Evidence and possible causes of velocity pulsing in a turbidity current in Lake Geneva

Abstract

Negatively buoyant riverine inflows plunge when entering lakes or reservoirs and form gravity-driven currents near the bed. When a high sediment load causes the density excess, such currents are called turbidity currents. They can supply momentum, heat, oxygen, sediments, nutrients and contaminants to deep lake basins and are the main cause of reservoir storage capacity loss. Even with steady inflow, turbidity currents have been observed to exhibit regularly pulsing velocity patterns, which are likely to enhance mixing between the inflowing and the ambient waters. Previously, literature linked this pulsing to several mechanisms, including interfacial waves such as Kelvin-Helmholtz instabilities and Rayleigh-Taylor instabilities related to surface lobes along the plunge line. However, to our knowledge, field measurements of the latter have not been reported.

In the present study, field measurements of the plunging inflow of the negatively buoyant Rhône River into Lake Geneva (Switzerland/France) were carried out. Vessel-mounted ADCP measurements of the longitudinal flow field of the plunging flow and the subsequent turbidity current were combined with remote time-lapse imagery capturing related surface patterns.

The ADCP measurements confirm that the inflowing river water plunges and forms a turbidity current. At the turbidity current-ambient water interface, regularly spaced “bulges” in the velocity pattern suggest pulsing. Simultaneously taken remote time-lapse images show that at the surface, the inflowing sediment-rich water forms a distinct plume with a triangular shape leading away from the river mouth in the downstream direction towards a sharp tip. At the edge and in the immediate surroundings of this plume, a variety of intermittent lobes and vortical structures whose periodicity might be related to that of the velocity pulsing in the turbidity current, is observed.