Hydro-sedimentary processes of a lofting turbidity current revealed by gridded ADCP measurements in the field

Hyperpycnal (negatively buoyant) river inflows into lakes or reservoirs plunge upon entry, generating gravity-driven underflows near the bed. When the density excess of these underflows is primarily due to high sediment concentrations, they are referred to as turbidity currents. If these underflows encounter a layer of equal density, they will detach from the bed and intrude into the water column, forming an interflow. In turbidity currents, such underflow-interflow transitions often happen through a process called ‘lofting’, whereby a flow that is initially negatively buoyant undergoes a buoyancy reversal due to sedimentation of particles. Direct observations of lofting are sparse, particularly in the field. As turbidity currents transport various constituents – not only sediment, but also contaminants, nutrients, and oxygen – originating from the river or eroded from the bed, their trajectory and final destination significantly influence the water quality of lakes and reservoirs. The latter highlights the importance of studying flow transitions such as lofting.

Field measurements of the turbidity current fed by the plunging Rhône River in Lake Geneva were conducted using a boat-towed ADCP along a grid of transects. The ADCP backscatter signal was used to achieve a first order estimate for the sediment concentration.

The measured velocity field reveals that in the longitudinal direction the Rhône River turbidity current initially breaks through the Lake Geneva pycnocline, detaches from the bed, rises vertically and intrudes into the pycnocline. Additionally, in the transverse direction the outermost parts of the current peel off and similarly rise and intrude into the pycnocline. This infers the presence of lofting in both longitudinal and transverse direction. In man-made dammed-river reservoirs, river valley walls provide a high degree of transverse confinement for turbidity currents, which might suppress the development of flow processes in transverse direction, such as transverse lofting. In most natural lakes, such confinement is not present. This infers a potentially significantly different underflow-interflow transition mechanism and resulting morphological impact between reservoir and lake settings.

The estimated sediment concentrations uncover a capacity of the lofting current to transport sediment-rich water away from the turbidity current centerline in transverse direction. This might influence the local bathymetry and support levee-building.