Field characterization of the negatively buoyant inflow of the Rhône River into Lake Geneva

Turbidity currents emanating from the Rhône River into Lake Geneva were first inferred by François-Alphonse Forel in the nineteenth century. This site remains attractive for several reasons. (1) Permanent measuring stations on the Rhône and Lake Geneva provide relevant river and lake data. (2) The bathymetry is known with bedform-resolved resolution, which is important since the negatively buoyant Rhône discharge has carved an active, 14 km long canyon in the lakebed. (3) Due to the variability of control parameters during a hydrological year, a variety of processes occur, including gravity currents (driven by temperature differences), turbidity currents (driven by suspended sediment concentration), and potentially even self-accelerating turbidity currents. The range of observable phenomena mean that insights gained on this site of intermediate size are probably relevant for smaller systems such as mountain reservoirs, and larger systems such as submarine canyons. The field investigation started in September 2016 and is on-going. Long-duration measurements with fixed, in situ instruments are combined with short-term, synoptic campaigns. The long-duration measurements make use of several vertical moorings consisting of an ADCP and an array of thermistors, a longitudinal array of thermistors installed on the bottom along the canyon axis, and remote sensing of the plunging area with an RGB and IR cameras. The event-wise measurements consist of a boat-towed ADCP, an echosounder and a CTD. The remote-sensing cameras reveal that the plunging process is often accompanied by intermittent large-scale horizontal vortices in the lake, and by the leakage of a non-negligible part of the Rhône inflow in the lake’s surface layer. The boat-towed ADCP measurements illustrate for the first time the three-dimensional pattern of a plunging flow in an unconfined environment. The fixed in situ instruments quantify the thickness and velocity of different kinds of turbidity currents, including turbidity currents with velocities of about 0.2 m/s that last for several hours, and short-duration turbidity currents with peak velocities up to 1 m/s.