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

Oysterbeds Mussel

farms

Max tidal range : 14 m

English Channel

Mont St-Michel

20 km

Max tidal range : 14 m

Oysterbeds Mussel

farms

English Channel

Oysterbeds

Photo : J. Mazurié

Musselfarms

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) ALTUSIn 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 velocities2.5 m.s-1

0 m.s-1

Sediment coverage

Influence of the mussel farms on the current magnitude

flood high tide

ebb low 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 musselfarms

without farms

Evolution of muddy sediment thickness over one month

with and without farms

with

without

Water level

Water level

with

withoutEvolution of S.P.M. concentrations over

one month with and without farms

Waves : SWAN computations (bottom velocity)PropagationRefraction (bathymetry & currents)

flood

hightide

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 2003

1 march 2003

Evolution of deposits after a 24 hour long storm

28 feb 2003

Storm27/28 feb

28 feb 2003

25 march 2003

14 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 ?