Wind And Surge Separation And Evolution Mechanism Of Nearshore Waves In Large Shallow Lakes

Waves are the main driving force for the resuspension of sediments in large shallow lake(especially in the nearshore sediment concentration area).In this paper,the nearshore waves of Taihu Lake were used as the research object to explore their characteristics and evolution laws,and to provide help for revealing the mechanism of cyanobacteria blooms and the prevention and control of water eutrophication in Taihu Lake.The nearshore waves of Lake Taihu include wind waves and swells,and their formation mechanisms,external characteristics,and ecological effects are all significantly different.A wind surge separation algorithm based on two-dimensional directional spectrum(2D separation method)was used to separate the nearshore waves of Taihu Lake in the spring,summer and autumn of 2018,and then the effective wave height and wave spectrum were used to analyze the evolution mechanism of the nearshore waves under the action of the wind field.The main conclusions are as follows:Measured wave characteristics: Wind field(wind direction,wind speed)is the key factor affecting the effective wave height,and the trigonometric function can characterize the quantitative influence of wind direction on the wave.The wind value(wind speed in the coupled wind direction)is weakly correlated with the measured wave effective wave height,which is inconsistent with the characteristics closely related to wind waves and wind fields,indicating that the waves in the nearshore area contain a certain proportion of swells.In addition,when the wind direction changes suddenly,the mixed wave energy presents an obvious multi-peak distribution,and the structural composition of the mixed wave changes significantly.Implementation and verification of the wave separation algorithm and analysis of separation results: Firstly,the 2D separation method is proved to be suitable for the shore waves of Taihu Lake through the verification of examples.Then the 2D method is used to separate the waves throughout the observation period,and the separation result shows that the correlation between the effective wave height and the wind value after separation is significantly higher than before separation,and the average direction of wind and waves has a significant linear relationship with the wind direction,which proves that the wind field is the key driving factor for waves.The evolution mechanism of wind wave,swell,and mixed wave in the coastal area under the action of wind field: In terms of the wind wave,wind direction has a significant influence on wind wave,and the influence of wind speed on wind wave is restricted by wind direction.The effective wave height of wind wave increases with the e-index.The growth rate first increases and then decreases with the wind speed.When the wind speed is greater than 6m/s,the growth rate slows down,which may be due to the energy dissipation caused by bottom friction and white cap breaking at high wind speeds.The occurrence frequency and intensity of swells under the action of onshore wind are high,and the effective wave height has a weak positive correlation with the wind value,which may be the reason why the reflected waves carry wind field information.In terms of the swell,the swell under the action of offshore wind has a poor relationship with the local wind field,which caused by long-distance wind wave propagation.Its low frequency and weak intensity are due to weak wind field,bottom friction and wave breaking induced by water depth.The effective wave height of the swell increases with the wind value in two trends: fast and slow changes with the e index,which may be caused by the difference in wave superposition caused by shore boundary reflections.In terms of the mixed wave,the average swell index of Taihu Lake is 0.176,which means the mixed wave is generally dominated by the wind wave.When the wind direction is 0 ? 90°,the average swell index is 0.65,and the mixed wave is dominated by the swell,which is related to the weak wind wave under offshore wind.The sudden change of wind direction will inhibit the development of wind and waves and contribute to the formation of low-frequency swells,which explains the phenomenon that the measured wave energy presents a multi-peak distribution when the wind direction changes suddenly.