Variable drivers of surface turbulence under condition of stratified density currents in a tributary bay of Three Gorges Reservoir

Abstract

Surface turbulence has a direct impact on thermal stratification and important ecological process. The dissipation rate of turbulent kinetic energy (ε), a key parameter to quantify turbulence, also significantly affects the hydrodynamic and biochemical processes. Continuous measurements of high-frequency surface (0.50 m depth) flow velocity, meteorological data, water level and water temperature conducted in Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR) during summer (2019/6/17–2019/6/24) and winter (2019/12/29–2020/1/4) observational period, have been used to explore the variability of surface turbulence and its possible driving factors. The results show that a two-layered flow structure was clearly visible in summer. The surface water flowed from downstream to upstream (surface intrusion currents), with an average longitudinal velocity of 0.05 m/s and 0.02 m/s during summer and winter observational periods, respectively. Meanwhile, the longitudinal velocity also showed periodic fluctuations at three different time scales, with periods of 1.80 h, 20 min and 1–3 s, respectively. The 1.80 h fluctuation was caused by the flow oscillation driven by daily discharge regulation of TGR. In addition, the surface turbulence dissipation rates varied from 10−8 W/kg to 10−4 W/kg in summer, and from 10−9 W/kg to 10−4 W/kg in winter. In XXB, the movement of surface intrusion currents is an important factor that dominates and maintains the surface turbulence, while the contribution of atmospheric forcing (wind shear and convective cooling) accounts for 27.17 % in summer but 58.95 % in winter. The diurnal water level regulation of reservoir and wind variations are co-driving forces for the diurnal differences of surface turbulence during the observational periods. Therefore, the atmospheric forces and reservoir regulation together determine the variability of surface turbulence. This study will help to deepen the understanding of the hydrodynamic process of large regulated riverine reservoirs.