In rivers like the Sélune, dams are physical barriers that affect geomorphology, water flow, sediment transport, and water chemistry. . Understanding current and future river dynamics can help predict the level of disturbance that will result from dam removal. Researchers must characterise river dynamics and key mediating factors (e.g., sources, transfers, hydrodynamic conditions, seasonal variability, and the frequency of extreme events) to distinguish the influence of dam removal from that of climate. The dams’ influence on the Sélune’s geomorphology extends beyond the upstream area covered by the reservoirs. Because they change water flow rates and sediment transport, the dams have direct consequences, particularly downstream, on river processes and the deposition of coarse sedimentary particles, which shape riverbed morphology. Reservoirs also change downstream flows because they promote mixing (e.g., of hotter/colder water, dissolved solids, or chemical compounds) and facilitate certain biological processes (that remove or produce specific compounds).
The river dynamics group is focused on three overlapping themes :
- Quantifying water flow, sediment transport, and water chemistry (e.g., interannual variability, base flow, and flow during flood events) (UMR INRA-Agrocampus ouest 1069 – SAS)
- exploring hydrodynamic conditions (UMR CNRS-Université Rennes 1 6118 – Geosciences Rennes, CNRS)
- studying current and long-term geomorphological dynamics (UMR CNRS-Université Rennes 2-Université Caen 6554 – LETG)
Taken together, the information gathered will contribute to the project’s interdisciplinary aim of using data on abiotic conditions to characterize and monitor changes in fish habitat (UE INRA 1036 – U3E and UMR INRA-Agrocampus Ouest 0985 – ESE).
Several methods will be used to gather the above data :
Water flow, sediment transport, and water chemistry will be characterized almost continuously at two stations located upstream and downstream from the dams. At other intermediate stations, several physicochemical variables will be measured by Électricité de France (EDF).
Geomorphological dynamics and sediment transport are quantified directly via field measurements as well as indirectly via aerial and terrestrial LIDAR surveys, passive integrated transponder (PIT) tag monitoring, and velocimetry.
Exchanges between the groundwater layer and the river are characterized by tracking water levels and temperature profiles. A transect of piezometers has been established, and both sensors and optical fibers probes have been placed in the riverbed.