Field measurements in the tidal bore of the Sélune River in the Bay of Mont Saint Michel (September 2010)

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. The bore forms during spring tide conditions when the tidal range exceeds 4 to 6 m and the flood tide flow is restrained into a narrow funnelled estuary. It is estimated worldwide that over 400 estuaries and shallow-water bays are affected by a tidal bore process. A well preserved macro-tidal environment such shallow-water bay system is the Bay of Mont Saint Michel in France, and some detailed field measurements were conducted in the tidal bore of the Selune River on 24 and 25 Sept. 2010. The turbulent velocity measurements were performed continuously at high-frequency (64 Hz) in the breaking tidal bore. The passage of the tidal bore was characterised by a strong turbulent mixing in the bore. A key feature was the rapid change in the channel cross-section as the tidal bore expanded over the sand banks over the left channel side. The turbulent velocity data showed the marked impact of the tidal bore roller. The longitudinal velocity component indicated some rapid flow deceleration during the passage of the tidal bore, associated with a sudden rise in the free surface elevation, and a flow reversal after the tidal bore front passage. The observations were consistent with a number of field and laboratory observations. The tidal bore passage was further characterised by some large fluctuations of all three turbulent velocity components. The Reynolds stress data indicated some large and rapid turbulent stress fluctuations during the tidal bore and flood flow. The passage of the bore and the ebb flow was always characterised by large backscatter intensity levels corresponding to large suspended sediment concentrations. The data showed some marked differences between the 24 and 25 Sept. 2010, possibly linked with some different initial flow conditions. On 24 Sept. 2010, the ebb flow was fast flowing and sediment-laden. The tidal bore arrival contributed to further sediment motion, and the BSI levels were significant both before and after the tidal bore. On 25 Sept.2010, the ebb flow was less energetic, and the backscatter data suggested little sediment suspension prior to the tidal bore. But a drastic increase in BSI was observed during with the passage of the bore linked with some sediment re-suspension. A specific particularity of the present field data set was the repeated problems experienced during both days and discussed in the report.