Research On Residual Characteristics And Its Driving Factors In Xiangshan Bay

As the famous aquaculture base in Zhejiang province, Xiangshan Bay draws much attention because of its water environment situation. As the dynamic foundation of the mass transportion, the tide current plays an important role in the transportion of the dissolved substances and the suspended materials. Although the magnitude of residual current is small, it indicates the transportion and exchange condition of water, besides, it is closely associated with the long-period transportion, diffusion and deposition. By studying the characteristics of tidal residual current, we’ll have a overall understanding to the transportation character mechanism and a new solution to solve the increasingly marine environment.In this paper, a numerical model is applied to study the distribution of residual current. In order to study the impact of tide, wind filed and the runoff to the residual current of Xiangshan Bay,6 calculation conditions are given and a comparison is made with the measured residual current. Besides, a typical point is selected to calculate the distrution characteristics of tide-induced, wind-driven and runoff-thermohaline residual current. The major archievement and conclusion are:(1) In the surface, the tide-induced residual current flows into the Xianshan Bay along the below of Fodu Channel, and flows out along the Niubi Channel(5-31cm/s). In the bottom, the direction of the tide-induced residual current is same to the top with the 60% in magnitude,meanwhile.In the surface, the direction of wind-driven residual current basically coincides with the the direction of wind, which flows out along the Fodu Channel (4-12cm/s). In the bottom, the wind-driven residual current keeps the opposite direction with the wind(2-8cm/s). Compared with the simple wind-driven residual current, the residual induced by wind and tide changes a lot in direction.In the surface, the direction of runoff-thermohaline residual current from the top to the mouth points to the out-bay, and flows out along the Niubi Channel(2-6cm/s). In the bottom, the tide-induced residual current is devided into 2 branches after entered into Xiangshan Bay by the Fodu Channel, one flows into the top along the channel, another flows to the sea along the Niubi Channel(1-4cm/s). Compared with the simple runoff-thermohaline residual current, the residual induced by runoff and tide keeps the same direction in the surface, the result is the increasing residual current(4-30cm/s). In the bottom, the direction of the simple runoff-thermohaline residual current keep contrast with the tide-induced residual in the area between the top and the mouth, the result is the decreasing residual current, meanwhile the increasing residual current exists outside the mouth (3-25cm/s).(2)By comparison with the 15 measured residual current data which is obtained by harmonic analysis of current data, we can find the distribution characteristics of residual current is basically the same. A circulation is generated in vertical in the main channel, while a horizontal flow outside the mouth.(3)To analysis the role of tide, wind and runoff played in the residual current of Xiangshan Bay, we choose a middle typical and calculate the single distribution characteristics. The statistical data shows that the proportion of the tide-induced, wind-driven and runoff-thermohaline residual current is 1:0.52:0.32 in the top, and 1:0.1:0.033 in the bottom, the vertical-integration is 1:0.27:0.22.With the analysis of the residual current, it’s known that the tide-induced residual current is bigger, followed by the wind-driven and runoff-thermohaline residual current. The wind and the runoff have greater influences in the top rather than the bottom. Hence it can be considered that tide-induced residual current is the main component of the residual current in Xiangshan Bay. With regard to the flow direction, the direction of the residual current induced by tide and wind-tide is essentially constant in vertical, the direction of the residual current driven by the additional 4 conditions change greatly meanwhile. The direction of residual current reverses at 0.4H in vertical.