Suspended sediments in Lake Geneva: Observations, settling behaviour and mechanisms in the Rhône River plume

In estuaries and marine environments, primary particles are frequently transported as large flocculated particles. This study provides, for the first time, evidence of in situ flocculation in Lake Geneva, a glacier-fed freshwater lake on the Swiss/French border. Measurements were focused in the nearfield of the Rhône River plume as it flows as an interflow into the stratified lake. Direct observations of flocculated particles in the whole water column with a digital holographic camera (LISST-HOLO 20-2000 μm), permitted estimation of the variability of sediment floc properties (size, nature and shape) with depth. Combined with full depth in situ laser particle sizing (LISST-100X), the measurements revealed that primary particles (< 4 μm) and microflocs (4-100 μm) are dominant in the Rhône River interflow, which exhibited the highest suspended sediment loads in the water column. In the hypolimnion below the interflow, where the sediment loads were the lowest, macroflocs (100-450 μm) were most frequent. The estimated fractal dimension (DF3D) of the flocs in the hypolimnion range between 2.0 and 2.6, highlighting a large variability in the floc shape. Estimates of the settling velocities (Ws) showed an increase from 0.05 to 3 mm s-1 when floc sizes increased from 30 to 500 μm. This emphasizes the important influence of flocculation of fine sediments on the increase of Ws. These estimates are similar to previous estimates of Ws for fluvial flocs (Thonon et al., 2005). However, they are slightly faster (up to 2 times) than Ws estimated for flocs in the vicinity of the Rhône River plume as it flows into the Mediterranean Sea (Many et al., 2019). This difference in Ws between the two Rhône River plume environments is explained by the shapes of the flocs, which overall are less complex and therefore flocs settle faster in Lake Geneva compared to the Mediterranean Sea. Furthermore, the influence of instantaneous turbulent kinetic energy as a factor limiting the maximum floc size within the Rhône River interflow was investigated. The observed turbulence level in the interflow corresponded to an estimated Kolmogorov microscale of less than ~200 μm. This results in the potential breakup of flocs larger than 200 μm into smaller primary particles and microflocs and thus can explain the smaller floc size in the interflow compared to that in the hypolimnion.

Plastic pollution in Swiss surface waters: nature and concentrations, interaction with pollutants

Luiz Felippe De Alencastro, Colin Demars, Florian Faure, Olivier Wieser

Marine microplastic (,5 mm) water pollution has met growing public and scientific interest in the last few years. The situation in freshwater environments remains largely unknown, although it appears that they play an important role as part of the origin of marine pollution. Apart from the physical impacts on biota, chemical effects are to be expected as well, especially with smaller particles. This study aims at assessing plastic abundance in Lakes Geneva, Constance, Neuchaˆtel, Maggiore, Zurich and Brienz, and identifying the nature of the particles, potential ingestion by birds and fishes, and the associated pollutants. Lake surface transects and a few rivers were sampled using a floating manta net, and beach sediments were analysed. Plastics were sorted by type (fragments, pellets, cosmetic beads, lines, fibres, films, foams) and composition (polypropylene, polyethylene, polystyrene, etc.); fish and water birds were dissected to assess their potential exposure, and analyses were conducted on the hydrophobic micropollutants adsorbed to the microplastics as well as some potentially toxic additives they contained. Evidence of this pollution is shown for all lakes, microplastics of all types and diverse composition having been found in all samples. Birds and fish are prone to microplastic ingestion, and all the tested chemicals (both adsorbed micropollutants and contained additives) were found above the detection limit, and often the quantification limit. The sources and their respective contribution need to be confirmed and quantified, and the ecotoxicological effects need further investigation. Other questions remain open, including the transport and fate of plastic particles in the environment.

