1. 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, in collaboration with MWH and the Guyana sea defence public group. We would like to thank these two groups for their support.

2. Outline Mangrove-mud coasts distributions Effect of hard structures hydrodynamic analysis biogeomorphological analysis How to mitigate the negative effects?

3. The total area of mangroves in the year 2000 was 137,760 km 2 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 inhabitants Carbon stock (1000 T/ha, Donato, 2011 ) Coast protection (not only for extreme events), Nursery habitats and biosphere diversity ( Allison, et al., 2011 ) ? 1600km long Mangrove-mud coasts

4. Mature mangrove community Human induced pressure on mangrove-mud coasts ~15km Hydrodynamic fringe oscillation ~10 m ~5km extreme mangrove fringe oscillation ~1km regular Sketch of the Guyana shoreline (from the gvt) Fish/shrimp ponds Mangrove-mud coasts natural equilibrium

5. 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

6. Friedrichs and Aubrey, 1996 The dam reduces the tidal prism (by few percents) and sediment input, by consequences hydrodynamic analysis : Winterwerp et al., 2013

7. 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

8. But a complex topography and some naturally convex profiles

9. How much physic can help understanding interactions between cohesive sediment and ecosystems? Adapted from Corenblit et al., 2007 Coastal landforms Coastal landscape dynamics Biogeomorphological approach

10. ~15km mudbank fringe oscillation ~10 m 1 Accreting feedback loops under natural conditions ~5km extreme mangrove fringe oscillation ~1km regular with seeds mangrove (dead or alive) gentle wave dissipation seeds anchoring flocculation lowest exposure to shear flow siltation bed accretion Anthony and Gratiot, 2012 Biogeomorphological approach: Anthony and Gratiot, 2012 Courtesy of C.Proisy

11. 2 Eroding feedback loops for disrupted mangrove fringe ~15km mudbank fringe oscillation ~10 m ~5km extreme mangrove fringe oscillation ~1km regular without seeds sea Wall abrupt wave dissipation highest exposure to shear flow high sediment load turbid underflow flocs breakage and bed erosion need of sea defences Anthony and Gratiot, 2012 Biogeomorphological approach: Anthony and Gratiot, 2012

12. 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

13. 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!

14. 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

15. 09/86 10/89 11/93 08/95 02/00 11/01 6 km