Tracing Unconfined Nearfield Spreading of a River Plume Interflow in a Large Lake (Lake Geneva): Hydrodynamics, Suspended Particulate Matter, and Associated Fluxes

Abstract

River inflows have a major influence on lake water quality due to their input of sediments, nutrients and contaminants. After leaving the river channel, river waters form a plume and interact with ambient lake waters. Strong hydrodynamic changes take place in the nearfield. To determine the nearfield hydro-sedimentary dynamics of the negatively-buoyant Rhône River plume in thermally stratified Lake Geneva, field campaigns were carried out at high and intermediate river discharge. High-resolution full-depth Acoustic Doppler Current Profiler (ADCP) transects were taken at 400, 800, 1,200, and 1,500 m from the river mouth, combined with profiles of temperature, turbidity and particle size distribution. These measurements provided, for the first time in a lake, detailed velocity fields across the full-plume cross-section in each transect. Furthermore, the unique combination of measurement techniques allowed the quantification of entrainment, Suspended Particulate Matter (SPM) concentrations and fluxes. They revealed that the negatively buoyant river inflow intruded into the metalimnion as a laterally unconfined interflow and continued flowing straight out in the streamwise direction, since currents in the lake were weak. At the same time, it mainly spread laterally due to entrainment of ambient water. The size of the interflow core and its velocity progressively decreased with distance from the mouth, as did SPM concentrations and volumes of particles (by a factor of 2–3 within 1,500 m), due to sediment settling and entrainment. The interflow momentum flux remained constant along the pathway, but interflow cross sections and discharge increased 2–3 times (within 1,500 m). The entrainment coefficient was >2 at 400 m and further increased non-linearly along the pathway. These values are much higher than those reported for laterally confined laboratory studies. The particle size distribution in the interflow was dominated by fine particles (<32 μm), which were transported up to 1,500 m from the mouth and most likely beyond, whereas larger particles (>62 μm) almost completely settled out before reaching that distance. The above processes occurred independent of river discharge; their intensity, however, changed with discharge.

From Particles to Flocs: Revealing Where Flocculation Occurs in the Nearfield of a Negatively‐Buoyant River Plume in a Large Lake (Lake Geneva)

“…It spreads laterally unconfined within the thermocline from ∼10 to 25 m depth (Figure 3), and flows northward, straight out from the river channel (Figure 1b). This plume pattern agrees with the one that was previously observed (Piton et al., 2022; Soulignac et al., 2021). The velocity profile slightly differs at S1 since the interflow is not yet fully established at that distance from the mouth (350 m).…”

“…The settling of SPM out of the interflow by flocs could explain the decreasing sediment flux along the nearfield interflow pathway reported by Piton et al. (2022).…”

Unconfined Plunging of a Hyperpycnal River Plume Over a Sloping Bed and Its Lateral Spreading: Laboratory Experiments and Numerical Modeling

“…The underestimation of x p may result from sediment deposition that is not included in the experiments or the numerical model. As the hyperpycnal plume develops, it loses momentum, which leads to the deposition of larger‐sized suspended sediment, reducing the relative density difference (Piton et al., 2022). Sedimentation may also reduce the bottom slope angle.…”

“…This investigation was motivated by ongoing ECOL field studies of the negatively buoyant Rhône River inflow into Lake Geneva (e.g., Piton et al., 2022; Soulignac et al., 2021). The river channel aspect ratio H 0 / B 0 = 0.04 and lake bottom slope of β = 8° were reproduced in the laboratory experiments.…”

“…In lakes or oceans, river inflow can develop laterally after leaving the inflow channel (Figure 2); thus, the plunging of a hyperpycnal inflow can be three‐dimensional (3D). Examples of unconfined plunging are the Lillooet River entering Lillooet Lake (Canada; Best et al., 2005), the Slims River flowing into Kluane Lake (Canada; Crookshanks & Gilbert, 2008), and the Rhône River plunging into Lake Geneva (Switzerland; Piton et al., 2022; Soulignac et al., 2021). Figure 2a gives an aerial view of the Rhône River mouth showing a triangular pattern on the water surface.…”