Csiro Publishing2015

Experimental Study on Bedload Transport and Bedforms: Behaviour and Interplay in Steep Turbulent Streams

Ivan Pascal

In mountain regions, steep streams play an important role in water and sediment connectivity. In these highly dynamic systems, water flow features, sediment fluxes and stream morphologies are tightly interlinked over a broad range of temporal and spatial scales. Understanding their interactions is key to forecast, prevent and mitigate the impacts of stream-related hazards on human population and facilities as well as to manage the resources connected to the rivers. Sediment clasts at rest on the bed surface may be eroded, transported and deposited by the water flow. Grains that move in rolling and saltation regimes close to the bed compose the 'bedload', which is considered the main mode of sediment transport when studying morphodynamics of mountain rivers. The fate of these particles is of primary interest for river scientists and engineers as bedload predominantly drives the river morphology evolution and responds to it in a complex manner. Therefore, measuring and predicting bedload transport rates are fundamental concerns for many works of research and applications. Bedload transport rates may exhibit large non-Gaussian fluctuations, even under steady-state conditions, especially when bedload transport intensity is weak. These fluctuations may originate from different phenomena and affect the accuracy of bedload transport rate estimates. Particles transported by the water flow can destabilise other particles at rest on the bed surface. Under weak bedload transport conditions, this positive feedback can lead to large bedload pulses. When grains settle, they often organise in small clusters or larger bedforms. Bedform migration has been identified as one of the potential causes of bedload transport fluctuations. Trains of upstream-migrating antidunes may develop in steep coarse-bedded streams when the water discharge is sufficiently high to ensure fully supercritical flow conditions. In this dissertation, I present an experimental investigation on the behaviour of bedload transport fluctuations and antidunes in steep supercritical flows. The experiments were conducted in narrow flumes under well-controlled conditions with high-resolution image-based bedload transport and bed topography monitoring. The dependence of mean bedload transport rate uncertainty on the averaging time interval was analysed. Furthermore, I performed experimental runs with increasing transport intensity and a nearly constant mean bed slope angle to study how antidune sequences composed by bedforms of variable shape typically responded to the mean bedload flux imposed under steady-state conditions. Spectral analysis of the bed elevation perturbation in time and space allowed us to quantify the variability ranges of antidune shape and celerity. Scaling the spectra with opportune reference scales made it possible to identify a consistent dimensionless relationship between antidune geometry and migration celerity, which includes the dependencies on flow and sediment supply forcings. I studied how the bedload transport rate fluctuates in presence of migrating bedforms. The effects on the bedload transport rate periodicity due to non-uniformities in the antidune migration period and to deviations from the steady-state transport conditions were also investigated. This work is a further step on the way towards a complete understanding of the interplay between bedload transport fluctuations and bedforms in mountain streams.

EPFL2022

Nearfield development of the negatively buoyant Rhône River inflow into Lake Geneva as an interflow: suspended particulate matter and associated fluxes.

David Andrew Barry, Benjamin Daniel Graf, Ulrich Lemmin, Violaine Piton, Frédéric Charles Soulignac, Htet Kyi Wynn

River inflows have a major influence on lake water quality through their input of sediments, nutrients and contaminants. It is therefore essential to determine their pathways, their mixing with ambient waters and the amount and type of Suspended Particulate Matter (SPM) they carry. Two field campaigns during the stratified period took place in Lake Geneva, Switzerland, in the vicinity of the Rhône River plume, at high and intermediate river discharge. Currents, water and sediment fluxes, temperature, turbidity and particle size distribution were measured along three circular transects located at 400, 800 and 1500 m in front of the river mouth. During the surveys, the lake was thermally stratified, the negatively buoyant Rhône River plume plunged and intruded into the metalimnion as an interflow and flowed out in the streamwise direction. Along the pathway, interflow core velocities, SPM concentrations and volumes of particles progressively decreased with the distance from the mouth (by 2-3 times), while interflow cross sections and plume volume increased by 2-3 times due to entrainment of ambient water. The characteristics of the river outflow determined the characteristics of the interflows: i.e. interflow fluxes and concentrations were the highest at high discharge. Both sediment settling and interflow dilution contributed to the observed decrease of sediment discharge with distance from the mouth. The particle size distribution of the interflow was dominated by fine particles (62 μm) have almost completely settled out before reaching 1500 m.

2021

Experimental Study on Bedload Transport and Bedforms: Behaviour and Interplay in Steep Turbulent Streams

Ivan Pascal

EPFL2022