1. Morphological evolutions of a macrotidal bay under natural conditions and anthropogenic modifications Mont Saint Michel Bay, France F. Cayocca, P. Le Hir, P. Bassoullet, H. Jestin, P. Cann IFREMER-Brest, France

2. Oyster beds Mussel farms Max tidal range : 14 m English Channel Mont St-Michel 20 km Max tidal range : 14 m Oyster beds Mussel farms

3. English Channel Oyster beds

4. Photo : J. Mazurié

5. Mussel farms Photo : F. Cayocca

6. Photo : J. Mazurié

7. (after Ehrhold, 1999) Sediment distribution sand mud muddy sands

8. Current vertical profile ADCP measurements on tidal flat Main tidal flow on the intertidal flat: Cross-shore & flood dominant

9. wind (intensity, direction) Wave index (m) S.P.M. (g/l) 20 cm above bottom OBS sediment level (cm) ALTUS In the western part - tidal influence -same relationship between wind / waves / turbidity -strong wave erosion - fast recovery Water level (m) time (days, since 2003/02/01)

10. Development of a 2DH numerical model in order to :  simulate the spatial and temporal variation of turbidity (related to the primary production)  constitute a physical basis for ecological modelling (trophic capacity)  investigate the effects of mussel farms on sediment patterns

11. - SiAM2D model (hydrodynamics & sedimentary processes) - cartesian irregular grid - tide & wind forcings - waves: under process (wave/current coupling) - sediment transport : suspension only - « on-line » consolidation Mussel farms: Strickler friction adjustment (function of posts size, spacing between rows, angle between current and rows (previous study: LCHF, 1987) dx = 200 m to 400 m dy = 200 m to 500 m

12. Model Validation Along shore flow Cross-shore flow

13. Computed maximum current velocities 2.5 m.s -1 0 m.s -1 Sediment coverage

14. Influence of the mussel farms on the current magnitude flood high tide ebblow tide

15. Maximum velocities Sediment deposits after 1 year (tide only) Initial uniform sediment thickness : 0.1 m Without mussels farm Deposits thickness

16. Sediment deposits after 1 year (tide only) Initial uniform sediment thickness : 0.1 m with mussel farms without farms

17. Evolution of muddy sediment thickness over one month with and without farms with without Water level

18. with without Evolution of S.P.M. concentrations over one month with and without farms

19. Waves : SWAN computations (bottom velocity) Propagation Refraction (bathymetry & currents) flood high tide low tide ebb

20. Sediment deposits after 1 year (tide + constant small waves) Initial uniform sediment thickness : 0.1 m With mussels farm Tide only Tide + waves

21. 26 feb 2003 1 march 2003 Evolution of deposits after a 24 hour long storm 28 feb 2003 Storm 27/28 feb 28 feb 2003

22. 25 march 2003 14 march 2003 Evolution of deposits after a 24 hour long storm 5 march 2003

23. Conclusions & discussion - tidal flow intensity controls the sediment coverage, but not the dynamics - although macrotidal environment, sediment erosion/deposition is strongly controlled by waves, wind-induced currents - reduction of flow intensity within the mussel farms, acceleration on sides - increased deposition around the farm and onshore - increased sediment dynamics in the area, and consequently increased turbidity - validation of mussel farms impact is required (ADCP campaign) - modelling under process : stabilization of sediment budget after 1y mixtures of mud and fine sand inclusion of biodeposits ?