The mangrove-mud coastsunder human pressures
Gratiot, N. and Anthony, E.J.
© Ctophe, 2004
The research was carried supported by EuropeAid/124705/D/SER/GY, in collaboration with MWH and the Guyana sea defence public group. We would like to thank these two groups for their support.
Outline
Mangrove-mud coasts distributions
Effect of hard structureshydrodynamic analysisbiogeomorphological analysis
How to mitigate the negative effects?
The total area of mangroves in the year 2000 was 137,760 km2 in 118 countries and territories in the tropical and subtropical regions of the world. Giri et al., 2011.
Living resource for tens of million of inhabitantsCarbon stock (1000 T/ha, Donato, 2011)Coast protection (not only for extreme events),Nursery habitats and biosphere diversity (Allison, et al., 2011)
? ? ?
1600kmlong
Mangrove-mud coasts
Mature mangrove community
Human induced pressure on mangrove-mud coasts
~15km
Hydrodynamic fringe oscillation
~10 m
~5km extreme
mangrove fringe oscillation
~1kmregular
Sketch of the Guyana shoreline (from the gvt)
Fish/shrimpponds
Mangrove-mud coasts
natural equilibrium
Many mangrove-coasts are eroding, almost everywhere!
Rice culture, F.Guiana
Gensac, 2012
Fish ponds, Thailand
Winterwerp, 2005
Why,what can we do?
Mangrove-mud coasts
Friedrichs and Aubrey, 1996
The dam reduces the tidal prism (by few percents) and sediment input, by consequences
hydrodynamic analysis : Winterwerp et al., 2013
Positive feedback loop (snowball effect) for coastal erosion
Too extensive land-use
Reduction in on-shore sediment flux & local increase in wave hegith due
to reflection
(more)erosion
Concave-up mudflat profile
Higher waves
Increase in gross erosion
hydrodynamic analysis : Winterwerp et al., 2013
But a complex topography and some naturally convex profiles
How much physic can help understanding
interactions between cohesive sediment and ecosystems?
Adapted from Corenblit et al., 2007
Coastallandforms
Coastal landscape dynamics
Biogeomorphological approach
~15km
mudbank fringe oscillation
~10 m
1 Accreting feedback loops under natural conditions
~5km extreme
mangrove fringe oscillation
~1kmregular
with seedsmangrove(dead or alive)
gent
le w
ave
diss
ipat
ion seeds
anchoring
flocculation lowest exposure to shear flow
siltationbed accretion
Anthony and Gratiot, 2012
Biogeomorphological approach: Anthony and Gratiot, 2012
Courtesy of C.Proisy
2 Eroding feedback loops for disrupted mangrove fringe
~15km
mudbank fringe oscillation
~10 m
~5km extreme
mangrove fringe oscillation
~1kmregular
without seeds
sea Wall
abrupt w
ave
dissipatio
n
highest exposure to shear flow
high sediment load
turbid underflow
flocs breakage and bed erosionneed of sea
defences
Anthony and Gratiot, 2012
Biogeomorphological approach: Anthony and Gratiot, 2012
1) Follow the recommendations of Winterwerp et al., 2013 & the ones of Anthony et al. 2012, i.e.
2) Restore some muddy (and not silty) conditions along the coast.3) Reconnect mature mangrove with mudshoals.
C.Acrkoyd, 9th EDF
How to mitigate the negative effects
there is often no need for planting (Lewis 2005)
→ dredging at good temporal windows→ facilitate seeds arrival from mature mangrove seed by passing, human transplantation
Guyana, Region 3 – Bovell – Rumzeight
Guyana without mangrove belt,
what would be the price to pay to maintain a sea defence ?
Solutions to restore 0(1km) of mangrove fringe / hard structure on 10th of kms
Why not go from aquaculture to agriculture (avicennia?)
Thank you for your attention!
Strategy of restoration synthesised by Winterwerp et al., 2013
1) Do not erect sea defenses too close to the waterline(in between the mangrove stands).
2. Do not deploy revetments to enforce the muddy coastline,
3. Do not erect coast-parallel breakwaters.
4. Do not plant the wrong mangrove species
09/86
10/89
11/93
08/95
02/00
11/01
6 km