Ecosystems services provided by coral reefs and mangroves against tsunamis: keys for New Caledonia

Abstract

This study concentrates on the role played by coral reefs and mangroves to protect population from tsunamis within the context of sea level rise and coastal anthropization.

Full text

This study concentrates on the role played by coral reefs and mangroves to protect
population from tsunamis within the context of sea level rise and coastal anthropization.
Bruce THOMAS |IRD –Nouvelle-Calédonie
bruce.eo.thomas@gmail.com
Jean ROGER |IRD –Nouvelle-Calédonie
jean.roger@ird.fr
Jérôme AUCAN|IRD –Nouvelle-Calédonie
jerome.aucan@ird.fr
1 –GENERALITIES
2 - MAPPING METHOD
Over the last 150 years, at least 37 tsunamis have reached the shores of New
Caledonia (Roger et al., 2019). From local to regional and far-field seismic sources,
waves showing a maximum of up to 2 m have been reported, sometimes causing
destructions and even once, killing people in Lifou.
New Caledonia is a sparsely populated island of circa 280000 inhabitants
surrounded by a great coral reef barrier and mangroves. Within the framework of
the TSUCAL project, it has been estimated that 85%of the island population is
living within the coastal fringe, below 20 m high: what about this population's
vulnerability towards tsunami future potential submersions?
The aim of the present study is to provide keys which could help risk managers
in terms of coastal development in New Caledonia by trying to answer the
question: where should we look at the waves more accurately?
with or without the coral reef barrier
with or without the natural gaps in the coral reef barrier
with or without mangroves by placing them strategically in front of gaps in the coral
reef barrier (reintroduction of mangroves ecosystems)
with variable friction parameters on the coral reef barrier
for multiple ranges of tsunamis (origin, depth, magnitude, amplitude)
highlight the ideal set of coral reef barrier and mangroves to protect from tsunamis
work on the mapping of population facing coral reef barrier / mangroves responses
to tsunamis: http://maps.oceanwealth.org/# (Beck and al., 2018)
MAPPING CORAL REEF
Slowing down tsunamis –filtering role of the reef
Coral reef barriers could reduce tsunami impact on the coast by decreasing the
wave energy by 80% thanks to frictional processes (Kunkel and al., 2006). However,
for a very large wave amplitude, empirical evidence in the 2004 Indian Ocean
tsunami highlights that the regions with coral reefs were impacted as much as
regions with no reefs.
Preserving coral reef in New Caledonia
The shape and features of coral reefs in New Caledonia help it to be a buffer
against tsunamis (i.e. zoom 2). Openings are the damper in this study but
highlights the importance of restoring and protecting much more reefs, as it can
be a cost-effective approach to enhance first line coastal defense (Brittin, 2014).
REFERENCES
3 –MODELING AND MAPPING PERSPECTIVES
Tsunami maximum
amplitude for the
model example
Coastal area below 20 m
elevation
Coastal population living
below 20 m elevation
function of the number
precise data of populated
areas from ISEE New
Caledonia (Institute of
Statistics and Economic Studies)
Slowing down tsunamis –filtering role of the mangrove
Mangrove attenuate directly waves by providing additional drag
against wave energy through their trunks, as well as the aerial roots
that can attenuate surge energy by adding friction (Laso Bayas and
al., 2011). Existing dense coastal vegetation in front of settlements
also reduces casualties by an average of 5%.
Preserving mangrove ecosystem reef in New Caledonia
On the west coast of New Caledonia, mangrove is widely present in
front of villages (i.e. zoom 1), reintroduction in the holes would help
protecting tribes. Mangroves also provide ecosystemic services,
especially in terms of fish development (Kathiresan, 2012).
Beck, Michael W., Iñigo J. Losada, Pelayo Menéndez, Borja G. Reguero, Pedro Díaz-Simal, et Felipe Fernández. « The Global Flood Protection Savings Provided by Coral Reefs ».
Nature Communications 9, no 1 (décembre 2018): 2186. https://doi.org/10.1038/s41467-018-04568-z.
Brittin, Rachel. « Coral Reefs Reduce Risks From Natural Hazards », A scientific summary from The Pew Charitable Trusts, mai 2014.
Kathiresan, K. « Importance of Mangrove Ecosystem ». International Journal of Marine Science, 2012. https://doi.org/10.5376/ijms.2012.02.0010.
Kunkel, Catherine M., Robert W. Hallberg, et Michael Oppenheimer. « Coral Reefs Reduce Tsunami Impact in Model Simulations ». Geophysical Research Letters 33,no 23 (14
décembre 2006): L23612. https://doi.org/10.1029/2006GL027892.
Laso Bayas, J. C., C. Marohn, G. Dercon, S. Dewi, H. P. Piepho, L. Joshi, M. van Noordwijk, et G. Cadisch. « Influence of Coastal Vegetation on the 2004 Tsunami Wave Impact in
West Aceh ». Proceedings of the National Academy of Sciences 108, no 46 (15 novembre 2011): 18612-17. https://doi.org/10.1073/pnas.1013516108.
Roger, Jean, Bernard Pelletier, et Jérôme Aucan. « Update of the Tsunami Catalogue of New Caledonia Using Adecision Table Based on Seismic Data and Maregraphic Records ».
Natural Hazards and Earth System Sciences Discussions, 26 février 2019, 1-22. https://doi.org/10.5194/nhess-2019-36.
Regional seismic sources of New Caledonia tsunamis
MAPPING MANGROVE
80 cm
0 cm
1
2
Seismic source of the tsunami model
South Vanuatu Trench
magnitude Mw = 7.5
earthquake
23 km depth
(Roger et al., 2019)
West coast
subsidence:
wall of mangroves
with some gaps
East coast
uplift:
mangroves only
near estuaries