River dynamics

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).

Figure 1: Main research topics in the river dynamics thematic area: water flow, sediment transport, and water chemistry, geomorphology, and hydrodynamic conditions
Figure 1: Main research topics in the river dynamics thematic area.

The river dynamics group is focused on three overlapping themes :

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).

Methods

Pont de Signy measurement station downstream from the dams (on left) and sample collection at the Pont de Virey measurement station upstream from the dams (on right)
Pont de Signy measurement station downstream from the dams (on left) and sample collection at the Pont de Virey measurement station upstream from the dams (on right)

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.

Monitoring pebble movement with PIT tags
Monitoring pebble movement with PIT tags

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.

Three piezometers near the Pont de Signy measurement station
Three piezometers near the Pont de Signy measurement station