Physical mechanisms of internal seiche attenuation for non-ideal stratification and basin topography

The dynamics of vertical mixing and the occurrence of basin-scale internal waves (internal seiches) in lakes and reservoirs are often classified and described based on the force balance of wind shear and horizontal pressure gradients resulting from wind-generated currents (the Wedderburn number). The classification schemes consider specific time scales that are derived based on a simplified vertical density distribution, a rectangular basin shape, and a constant water depth. Using field measurements and numerical simulations with a validated hydrodynamic model, we analyzed the transfer of energy from wind to the internal seiche field in a small reservoir. Our results demonstrate that the basin shape has a strong influence on the energy dissipation and on the transfer of energy to propagating high-frequency internal waves, thereby attenuating the generation of basin-scale internal seiches. Most of the energy loss of the internal seiche occurs at the sloping boundary, where the internal seiche is susceptible to shoaling and breaking. These findings suggest that the Wedderburn number can be used to predict the occurrence of internal seiche activity in continuously stratified systems. As the Wedderburn number and derived mixing classifications are widely applied also for the interpretation of observed ecological and biogeochemical processes, its application to basins with sloping bathymetry and complex shape should be critically scrutinized, and deviations from predicted dynamics, including the presence of hotspots of turbulent mixing, should be considered.