The mangrove-mud coasts under human pressures Gratiot, N. and Anthony, E.J. © Ctophe, 2004 The research was carried supported by EuropeAid/124705/D/SER/GY,

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