in Tonkin Gulf with a hydrodynamic model. The obtained water exchange matrix and residence time show the similar seasonal variation between water exchange and circulation; The water residence time is short and exchange in each subarea is more active under monsoon; Guangxi Coastal Water residence time is longest and Honghe River Water residence time is shortest under northeast wind in winter; Guangxi Coastal Water residence time is shortest and stretches to the middle part of Tonkin Gulf.Secondly, the RWM is used to investigate the water transportation in Tonkin Gulf. The simulation result indicates that the two most important channels are Qiongzhou Strait and the southern mouth of Tonkin Gulf connecting open waters. In winter, with the influence of circulation and tidal current, water sourced from Qiongzhou Strait could affect the northeastern of Tonkin Gulf where Guangxi Coastal water characters are found, while water sourced from the west area of Hainan Island is restrained in Lin’gao subarea. In summer, water from Qiongzhou Strait moves anti-clockwisely with narrow flow width along the coast of Leizhou Peninsula and Guangxi. The water sourced from western area of Hainan Island flow northward40-50km, then turned west and northwest. Streamflow has significant influence on western local waters in the Tonkin Gulf. River plume effects waters20-70km off Vietnam coast with expanded range in winter. The river low discharge from Guangxi Province is restrained within20km offshore. Finally, the RWM is run to study the influence of typhoon on mesoscale migration of Tonkin Gulf water. The results showed that a significant deviation from the original trajectory is found on the right waters of typhoon path but little difference on the left water. Comparison of trajectories of water movement in the Tonkin Gulf during typhoon Haiyan with three different driving force schemes, tide only, tide plus monthly mean wind, and tide plus typhoon, indicate that typhoon Haiyan has strongest influence on transitional water and minimal impact on Vietnam southern coastal water. In addition, the tidal current is revealed to play a decisive role in water exchange in Tieshan Bay. Higher exchange rate was found at the upper layer comparing the bottom layer, as well as at the mouth and middle of Tieshan Bay with comparison of its top. Also, higher exchange is found in ebb tide than flood tide. Single neutral particle trajectories shows that water exchange in Ruliang Reserve is active. Shankou Mangrove Natural Reserve is affected by water from Shatian, Shitoubu and Baisha river.