Mangroves ‘no defence v. Tsunamis’

Coastal mangroves and green belts offer little or no protection against the deadly might of a tsunami.

This is the finding of a controversial new scientific report, just published in the international journal, Estuarine and Coastal Shelf Science.

Lhoknga Beach, April 2005. Five metre sand dunes and coastal forest provided no protection for Lhoknga.

Research from the ARC Centre of Excellence for Coral Reef Studies (CoECRS) at James Cook University, the University of Guam (UoG), and the Wildlife Conservation Society-Indonesia Programme (WCS-IP) has overturned claims that death rates were lower in villages shielded by mangroves during the 2004 Boxing Day tsunami.

The researchers’ findings challenge current advice by the United Nations Environment Program, non-government organisations, and other scientists that ‘green belts’ and buffer zones should be incorporated into reconstruction efforts to protect villages from future tsunamis.

In fact, the ecologists warn, these green belts may give a false sense of security, leading to greater loss of life should such a tragedy recur in future.

When Dr Alex Kerr of UoG, Dr Andrew Baird of CoECRS and Dr Stuart Campbell of WCS-IP, reanalysed data from a recent Indian study, they found it was height above sea-level and distance from the shore that protected the inhabitants of some villages, rather than vegetation.

“Our re-analysis revealed that the distance of a village from the coast and the height of the village above sea level explained 87 % of the variation in mortality among villages. Once these two variables were taken into account, vegetation area provides less than a 1% increase in explanatory power,” says Dr Kerr.

“The apparent link between vegetation area and mortality was actually due to the fact that more vegetation grows at higher elevations above sea-level – and the greater the distance from the sea, the greater the area of vegetation.”

“In short, if you had hamlets of equal elevation and distance from the sea, differences in vegetation area would make little difference to the death toll from a tsunami.”

Dr Baird says the new analysis means there is genuine danger in overstating the protective capacity of vegetation in the event of another tsunami. “Mangrove forests provide coastal communities with many valuable goods and services. However, expecting them to provide protection from tsunamis is unrealistic. Following the Krakatoa eruption the resulting tsunamis penetrated 8 kilometres through primary rainforest.”

“Right now buffer zones are being enforced through a combination of government legislation, and a refusal of aid to people that wish to move back to their homes, in the belief they may provide protection from some future huge event. These buffer zones are only between 100 and 500 metres, yet they may dispossess over a million people in India and Sri Lanka.”

“It’s a beautiful idea that green belts can stop a tsunami, and its aims are commendable.  But it isn’t true, and it won’t work.”

Logically, the safest course is to build farther from the sea or on higher ground. However, this cannot be considered in isolation from the social economic and emotional cost of shifting entire communities and their livelihoods, the researchers say.

“Tsunamis are catastrophic but, fortunately, they are rare – and a well-organised early-warning and evacuation plan may be far more effective in saving lives.”

“Even if property is destroyed, this may still be preferable to the social dislocation and potential conflict involved in removing entire communities from their coastal existence.”

Dr Baird adds that a further dimension of the problem is the fact that in many cases these buffer zones are being used by developers and local authorities as a pretext for moving communities, so they can then build tourist facilities on the vacated shoreline.

Article:

Kerr, AM, Baird, AH and Campbell, SJ (2006). “Comments on “Coastal mangrove forests mitigated tsunami” by K. Kathiresan and N. Rajendran [Estuar. Coast. Shelf Sci. 65 (2005) 601-606].” Estuarine, Coastal and Shelf Science 67(3): 539-541.
Link to Full Text or pdf

More information:

Dr Andrew Baird, CoECRS and James Cook University, Tel +61 7 4781 4857; andrew.baird@jcu.edu.au
Dr Alex Kerr, University of Guam Tel +1671 735 2182; akerr@guam.uog.edu
Dr Stuart Campbell of the Wildlife Conservation Society, Tel +62 251 321 527 s.campbell@wcsip.org
Professor Terry Hughes, Director, CoECRS, + 61 7 4781 4000
Jenny Lappin, CoECRS, + 61 7 4781 4222
Jim O’Brien, James Cook University Media Office, + 61 7 4781 4822