Morphological evolutions of a macrotidal bay under natural conditions and anthropogenic modifications Mont Saint Michel Bay, France F. Cayocca, P. Le Hir,

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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

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

English Channel Oyster beds

Photo : J. Mazurié

Mussel farms Photo : F. Cayocca

Photo : J. Mazurié

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

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

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)

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

- 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

Model Validation Along shore flow Cross-shore flow

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

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

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

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

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

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

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

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

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

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

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 ?