MANGROVES AS A TSUNAMI BUFFER- IS IT A MYTH?
de Silva Wijeyeratne, G. (2005). Mangroves as a Tsunami Buffer- Is It a Myth? Adoh. February. Pages 60-64.
(This article was published in the February 2005 issue of Adoh Magazine. The article may be reproduced in part or in whole with the permission of Adoh Magazine and the author. But please credit appropriately the source “Adoh Magazine, February 2005”).
In the first two weeks after the Tsunami, I found myself spending a lot of time on the coast of Sri Lanka. Initially it was with search teams looking for survivors and the dead, followed by visits with the media to carry the message of re-building the nation. At first, as with many others, my feelings and thoughts were pre-occupied with those who had suffered the loss of lives or had their livelihoods destroyed. After the search for the survivors and the dead, my time was occupied with taking action to restore the livelihoods of people. Although I received many phone calls from the media on environmental issues, preparing more detailed answers on environmental issues was a second priority, in the days soon after the Tsunami.
A frequent question to me, which I would like to address now, was on a story popular with some environmentalists, that the presence of mangroves saved lives from the Boxing Day Tsunami. Is this true or is it a myth contrived by the environmental lobby.
The answer is yes or no, depending on how one attributes the presence of mangroves to saving lives.
The simplest reason why mangroves saved lives is, if there were mangroves, there was often no town or village in the way of the Tsunami. I don’t believe there is something inherently special about mangroves as an eco system that saved lives or blunted the force of the Tsunami. Where fewer lives were lost it was due to what I would describe as the ‘coastal reservation’ effect.
First of all, let me counter the idea that the presence of mangroves reduce the force of an incoming Tsunami.
The force with which the waves hit the shoreline, did not depend on the type or amount of terrestrial vegetation or eco system on the shore line. It depended entirely on the oceanic topography, currents and other physical factors. A physical barrier such as a reef, would have blunted the force of the on-coming waves. A gently sloping sea bottom, extending well out from the shore, would also normally be expected to gradually dissipate the energy of the on-coming waves.
In certain places where mangroves were present, for example in and around Yala National Park, the waves were very strong. Where there were no physical barriers present such as sand dunes, whatever was on the shoreline, mangrove or scrub jungle, bore the full fury of the Tsunami. In Yala, I noticed that in some places, there was no evidence left of the modest area of mangroves that once existed. The waves had ripped them off.
North of Colombo, in the stretch from Chilaw to Puttalam, the extensive mangroves have now almost entirely been removed and replaced with prawn farms. In this area, there was little or no damage from the Tsunami, as the area was on the leeward side of the seismic origin. If one were to engage in fitting the facts to commercially advantageous theories, the prawn farming lobby could argue that all mangroves should be converted to prawn farms to act as a buffer for Tsunamis. This is patently absurd. In Negombo, a string of hotels stud a wide sandy beach. There were no mangroves on this stretch. The sea did rise and water came inland over a hundred meters from the shoreline and flooded the ground floor of the five star hotel, The Beach. But it was not a destructive wave. A few hundred meters away is the Seashells Hotel, no more than a few tens of meters away from the shoreline. The rising sea stopped at the edge of the hotel. Once again one cannot contrive theories where mangroves don’t exist and hotels do, on sandy beaches, wide or narrow, it stops a Tsunami in its track. To re-iterate, the force of the waves from the Tsunami had to do with coastal topography and physical factors, which at times over a few hundred meters, could produce widely varying strengths of the impact.
If we agree that the force of the waves is due to oceanic factors, the next question is how good a buffer or natural shock absorber, are mangroves. The most effective buffer is a strong, man made or natural physical barrier. Where there were large sand dunes, around 30 to 40 feet in height and broader in width, the force of the waves was broken. The colonial Dutch fort of Galle, is a good example of an effective man-made Tsunami defence. Although it was originally built for military reasons, anyone atop its high ramparts, some which are over thirty feet wide and as tall, would have escaped unscathed. Vegetation, building and people behind big sand dune complexes survived. A good example of a survivor is the Yala Village hotel, tucked behind extensive sand dunes. A few kilometers away, the Yala Safari Game Lodge, built between one of the many long gaps between sand dunes, was totally destroyed. Mangroves and scrub jungle would also slow down the passage of water. But the waves would need to pass several hundred meters of mangrove or scrub jungle (or even bare land) before the force of water was abated to the same degree as a forty foot high and forty foot wide sand dune system.
Clearly the saving of lives by mangroves was a secondary effect by creating a coastal reservation belt. Towns and villages which were several hundred meters inland behind a mangrove complex, were safe. The Galle Road heading south, passes behind a vast complex of mangroves in the Rekawa-Kalametiya area. The coastal area is thinly populated, on certain stretches, as much of the villages and towns are beside the road, well behind the mangroves. The coastal reservation system so created meant that the water did not reach these road-side villages and towns. So lives were saved. However in the same area, just a few villages were right beside the shore. These did suffer a loss of lives. But the total number of people besides the sea was relatively small, in these mangrove rich areas. Therefore, the overall number of casualties was very small relative to a large sea-side town such as Hambantota or Galle. The bottom line is that anything that was a few hundred metres inland, whether buffered by mangroves or bare land, was safe.
If in Sri Lanka, the first five hundred meters of coastline had been scrub jungle, mangrove or even a cash crop, with a rule that all human habitation had to be behind this belt, there may have been little or no human casualties on land. This does not of course mean that for human safety, such a wide coastal reservation system has to be implemented. Although a risk remain of a Tsunami, the degree of risk may not warrant such extreme measures. However, where there is already a belt of natural vegetation in the form of mangroves or scrub jungle, it makes sense to preserve these. Not just as a defence against tsunamis, but for preserving the integrity of the island’s rapidly degrading eco-systems.
With mangroves, there are other reasons for preserving them. They are a vital part of the oceanic food web. Destruction of mangroves result in the inevitable destruction of fish stocks. Sri Lanka’s marine fisheries, on which so many people depend for a livelihood, will be severely damaged if the remaining mangroves are damaged any more. Mangroves and other wetlands also act as buffers against freshwater floods, when there is heavy rain. Mangroves, acting as a coastal reservation, which serves as a tsunami buffer is a bonus. But there are more over-riding physical, commercial and scientific reasons for holding onto what is left of our precious mangroves.