oyster beds
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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
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 ?
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