Turn to nature

Mangroves and other vegetation can help resist tsunamis

By Arun Bapat
Published: Tuesday 15 March 2005

Turn to nature

-- The new millennium began disastrously: an earthquake struck Bhuj, Gujarat on January 26 2001. India had barely recovered from the seismic calamity when a tsunami hit the southern coasts on December 26, 2004. In this context, let us not forget that other coastal areas are equally vulnerable to tsunamis. Scientific archives tell us that a tsunami had struck Kandla in Gujarat in 1945 -- it was precipitated by an earthquake that measured 8.25 on the Richter scale, and had its epicentre 100 kilometres south of Karachi. The height of this tsunami at Kandla was about 12 meters. Kandla was not a very developed port then; so the destruction the wave wrought was not very large. But things are very different today: Kandla is one of the largest ports on the west coast and a big financial, commercial and navigation hub. A tsunami like the one of 1945 would create another Nagapattinam here. It is therefore essential that appropriate steps are immediately taken to protect this port city, and other cities as well.

Walls don't help After the December 26 calamity, several people have advocated constructing protective walls along coasts. But will these keep tsunamis at bay? No. Because we really do not know how high the killer waves would be: a 5-metre high wall can be swept away by 7-metre high waves. Humankind must remember that it cannot conquer nature.

Some experts also moot a tsunami warning system along the lines of the Pacific Tsunami Warning System for the Bay of Bengal and the Arabian Sea. The Pacific region gets one to three tsunamigenic earthquakes, every year. This keeps the system activated. Even then, this system issues two or three false alarms, every year. But the Indian scenario is somewhat different: tsunamis occur after several decades along Indian coasts. It quite likely this alarm system will lie unused for long. One wonders how effective such an idle system will be when a tsunami actually strikes. Can our engineers and technicians maintain a warning system that might have to function once in 50 years? A better way of dealing with tsunamis is educating people about the killer waves. It is well known that about an hour before a tsunami hits the coast, the sea recedes between a half to two kilometres. A day or two before the tsunami, big fish swim away to the deep seas. Minimum temperatures suddenly drop by 2 cto 4 c one or two days before a tsunami. Informing people about such stark signs will help.

Helpful vegetation Moreover, we could do well to understand that nature offers us resources to deal with the killer waves. Mangroves are one such resource. The total surface area of a small mangrove leaf is about 2-5 square meters. When the tsunami hits the bush, the energy of the waves is absorbed by the leaves. The blades bend when hit, but they do not break -- they spring back immediately to resist the next onslaught.

Planting of tress near shores is effective in reducing the amplitude of waves. For example, Nipa fruticosa and Spinifix squarosus trees are known to offer resistance to tsunamis. And so do, the well known Phoenix paludosa (khajoor) and Casuarina (suru) trees -- their stems are very strong and so they are excellent protection against severe winds. They are also effective sand binders. Even high tide water does not expose their roots or take away sand from the ground in the vicinity. The advantage of planting mangroves or other trees is that once planted, they do not need regular maintenance.

All of us must act immediately. We cannot allow our ports to be at the mercy of tsunamis. There should not be another Nagapattinam. Similarly, other major ports on the east coast, such as Paradip and Haldia, and Ennore and Chennai, also need appropriate protection measures. Clubbing modern science and traditional knowledge is the best mitigation of tsunamis.

Arun Bapat is a Pune-based seismologist

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