P_Le Hir et al.mp4

Recording of presentations at the Suricates project closing conference

Title : New insight into morpho-sedimentary dynamics in the Rance modified-estuary: propositions for managing sediments and reducing mud deposition

Authors : Pierre LE HIR*1, Rémi BARBIER1 , Bruno CALINE1 , Sylvain HAUVILLE1 , Carine HEITZ1 , Isabelle LAJEUNESSE1 , Mario LEPAGE1 , Eric MASSON1 , Marc SAS1 , Pierre SCEMAMA1 , Paul SCHERRER1 , Eric THIEBAUT1 , Marion CHAPALAIN2

1 Scientific Board for the Sediment Management Plan of the Rance estuary (France) 2 Etablissement Public Territorial de Bassin Rance, Frémur, Baie de Beaussais, Rue Gambetta, 22100 Dinan (France) * Corresponding author e-mail: pe.lehir@gmail.com

Summary :

The macrotidal Rance estuary is deemed to be subject to excessive sedimentation since the implementation of a tidal power station (TPS) at its mouth, close to St-Malo. In order to analyse sediment dynamics and to identify measures likely to reduce sedimentation, a plan for managing sediment has been set up, supported by a scientific committee (board). This communication synthesizes the committee’s conclusions and its propositions.

After investigations based on field measurements and numerical modelling, a diagnosis on hydrodynamics and sediment features was established. The comparison of bathymetries before and after the installation of the power plant shows a dominant erosion on intermediate and upper intertidal areas: this is probably due to wind waves strengthening by the extension of high water slack, which is required for running turbines. But in recent years, a global sedimentation is observed, around 120 000 m3 /year. In particular, the upstream reach experiences a rapid mud sedimentation since the TPS building, partly due to the shortening of the tidal basin induced by the upstream weir raising, and reinforced by the reduction of tidal amplitude and tidal flows induced by the TPS running.

The Scientific Board suggests 5 levers/measures to reduce fine sediment deposition in the estuary:

- As main sediment input is shown to occur from the sea during storms, it is proposed to close the dam and stop the TPS for full tides when the waves in nearby coastal area are predicted to excess a given threshold.

- The TPS reinforced sediment trapping in the tidal basin because of increased tidal asymmetry, reduced tidal amplitude and extended high water slack; alternative running of the TPS is looked for, either by increasing ebb currents through the opening of gates, or by extending the flood duration; in case of excessive loss of electricity production, such an alternative TPS running would be introduced at short periods that should be optimized.

- Previous experiments of sediment trapping and dredging in the upstream reach of the estuary were proved to be efficient; then dredged sediment was temporary stocked on a transit site for dewatering and desalinating before application on agricultural parcels as soil-enricher; accounting for the possible increase of agricultural use of sediment, as demonstrated in Suricates project, it is proposed to develop and repeat the procedure more often, by increasing the capacity of the transit site.

- Whatever the efficiency of previous perspectives, the need of local dredging for maintaining specific uses or protecting cultural heritage, especially near the shore, will remain: for selecting and planning operations, a permanent representative board has to be set up.

- The above strategy aims at drastically reducing the future sediment input and deposition in the estuary ; a significant and extended dredging of present-day deposited mud can be wished by stakeholders, for instance in upstream reaches: but numerical modelling shows the risk of further increase of sedimentation following such a massive dredging operation; the Scientific Board recommends that the first levers for action should be put in place before such an operation is launched, in order to reduce sedimentation in the future.