Talk:French case studies: Upper tidal flat evolution in the bay of Mont-Saint-Michel (NW France)

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Review by Job Dronkers (January 2013)

The changing course of the Couesnon tidal channel in the period 2009-2010



The article describes nicely some aspects of tidal flat morphodynamics in the Baie du Mont Saint-Michel. Progradation and erosion of tidal flats is a highly dynamic process in the eastern part of the Baie, related to the fast migration of the tidal channels of the Sée, Sélune and Couesnon rivers[1][2]. Major causes of the exceptionally fast morphodynamics of the Baie are (1) the high mobility of the substrate, which consists largely of fine non-cohesive bioclastic sediments (so-called 'tangue'), and (2) the very high tidal range, due to resonance and funnel effects.


The fast migration of the Couesnon channel in the period 2009-2010 is illustrated in the opposite figure. It pictures the changing course of the Couesnon tidal channel around the Mont Saint-Michel (at the centre) at different times during the period 2009-2010 (taken from the thesis of Lucile Gluard, Université de Caen, 2012).






The fast rising tide with high flood velocities is probably a major cause of the overall net sedimentation of the Baie du Mont Saint-Michel. At high spring tides, a tidal bore propagates into the Baie, shown in the figure below (taken at sunset on the 1st of September 2011).

The tidal bore (at the left, blue) advancing into the upper bay area (at the right, red). The red colour is the reflection of the sky at sunset by the still water surface in front of the tidal bore. The blue colour is due to red absorption by the turbulent surface waters of the bore.


Since a few years, the tidal flat progradation around the Mont Saint-Michel is counteracted by artificial flushing. For this purpose, a mobile barrier has been built in the Couesnon river, which has become operational in 2009. The river behind the barrier is used as flushing reservoir, which is filled at high water and emptied at low water.

Figure 4 is unfortunately missing in the article.


Related articles

Tidal bore dynamics

References

  1. Levoy, F., Anthony, E.J., Dronkers, J., Monfort, O., Izabel, G. and Larsonneur, C. 2017. Influence of the 18.6-year lunar nodal tidal cycle on tidal flats: Mont-Saint-Michel Bay, France. Marine Geology 387: 108-113
  2. Levoy, F., Anthony, E.J., Dronkers, J., Monfort, O. and Montreuil, A-L. 2019. Short-term to Decadal-scale Sand Flat Morphodynamics and Sediment Balance of a Megatidal Bay: Insight from Multiple LiDAR Datasets. Journal of Coastal Research SI 88: 61–76