Hydrodynamics and mixing mechanisms in a subtropical reservoir

Abstract

ABSTRACT With the aim to improve current knowledge on physical processes in tropical and subtropical reservoirs, we explored the dynamics of prevailing hydrodynamic processes in a small- to medium-sized reservoir based on 1-year continuous measurements of temperature, flow velocity, and turbulence. The mixing regime of the reservoir is characterized as warm polymictic, and extended periods of comparably strong density stratification were observed during all seasons, interrupted by several mixing periods between autumn and winter. The stratification was affected by lateral, density-driven flows from the main river inflow. A forebay through which the river enters the reservoir influenced the density currents and reduced the occurrence of underflows. Flow velocities in the middle of the reservoir were generally small, and higher velocities (currents >3.5 cm s−1) were driven by wind. These currents were more frequent during stratified than during mixed periods. No basin-scale internal waves (seiches) were observed, but, high-frequency internal waves propagating, with periods between 2 and 17 min were present in all seasons. Dissipation rates of turbulent kinetic energy were generally small (10−10–10−9 W kg−1). Wind-driven currents were associated with enhanced dissipation rates at the bottom-boundary layer but represented only 10% of the total monitored period. We identified several aspects in which the observed hydrodynamics of the reservoir differed from those of more commonly studied temperate aquatic systems. We expect that our findings are representative for a large number of small- to medium-sized lakes and for a growing number of reservoirs located in the tropical and subtropical zone.