Regulation Of Tidal Straining On Vertical Spatial Structure Of Lateral Currents And Residual Circulations In The Upper Xiangshan Bay

In the 21st century,our country attaches great importance to the development and utilization of marine resources,which promotes the rapid development of mariculture,but also brings a series of problems to the water environment.The sea-land interaction in the bay is often accompanied by the transport of water,sediment,pollutants,nutrients and other substances.The material transport process in the bay is mainly affected by the dynamic field,so it is of great significance to understand the hydrodynamic process.Because the hydrodynamic field is affected by tidal current,salt and fresh water,runoff and other factors,there are significant differences in hydrodynamic processes in different regions.Previous studies mainly focused on the hydrodynamic process in the main channel direction,and less attention was paid to the transverse process.With the development of research,scholars have found that lateral currents（flow perpendicular to the main channel direction）plays an important role in the offshore lateral momentum and mass transport.Tide is one of the most important driving forces in the offshore dynamic field,and the periodic tidal current movement usually occurs with the lateral currents,so the material transport process of the lateral currents depends largely on its vertical spatial structure.The vertical spatial structure of the lateral currents is mainly affected by its driving mechanism,and stratification can also affect the vertical spatial structure of the lateral currents.In recent years,much progress has been made in the study of the lateral currents structure in coastal waters under different stratification conditions,and it is found that tidal straining can affect the mixing and stratification state of the water body,and that tidal straining can regulate the vertical spatial structure of the lateral currents under strong stratification conditions.However,there is still a lack of understanding of how the tidal straining regulates the vertical spatial structure of the crosscurrent in the bay with weak stratification intensity.Xiangshan Bay is a bay with weak stratification intensity,and the tidal straining phenomenon in the bay is significant.Xiangshan Bay is an important aquaculture area in China.The hydrodynamic process in the bay is related to the transport of pollutants,nutrients and other substances,so the water environment and dynamic field of the aquaculture area are important contents of the bay research.Taking Xiangshan Bsy as an example,the study of the lateral process of Xiangshan Bay can enrich the research content of offshore dynamics.Based on the field observation data,this study analyzes the tidal phenomenon and its stratification intensity in the upper Xiangshan Bay,and studies the variation characteristics of the lateral currents,and further discusses the variation characteristics of the lateral residual circulation,and discusses the regulation mechanism of the tidal straining on the vertical spatial structure of the lateral currents and residual circulation.The conclusions are as follows:（1）There is an obvious tidal phenomenon in the upper Xiangshan Bay.The maximum tidal range during spring tides can be up to 5.0 m.During the spring tide（4M₂）,the tide shows the phenomenon of diurnal inequality,and the maximum tidal range is about 4.6 m;During the spring tide（2M₂）,the tide does not show the phenomenon of diurnal inequality,and the maximum tidal range is about 3.8 m;During the transition of spring-neap tide,the tide shows the phenomenon of daily inequality,with the maximum range of about 3.7 m.（2）Xiangshan Bay is a sea area with weak stratification intensity.During the spring tide（2M₂）,the salinity varies between 25.30 and 26.20,and the maximum salinity difference between the surface and bottom layers is about 0.24;During the transition of spring-neap tide,the salinity varies between 23.50 and 24.20,and the maximum salinity difference between the surface and bottom layers is about 0.33.（3）The vertical spatial structure of the lateral currents in the upper Xiangshan Bay is obviously controlled by the tidal straining.Because the tidal straining phenomenon is mainly affected by the tidal current,when the tidal current is about0.80 m/s at the maximum flood,the tidal straining phenomenon is significant;When the tidal current is about 0.60 m/s at the maximum flood,the tidal straining is suppressed.The results show that during the spring tide（4M₂）,when the tidal current is about 0.80 m/s at the maximum flood,the strong vertical mixing at the flood tide causes the lateral currents to show a one-layer structure at the high high tide;When the tidal current is about 0.60 m/s at the maximum flood,the weak vertical mixing at the flood tide causes the lateral currents to show a two-layer structure at the low high tide;The vertical mixing is weak at ebb tides,and the lateral currents shows a two-layer structure at low tides.During the spring tide（2M₂）,the tidal current is about0.80 m/s at the maximum flood tide,and the strong vertical mixing at the flood tide makes the lateral currents show a one-layer structure at the high tides;The vertical mixing is weak at the ebb tides,and the lateral currents shows a two-layer structure at the low tide.During the transition of spring-neap tide,the tidal current is about 0.60m/s at the maximum flood tide,and the vertical mixing is weak at the flood and ebb tides,so the lateral currents shows a two-layer structure at both high and low tides.In addition,during the spring tide（4M₂and 2M₂）,the intensity and duration of the lateral currents show obvious asymmetry between high and low tides.During the transition of spring-neap tide,the asymmetry of high and low tides in the duration of the lateral currents is broken,but the asymmetry of high and low tides in its intensity still exists.（4）Influenced by the asymmetry of high tides and low tides of the duration and intensity of the lateral currents,the lateral residual circulation in the upper Xiangshan Bay presents a two-layer structure which is southward in the upper layer and northward in the lower layer.Due to the difference of tidal straining in different tidal periods,the reverse depth point of lateral residual circulation ascends from the spring tide（4M₂and 2M₂）to the transition of spring-neap tide.The reverse depth of the lateral residual circulation is near the middle of the water body during the spring tide（4M₂and 2M₂）,and the reverse depth is at the upper 1/3 of the water body during the transition of spring-neap tide.