Study On The Thermal Stratification Characteristics Of Xiangxi Bay And The Physical Mechanism Of Evolution

Since the construction and impoundment of the Three Gorges Reservoir in 2003,it has brought enormous comprehensive benefits in terms of flood control,power generation and shipping,it also has a substantial impact on ecological and environmental issues in the reservoir area.Reservoir water temperature is a vital part of the water environment,and is an important parameter affecting reservoir water quality and an important water quality indicator for evaluating the impact of reservoirs on the ecological environment of the reservoir area.In order to analyze the thermal stratification characteristics of the Xiangxi Bay and the physical mechanisms of its evolution,this study takes the Xiangxi Bay,the first major tributary at the head of the Three Gorges Reservoir,as the object of study.Through extensive field meteorological,hydrological and vertical water temperature monitoring,the characteristics of hydro-meteorological changes in the Xiangxi bay were analyzed.At the same time,the thermal stratification characteristics of Xiangxi Bay and its evolution characteristics are analyzed in conjunction with water temperature data from the monitoring platform of Xiangxi Bay,and the influence of water-air heat balance on the water temperature structure in terms of thermal contribution is explored.The influence of typical meteorological processes on the water temperature structure was analyzed by calculating the surface turbulence scale indicators and thermal balance characteristics of the Xiangxi Bay.By comparing the characteristics of different forms of density current in the Xiangxi Bay and analyzing their effects on the water temperature structure,and by monitoring the flow condition and vertical temperature,the effects of tributary oscillations on the water temperature structure and bottom energy dissipation were analyzed.The specific conclusions are as follows.（1）There were a total of 460 small water level fluctuations（≤1m/day）during the monitoring period,accounting for 93.50%of all days counted,moderate water level fluctuations（1-2m/day）occurred 25 times and large water level fluctuations（>2m/day）occurred 7 times.The surface temperature in the Xiangxi Bay ranged from 12.91-33.24°C and the bottom temperature ranged from 11.02-26.28°C.Water temperature of the upstream inflow was relatively low during most of the monitoring period,with water temperature variation ranging from 4.91-26.04°C and a significant diurnal temperature difference.Mainstream backflow was high and the water temperature was evenly mixed,with water temperature variation ranging from 11.42-27.29°C and small diurnal variation.Throughout the monitoring period,solar shortwave radiation was generally stronger in summer and autumn than in winter and spring.Wind speed on the water surface of the Xiangxi Bay ranged from 0-12.8m/s with a mean value of 1.23±1.01m/s.A breeze of 1m/s was the most frequent throughout the monitoring period,while wind speed varied considerably from day to day,generally showing a higher wind speed in the afternoon and almost no wind at night and in the morning.（2）Xiangxi Bay is a typical single-circulation reservoir water body.The range of water temperature variation during the monitoring period ranged from 11.02-33.24°C,with a wide range of surface temperature variation and complete vertical mixing occurring only once per year.During the cooling period,the water column within 41meter water column was in a seasonal cooling state.As a whole,a cooling rate of 0.069°C/day was observed in the surface layer,0.059°C/day in the layer of 36 meter,while the cooling rate in the bottom 41 meter was almost zero;during the warming period,all layers maintained a large rate of increase.25 meter temperature showed an increasing trend,with an average rate of increase of 0.143°C/day.The rate of increase in water temperature corresponding to the surface heat flux was 0.081°C/day,contributing 56.6%of the heat to the water column warming.（3）The heat flux in the Xiangxi Bay varies significantly from day to day,with an overall effective heat flux range of-220-900 W/m².The diurnal scale of turbulence at the water surface influences the heat distribution,with effective solar radiation distributed within 4 m of water depth.A decrease in latent heat flux from evening to night and wind stress enhance the evaporative capacity of the water surface,with turbulent kinetic fluxes at the water surface generally ranging from 1.4 x 10^-7m³/s³ to 2.3 x 10^-7m³/s³.Convection and wind stress dominated turbulence generation.the most dramatic fluctuations in temperature and wind speed were observed throughout the process from the afternoon of 29 December,2020 to the morning of 1January,2021,with temperature variations ranging from 0.89 to 13.6°C.The analysis of the near-surface turbulence characteristics showed that the flow pulsations in the vertical direction of the water column were much less intense than the north-south and east-west flow pulsations,and the turbulent kinetic energy showed a significant positive correlation with the wind speed,indicating that the near-surface turbulent kinetic energy mainly came from the wind stress effect,and it was mainly horizontal shear.（4）Under the surface density current,the flow velocity in the surface layer of the tributary increases,mixing is enhanced and the mixing layer is relatively deep,sometimes reaching 10m or less.The stratification of the surface water column gradually increases during the day,making the mixed layer shallower and showing obvious diurnal variation;while in the middle density current mode,the mixing of the water column in the middle layer of the tributary is stronger,while the surface water column is subjected to the top-support effect and the retention time of the water column is enhanced,which continuously absorbs heat and accelerates stratification.There are 2-hourly tributary oscillations in Xiangxi Bay.The tributary oscillation has a significant effect on the vertical water temperature structure.The tributary oscillation changes the vertical water temperature structure by producing periodic changes in flow velocity and direction in the vertical direction,and has the most significant effect on the vertical water temperature distribution in the bottom water column.The increase in inflow flow in the upper reaches of the Xiangxi Bay increases the water temperature increase by increasing the vertical temperature gradient in the bottom layer of the Xiangxi Bay.At the same time,it enhances friction and energy dissipation at the water-sediment interface,which is expected to have a significant impact on sediment biochemical reactions and substrate resuspension.