Basic Characteristics And Seasonal Variation Of Tidal Current Vertical Structure In Yangtze River Estuary

The Yangtze Estuary area is densely populated,economically developed,and there are many large-scale projects.The study of the tidal current characteristics of the tidal reach in this area has extensive social and scientific significance for shipping safety,project implementation and estuarine ecological environment protection etc.Due to the limitation of observation conditions and equipment,the previous research on tidal current in the Yangtze River Estuary mainly focused on understanding its basic characteristics,and qualitatively explained the difference of tidal current in vertical direction(vertical structure)based on short-term observation data,and high-resolution vertical structure of tidal current and its variation law cannot be obtained.Therefore,this paper makes a tidal current harmonic analysis based on the long-term fixed ADCP(Acoustic Doppler Current Profilers)velocity data of three stations in the Yangtze Estuary in 2011,which is used to obtain the vertical structure of tidal current and residual current in the tidal reach of the Yangtze River estuary in different seasons,and then combined with the vertical structure of residual current to analyze the influence of runoff on tidal current in different seasons,so as to obtain the seasonal variation law of the vertical structure of tidal current.It provides data and method support for physical oceanography research and ocean monitoring in the waters of the Yangtze River Estuary.The main conclusions of this paper are as follows:(1)the basic characteristics of tides in the Yangtze River Estuary are as follows:among the three seasons,the monthly mean tidal range in summer is the largest,and gradually increases with time,and the maximum tidal range appears in September;among the tidal extremes,the highest high tide level and the lowest low tide level are the highest in summer and the smallest in spring.the results of harmonic analysis show that the tidal components of the three seasons are dominated by M₂ semidiurnal components,followed by S₂ and N₂semidiurnal components,and K₁ and O₁ diurnal components;The amplitude of the shallow water component M₄is the largest in summer,the second in autumn and the smallest in spring,indicating that the Yangtze Estuary is significantly affected by the shallow tide;According to the tidal regulation coefficient,it is known that irregular semidiurnal tides are easy to form in the estuarine reach of the Yangtze River.(2)The basic characteristics of the tidal current in the Yangtze River Estuary are as follows:the results of harmonic analysis show that the tidal components of the three seasons are dominated by M₂ semidiurnal components,followed by S₂ and N₂ semidiurnal components,and K₁ and O₁ diurnal components.The amplitude of the shallow water component M₄ is the largest in summer,the second in autumn and the smallest in spring,indicating that the Yangtze Estuary is significantly affected by the shallow tide.According to the tidal regulation coefficient,it is known that irregular semidiurnal tides are easy to form in the estuarine reach of the Yangtze River.there is the most wonderful part of diurnal inequality,and the duration of ebb tide is longer than that of flood tide.(3)The characteristics of the vertical structure of the M₂ component are as follows:the amplitude of the semi-major axis basically decreases in the vertical direction,but the difference between the surface layer and the bottom layer is small,and there is a great difference between the surface layer and the middle layer;while the semi-minor axis increases gradually from top to bottom,and the law of each station is inconsistent.The difference of ellipse inclination angle from surface to bottom is small,but for different seasons,the difference between C1 and C3 station is small,the maximum difference is less than 5°,However,at C2 station,the difference of ellipse inclination angle at the same water depth is obvious,which is the smallest in spring,the second in summer and the largest in autumn.The ellipse phase basically shows a unified law in different seasons of the three stations,that is,it decreases slowly from the surface to the bottom,and the difference between the phase values of the surface and the bottom is about 10°.(4)The fitting results of M₂ tidal current ellipse model values and measured values are as follows:the fitting results of M₂ tidal current ellipse parameters of M₂ component in different seasons by using friction model is not good,especially in the near bottom and surface layer,there is a great difference between the model values and the measured values.Therefore,the average velocity correction model is used to fit the vertical structure of tidal current,and the results show that the influence of the average velocity on the semi-major axis in this area can reach 0.1m/s to 0.2m/s,and the fitting result is better than that of the friction model.In addition,when the average velocity shear force is positive,it has a clockwise rotation effect on the ellipse inclination angle,that is,the average velocity makes the ellipse inclination angle to rotate in the direction of the average flow direction.It is the same as the variation law of the ellipse inclination angle and average flow direction of the measured data,which indicates that the average velocity correction model can be used to explain the variation law of the vertical structure of the tidal current in the tidal reach affected by runoff.