Study On The Effect Of Wind Field On Turbidity Of Water Source In Shallow Lake And Control Measures

Shallow lakes are important sources of drinking water,the open surface and flat bottom make the wind disturbance easily cause sediment resuspension at the bottom,thus leads to the increase of suspended matter concentration(SSC)in water body.Since the SSC of water source plays a direct and important role for raw water turbidity of waterworks,evaluating,predicting and controlling the SSC of shallow lakes are of great significance to optimize the operation process and improve the quality of water supply quality.However,continuous and high frequency data of SSC is always difficult to obtain,while turbidity data can usually be collected from automatic monitoring equipment or daily test reports of waterworks.Therefore,this paper intended to study the dynamic mechanism of wind field on the change of SSC in shallow lakes by investigating the response relationship between SSC and the raw water turbidity of waterworks,further analyzed how wind field impacted the turbidity and the measures to control it.This paper focused on the water intake area of Taihu waterworks,and the works and main conclusions were listed as follows:(1)The correlation between raw water turbidity and wind field(wind speed and direction),temperature,rainfall were analyzed through collecting high-frequency and continuous monitoring data.The results showed that the influence of rainfall was negligible,and the seasonal cycle of turbidity was highly similar to that of temperature,while the daily and even higher frequency variations were provided by wind.Wind field made a complex nonlinear effect on turbidity through wind direction,wind speed and duration.For the study area,northwest winds with speed more than 3m/s can sudden rises of turbidity,and would play a lasting effect in nearly 7 hours.(2)The random forest model was introduced for turbidity prediction,and the preprocessed temperature and wind fields data were selected as input variables.The reconstructed temperature series was obtained by empirical mode decomposition(EMD)and superposition of high contributing components.The wind field data were modified according to the related intensity with turbidity in different wind directions and speeds,the effective duration was also considered.The preprocess of temperature and wind field data significantly improved the accuracy of the model.(3)The causes of the spatial and temporal distribution of suspended solids in Lake Taihu under different wind conditions were analyzed by 3D hydrodynamic and sediment transport simulations driven by steady winds.It showed that the sediment resuspension caused by wind-induced waves was an important factor affecting the spatial distribution of suspended matter when the wind force was weaker than 3m/s.While for strong winds,the flow structure generated by wind-induced currents was the dominant driving force.And external input was found to be the main source of suspended matter in the study area by comparing the modeled SSC before and after the wave effect here.The simulated values had a highly similar trend with measured turbidity,and the lag effect of 6-8 hours between the two series further proved the time difference in turbidity response to wind field changes.(4)The measure of floating dam was proposed to block the sediment input then after the main effect of sediment transportation on turbidity variation in water intake area was confirmed.The effect of reducing the SSC,the hydrodynamic environment of the project region and the inflow conditions of the inlet by solutions of different offshore distance and draft depth were assessed based on numerical model.The recommended scheme was finally given by the modeling results and considering the economic costs as well.This paper intended to establish the responds relationship between raw water turbidity of shallow lake sources and weather,especially wind field from two aspects: the non-mechanism prediction model and the influence mechanism of wind field on turbidity.In former,the data preprocessing method that considering the response pattern between input and out variables not only improve the prediction accuracy effectively,but also increased the model interpretability.The current work broke through the limitation of existing models that requiring a large number of water quality and history turbidity information,which provides a new direction for the model optimization of water quality prediction.The mechanism study qualified the effect intensity of local sediment resuspension caused by wind-induced waves and external sediment input caused by wind-induced currents on SSC in the study area based on numerical simulation.And a database of modeled SSC under different wind directions and speeds covering all conditions of study area was established.The estimation of variation trend of turbidity was then achieved through the combination of limited modeling results,which laid a foundation for the accurate in shallow water environments.This paper achieved the forecast of turbidity trend from two aspects: the non-mechanical turbidity prediction model based on meteorological data and the influence mechanism of wind field on turbidity based on numerical model.As the meteorological data was easy to obtain and predict,both methods in current work had effectively extended the prediction time and improved the practicability of the model compared to previous studies.That made technique foundations for turbidity monitoring,forecast and early warning of waterworks,and provided ideas for raw water turbidity prediction shared similar water sources.