Adaptation Of Wind Drag Coefficient To Hydrodynamic And Water Quality Modeling In Large Shallow Lakes

Wind stress plays a decisive role in the formation of wind-driven current.For large shallow lakes,wind stress not only affects the flow structure,but also contributes to the resuspension of sediment.In the numerical simulation of lake hydrodynamics,wind drag coefficient as a important parameter directly contributing to the wind stress determines the momentum transfer between the atmosphere and lakes.It is of great significance to quantify the impact of wind drag coefficient on the hydrodynamics of inland lakes.In mathematical models,compared with treating the coefficient as a constant,using empirical formula to calculate the wind drag coefficient can better reveal the spatial distribution characteristics of the wind drag coefficient and improve the simulation accuracy to a certain extent,but the adaptation of wind drag coefficient based on the sea area observation test directly applied to the numerical simulation of inland lakes needs further study.Considering the availability of long-term monitoring data,We selected Upper Klamas lake,the second largest shallow lake in south-central Oregon,as the research area,and used the Environmental Fluid Dynamics Code(EFDC)to simulate the hydrodynamic and water quality characteristics of the lake.Aiming at the problem that the flow velocities in the hydrodynamic simulation was seriously underestimated,the empirical formula of the wind drag coefficient was modified.At the same time,the sensitivity analysis of three dimensionless coefficients in the modified wind-drag formula and the bottom roughness height was carried out.In addition,based on the collected water quality data,the calibration and validation of water quality model of Upper Klamath Lake were completed,and the response of shallow lake water quality simulation to wind speeds based on the modified model was preliminarily analyzed.The results showed the calibrated modified model resolved the underestimation problem,e.g.,at three sites in UKL the water velocity increased by 59-85%,and the relative error for the model decreased by 15-32%.Sensitivity analysis showed the modeled current velocities were more sensitive to the?coefficient than to the bottom roughness height₀ and the coefficients in the original wind-drag formula.The preliminary evaluation results showed the adaptation of the modified wind drag coefficient to flow velocities simulation was much higher than the original empirical formula.For TP and DO concentration simulation,the adaptation did not show a significant difference when the the modified wind drag coefficient was used in the model.Under effects of the actual wind field,the average error,root mean square error and Nash-Sutcliffe model efficiency for DO and TP concentration at the same water quality monitoring site by using the modified wind-drag coefficient formula in the model are different from those using the original wind-drag coefficient formula.By comparison,the changes in DO concentration at EPT site was more obvious in the modified model.Under effects of different design wind fields,the distribution of DO and TP concentration by using the modified wind-drag coefficient formula in the model is also different from that using the original wind-drag coefficient formula.