Effect of turbulence driven by wind on sediment suspension under different submerged vegetation density in Lake Taihu

Abstract

Wind direction and velocitydetermines the hydrodynamic structure in the shallow lakes. Notably, submerged vegetation also affect the hydrodynamic structure of the thermocline region of the lake which in turn affects the resuspension of benthic sediments. Therefore, based on meteorological and high-frequency hydrodynamic data, this study analyses the effect of wind field on the vertical shear of the flow field. Besides, study used Pierson–Moskowitz (PM) spectra to strip wind and surge waves to elucidate the response mechanism of turbulence in Meiliang Bay of Taihu Lake. Based on indoor experiments, we simulate the “wind value” (joint effect of wind speed and direction) on hydrodynamics and sediment release under different hydrophyte density cover. The results show that both wind and wave fields affect the hydrodynamic structure. In summer, the vertical shear layer of the flow field will shift downwards due to the wind field. The wave field consists of wind waves and surge waves, the wind waves having a greater impact on turbulence energy. A characteristic quantity 'wind value' is constructed using trigonometric functions which can characterize wind speed and wind direction of arbitrary magnitude. The correlation between 'wind value' and near-bottom turbulence is significantly better than that of wind speed alone, indicating that the effect of wind direction on turbulence cannot be ignored. Although, within a certain range, sediment re-suspension is not necessarily proportional to flow velocity but to turbulent kinetic energy due to the effect of wind direction. High-density submerged vegetation can form a wall boundary protection, which changes the flow pattern of the lake from mixed laminar flow to boundary laminar flow, reducing the intensity of turbulence and impeding sediment re-suspension. Findings of this study can provide a reference for endogenous control and aquatic ecological restoration in shallow lakes.