Consideration of sediment transport from the source to the sink, using the Rhone as an example, from the main sediment catchment area of the Rhone in the Illgraben to the sink in Lake Geneva. The colder and, due to the transported sediment, denser water of the Rhone flows downwards along the lake bed in the delta of Lake Geneva due to the difference in density; this process is called plunging

Sediment transport is a catalyst of water related hazards that enhances flood risks, causes local erosion that endangers structures (bridge piers, dikes), increases the flow forcing on infrastructures and their risk of breaking (pipelines, telecommunication cables). Reservoir sedimentation is one of the main unresolved problems in hydraulic engineering, and sediment transport is a vector of contaminants.

The restoration of natural-like sediment dynamics is a core component in river restoration schemes. In spite of their importance, sediment transport processes are commonly neglected in hydraulic engineering problems, and in particular in the assessment of water-related hazards. This can, at least in part, be attributed to the difficulty in accounting for sediment transport processes: engineering methods for sediment transport are characterized by notoriously high uncertainties. This, in turn, can be attributed to a lack of insight in sediment transport processes. At present, our research focusses on measuring and unravelling sediment fluxes from the source in the mountains to the sink at the bottom of lake.

Fig. (top) The Upper Rhône watershed ranging from the high mountains to the bottom of Lake Geneva; (bottom) Conceptual representation of the pathway of the Rhône River in Lake Geneva.

Field experiments

Field experiments in the Upper Rhône watershed (Switzerland) play an important role. This watershed is unique, in that it ranges from sources of sediment high in the mountains to sinks of sediment on the bottom of the deep Lake Geneva (Figure 3). The processes encountered in this watershed have global relevance.

In the framework of his PhD (started 04.2020), Jakob Höllrigl monitors the fluxes of sediment in the Rhône River just upstream of Lake Geneva and the Brienzwiler Aare just upstream of Lake Brienz (Switzerland). The purpose is to gain insight in the relation between meteorological events and river discharge at the one hand, and sediment flux and grain size distribution at the other hand. This research is done in cooperation with BAFU (Swiss Bundesamt für Umwelt), which makes its hydrological measuring stations available.

Little is still known on sediment transport in stratified environments and stratification effects are mostly neglected in hydraulic engineering. Recent research has shown that very small density differences, induced for example by slight differences in suspended sediment concentration or temperature, have surprisingly large effects of flow and sediment transport processes. Such stratification effects occur at the inflow of a river in a lake, reservoir or ocean, and also at the confluence of two rivers. The PhD of Stan Thorez (started 02.2018) focuses on field measurements of sediment transport at the inflow of the Rhône River into Lake Geneva (Fig. 2). This research is done in cooperation with the team of Prof. Barry at EPFL (Switzerland). A 36-month Austrian-French follow-up project funded by FWF-ANR involving 2 Postdocs and 1 PhD starts in May 2023 (TISS link). It extends the field measurements, and complements them with laboratory experiments and numerical modelling. The French partners are Dr. Negretti and Dr. Chauchat at LEGI, opens an external URL in a new window (France).

Turbidity currents are sediment-driven currents that flow in canyons they have shaped at the bottom of lakes, reservoirs or oceans. They are the main cause of the sedimentation of reservoirs. In ocean canyons, they constitute the largest fluxes of sediment and carbon on earth. They are extremely destructive and often cause the breaking of pipelines and telecommunication cables that traverse these canyons. The number of telecommunication cables on the ocean floor is growing exponentially, and the economic losses related to cable breaking are colossal. The aim of the research is to gain insight in morphometric characteristics of subaqueous canyons, understand the processes that shape them, quantify the fluxes of sediment, and have a better assessment of forces exerted by the turbidity currents on infrastructure. A new PhD (2023) focuses on field measurements and the Postdoc of Gauthier Rousseau (started 03.2022) focusses on complementary laboratory experiments. This research is done in cooperation with the teams of Prof. Barry at EPFL , opens an external URL in a new window(Switzerland), Prof. Talling at Univ. Durham, opens an external URL in a new window (UK), Dr. Bouffard at EAWAG , opens an external URL in a new window(Switzerland) and Prof. Kremer at Uviv. Bern, opens an external URL in a new window (Switzerland).