Indonesia: Mangrove forest planted as tsunami shield

Fred Pearce New Scientist 17 Dec 14;

THE worst tsunami on record hit South-East Asia on 26 December 10 years ago, killing more than 227,000 people in total. I recently toured villages on the west coast of Aceh, the Indonesian province that bore the brunt of the impact, where 167,000 lost their lives when a 20-metre wave crashed ashore that morning. I also saw the results of heavy mangrove reforestation since the tsunami.

Can replanting mangrove forests on tropical coastlines really protect communities from the immense destruction of a tsunami such as the Indian Ocean killer wave that struck 10 years ago?

My guide was environmental scientist Agus Halim whose wife and two children died in the disaster. He helped mastermind ecological reconstruction of the damaged coastlines for the Indonesian government, and was clear in his answer. "They are very important for protecting coastal areas, because they can absorb wave energy," he says. And, thanks to the thousands of hectares of mangroves planted round Aceh since the tsunami, next time they would do the job – or at least, that's the hope.

The tsunami was caused by an earthquake just offshore. In the fishing village of Keude Unga, an old man in a cafe told me that the wave was higher than the trees. "Every building in the village was destroyed. The only people who survived were those who ran for the hills," he says. In some of the villages, nobody survived.

Nothing could have prevented massive destruction, nor stopped the permanent sea invasion that followed. The quake caused widespread land subsidence that shifted the shoreline inland by up to 800 metres and left the sites of villages, such as nearby Gle Jong, permanently submerged.

There is growing evidence that even under such extreme circumstances, mangroves' dense root and branch networks could help dissipate tsunamis, reducing their devastation.

Some early claims for the protective effects of mangrove forests in 2004 have been debunked as statistically unreliable, because there are too many variables to allow easy conclusions, such as difference in beach slopes or coast angles (Estuarine, Coastal and Shelf Science, doi.org/bvd8ms).

But others are more robust, says Halim. A study found villages behind mangroves survived best (Science, doi.org/b564c7). And detailed analysis of satellite images of the west coast of Aceh by Juan Carlos Laso Bayas of the University of Hohenheim, Stuttgart, Germany, suggests that forests in front of settlements resulted in 8 per cent fewer casualties during the tsunami (PNAS, doi.org/c6wrbg).

That figure sounds modest, but it represents perhaps 10,000 lives. And it could be conservative. Experiments by Tetsuya Hiraishi and Kenji Harada of Kyoto University, Japan, showed that a 100-metre belt of dense mangroves could reduce the destructive energy of a tsunami by as much as 90 per cent.

The tragedy is that, as in many places across the tropics, most of Aceh's mangroves had been uprooted before the tsunami, to make way for shrimp ponds.

Since 2004, many NGOs have tried to replant the trees. Perhaps the most successful has been the Green Coast project supervised by Halim and run by two NGOs based in the Netherlands, Wetlands International and Oxfam Novib. In return for planting trees, survivors were offered collateral-free loans to help start post-tsunami businesses in their villages. Wetlands International agreed to write off the debts if 75 per cent of the trees survived for two years. This was critical for the project's success, says Wetlands' Indonesian director Nyoman Suryadiputra.

That target was achieved almost everywhere. Close to 2 million trees have been planted around 70 villages. This included mangroves on mud, and fast-growing native casuarina pines on sandy areas.

The foreign aid groups left Aceh five years ago. Most of the projects started with Green Coast credit, including cafes, fishing boats and cattle-rearing farms, are doing well. Life is returning to normal. There seems to be a baby boom. Halim married again and has a child. We watched the boy play in the sand as we ate lunch at a beachside restaurant.

The science, reiterated in a UN report on mangroves in September, suggests the villagers of Aceh can be reassured that the trees will provide some protection from the ocean. But if they ever hear the roar of an approaching tsunami again, they should head for the hills as fast as they can.

Fred Pearce's trip to Aceh was funded by Wetlands International

This article appeared in print under the headline "Ten years after the tsunami"