Use of electromagnetic tools to evaluate the risk of silting of the ‘Sélune’ river bed following the dismantling of the 'Vezins' and 'Roche Qui Boit' dams.

The hyporheic zone is a thin porous sedimentary interface that connects the river to the water table. It is a place where a large part of the groundwater transits and mixes with surface water. Recent studies point to the key role of this zone, a natural biological reactor at the groundwater-river interface, in altering the nitrogen and carbon cycles, capturing and releasing contaminants and buffering river temperatures. Past studies have suggested that, locally hyporheic fluxes can overtake groundwater-river exchanges, although the physical conditions (permeability, roughness, head, ...) under which such scenario occurs remains unclear. In this study, geophysical monitoring of electromagnetic conductivity along a river reach (Sélune, France) was used to identify longitudinal variations in bed permeability, and identify potential hotspot of hyporheic exchanges.
In practice, we will measure electrical conductivity at different depths into the river sediment using the electromagnetic instrument “CMD explorer”. The instrument is displaced over the river surface using a floating board. A two-layer model is then considered to separate the contribution of sediment bed and water column conductivities. Based on the law (Archie, 1942) in a saturated environment, we link measured variations in electrical conductivity to variations in sediment properties such as pore volume change. In parallel, manual permeability measurements of the riverbed were performed to compare and validate the permeability deduced by the electromagnetic method.
The recent removal of two dams over the Sélune river which was associated with an abrupt change of transported sediment load motivates us to perform time-lapse measurements of electromagnetic conductivity in several sectors upstream and downstream of the dams, before and after removal. Such repeated measure provides interesting data on the time scales of bed clogging processes and its impact on hyporheic exchanges.