The devastating tsunami of 26 December 2004 moved the governments of the affected states in peninsular coasts of India and efforts to replant the mangroves were augmented. This move was triggered more by the panic caused by the tsunami. In Tamilnadu alone steps were taken to plant 4000 hectares (ha) of Casuarina and 1400 ha of mangrove plants. Casuarina being cheaper, hardy and easily available species was a preferred species to save the coastal habitations.
Are mangroves and such plantations effective shields against tsunamis? Or are there other reasons for lesser submergence in some parts of the coasts? In fact scientists are divided over these two issues. Mangrove vegetation specialists have reasons to believe that mangrove plants are the saviors of coasts. Whereas geologists and oceanographers give evidences of the ground beneath that is, geomorphology, type of the coast and landforms like sand dunes as effective barriers against tsunami inundation (Ref. 'Bathymetry helps to fathom tsunami power').
An insight in to both the views will be useful for the planners, developers and the residents of the coastal regions.
Mangroves are an interface between the sea and the land. The mangrove forests provide services to humans. They are the breeding, spawning and nursing grounds for marine and pelagic species. Mangroves provide a daily livelihood to the local population. Some of the mangrove species like Rhizophora spp. with their above ground wide network of roots act as barriers against the onslaught of tidal waves and other ocean influences.
In the greed for land and also to make a quick bounty like the forests, mangroves too have been destroyed. As per one estimate in the second half of the 20th century, 50% of the world's mangrove forests have been destroyed. The current rate of loss of mangroves varies from 1 to 20% the study says.
The Indian Ocean tsunami took a toll of nearly a quarter million people and rendered millions homeless. After the tsunami biologists and local residents felt that had the bio-shield (mangrove) been there many a lives could have been saved! There is a belief that mangrove forests play a crucial ole in protection of the coasts against the onslaught of tsunami waves. However, there is very little scientific data to support the belief.
R.S. Bhalla of Foundation for Ecological Research, Advocacy and Learning, Puducherry, India has analysed and published his study in Current Science about the controversy whether mangroves act as a savior or it is just a coincidence! He has given detailed documentary evidences and also quoted other workers in favor or against the issue. Quoting Farid Dahdouh-Guebas a Belgian biologists and a protagonist of mangroves and K. Kathirsen and K. Rajendran's work published in Estuarine and Coastal Shelf Science in 2005 says that vegetative shelter belts, particularly mangroves were effective defences against tsunami.
A.M. Kerr and his co-workers responded against the hypothesis of those supporting mangroves as a bio-shield against tsunami. Kerr et al's reanalysis of the data shows that mangroves explain for less than 1% of human mortality in the affected areas. Bhalla has drawn attention to the work of B. Chatenoux and P. Peduzzi, the Swiss Geophysicists who carried out a spatial and statistical analysis of the tsunami affected areas in the eastern peninsular coast of India and found that the width of the land strip was, in vast majority, influenced by the distance to fault lines as well as inclination and length of proximal slope. However, biologically they found that the areas covered by seagrass beds were less impacted, whereas areas behind coral reefs were more affected. Coincidentally, mangroves are present in areas naturally sheltered by the geomorphology. Thus it is premature to say if the mangroves were really the saviors against the onslaught of the fury of the sea!
Bhalla says Nagapattinam suffered greater havoc because of the shallow coast, compared to Puducherry where the coast is steeper.
Geological factors account for a greater impact of the ocean waves has been amply proved by many evidences. Harry V. Andrews and his Herpetologist colleagues from Chennai, after the tsunami observed in Andaman and Nicobar Islands that the M 9 quake caused subsidence of Nicobar Islands areas around South Andaman Island and also upheaval by an average one metre of Little Andaman Island. This subsidence and upheaval drastically affected the habitats of the Olive Ridley turtles and other fauna.
Bhalla using satellite data, digitized imageries and statistical methods carried out a detailed study of the area of Coromandal coast between Kalpet, Puducherry and Vedarniyam in Nagapattnam. He collected data from 69 points from these areas a developed a Normalized Difference vegetation Index (NDVI) and found that vegetation did not have a significant impact on inundation distance. However, his study was not able to differentiate between the type of vegetation, whether it was mangrove or otherwise, therefore it can not be said with certainty whether Mangroves were a better defence against inundation or not.
However, the study has positively pointed that sand dunes are the most immediate form of coastal protection. Though a few agencies involved in the protection of Coromandal coast against possible future tsunamis, says Bhalla have considered beach and dune conservation. But the extensive dune plantation and construction of permanent settlements, sea walls and groynes is harmful, because then the natural beaches disappear. Puducherry is a classic example of disappearing beaches. Dunes act as windbreaks, protect against storm surges and tsunami inundation. Many of the coastal settlements constructed behind the beach dunes escaped the wrath of tsunami. Dues are integral to the livelihood of the fishing communities and also habitat of Olive Ridleys. The best feature of dunes is that they are wonderful aid to ground water recharge.
Whether the mangroves save the coastal habitation or not, they need to be conserved. Mangroves are part of natural eco-system. How much protection they give against tsunami inundation sensu-stricto is a matter of detailed study, but it is a fact that they do act as a natural barrier. Local folks anywhere in the coastal regions of the world affected by tsunami are always in favor of mangroves. That is why in Malaysia and East Africa mangroves have been successfully resurrected. On the other hand areas drastically cleared of mangroves like Banda Aceh in Indonesia suffered the most. Call it a coincidence or nature's wrath!
The studies so far point out that there is insufficient evidence to prove the role of mangroves in saving the coasts. However, a complete data base regarding the bathymetry of the coasts, versus tsunami damage is also drastically needed. It is a fact that gently sloping beaches of the east coast are more prone to tsunami than the abrupt and vertical coast lines of the west. Damage from tsunami and other climatic impacts of the seas can be considerably reduced if the coastal regulations are strictly implemented, dunes are left undisturbed and mangroves are resurrected. Mangroves may not be a strong or useful barrier against tsunami, but they are strong enough against high tides etc.