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Fluvial dynamics: Study of water, sediment and chemical fluxes and the geomorphology of the Selune before the removal of dams

Coordinator(s)

Alain Crave, C Le Gonidec, Ophélie Fovet, Anne-Julia Rollet, C Grimaldi

Scientific context

As with biological flows, dams constitute a physical barrier affecting water, sediment and chemical flows, as well as the geomorphological functioning of rivers. Dams act as sediment traps, which are settling at the bottom of the reservoirs. Similarly, nutrients of terrestrial or organic origin become trapped and are no longer transported downstream.

This results in a sediment and chemical deficit downstream. The sediment deficit, particularly in coarse elements (sand, pebbles, etc.) has an impact on the geomorphology of the watercourse. The impact of the dams on the geomorphology of the Selune is not confined to the flooded zone. The lack of coarse sediment downstream leads to channel incision, which modifies not only the shape but also the functioning of the watercourse.

Subject(s) of study

This project is studying sediment, chemical and water fluxes in the Selune, the transport of fine and coarse sediments, and the geomorphology of the river.

Objectives

Removal of the dams on the Selune will change the flow of materials and the geomorphological functioning of the river in the flooded areas and downstream. The aim of this project is to provide detailed knowledge of the water, sediment and chemical fluxes in the Selune in the presence of the dams. A good knowledge of these fluxes and of the geomorphology of the Selune with the dams will help highlighting the intensity of the changes brought about by the restoration of ecological continuities.

This knowledge comes through:

• A precise quantification of the flows measured and the morphology of the watercourse,
• An understanding of their dynamics (upstream-downstream dynamics, seasonality),
• In order to establish an initial morpho-sedimentary state of the Selune with the dams, which can be used as a basis for future monitoring during the restoration phase.

Methods

Three monitoring stations have been equipped to quantify sediment, chemical and water fluxes on the Selune. One station (Virey) is located upstream, without the effect of the reservoirs, and two stations (La-Roche-Qui-Boit and Signy) are located immediately downstream of the dams.

The Virey and La-Roche-Qui-Boit stations were installed by EDF. The Signy station was installed by INRA for scientific monitoring.

The geomorphological study of the watercourse is based on different methods applied upstream and downstream of the dams:

• Direct measurements on the downstream section of La-Roche-Qui-Boit, the section most sensitive to the presence of the dams: measurement of bank height, longitudinal profile of the watercourse, study of mobility in plan,
• LIDAR measurements over a wider area: use of airborne and ground-based LIDAR to cover the areas upstream and downstream of the dams.

Laboratories involved

• SAS - UMR Soil Agro hydro system Spatialisation, INRA/Agrocampus Ouest, Rennes
• Geosciences, CNRS, Université Rennes 1
• GEOPHEN - UMR LETG Physical and Environmental Geography, CNRS/Université de Caen Basse-Normandie