Wetlands International

Mangroves protect our coasts against wind and swell waves

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Source: Wetlands International

Ede, the Netherlands - A new report by The Nature Conservancy and Wetlands International proves that mangrove forests protect coastal populations and infrastructure against wind and swell waves. Preventing damage to coastal infrastructure and flooding, mangroves reduce wave height by as much as 66% over 100 metres of forest. With coastal populations vulnerable to the impacts of extreme events such as storms and hurricanes, these organisations say mangrove management needs to be included in climate change adaptation and disaster risk reduction efforts in coastal areas worldwide.

Coastal populations are particularly exposed to extreme events such as storms and hurricanes. These pressures may increase with climate change and sea level rise. Coastal wetlands such as mangrove forests strongly contribute to the safety, food security and income of tens of millions of people throughout the tropics. This new report by global NGOs The Nature Conservancy and Wetlands International brings together the latest scientific research from leading engineers and ecologists on coastal protection against waves. The report concludes that “mangroves can reduce the height of wind and swell waves over relatively short distances: wave height can be reduced by between 13 and 66% over 100 m of mangroves”.

Lead author Anna McIvor (TNC) says: “Waves are most rapidly reduced when they pass through a greater density of obstacles. This means that mangroves with aerial roots attenuate waves in shallow water more rapidly than those without”. She adds: “When the water gets deeper, waves may pass above aerial roots, but then the lower branches can perform a similar function.” The report outlines that the slope of the shore and the height of the waves also affect wave reduction rates through mangroves.

While narrow mangrove belts are able to reduce wind and swell waves, much larger areas of mangroves are needed to protect against storm surges. Additionally, mangrove forests provide many other vital ecosystems services, such as timber, food, nursery areas for fisheries and carbon sequestration, so wider bands of mangroves should be maintained or restored where possible.

Wetlands International has more than a decade’s worth of experience in protecting and restoring mangrove forests throughout Indonesia. Wetlands International Indonesia’s director Nyoman Suryadiputra has seen first-hand the benefits of wave reduction by mangroves: “Coastal villages in Aceh and Central Java experience greater protection because of the mangroves we restored together”.

Mangroves for coastal defence

To strengthen the role of mangroves in coastal defence, they need to be managed appropriately. The report points out that coastal engineers have started modelling the passage of waves through mangroves to understand the level of protection provided and to plan how to increase it. Appropriate management involves the protection of mangrove areas in key settings, and the restoration or planting of mangroves in degraded and deforested settings, where local conditions have been shown to support the establishment of mangrove seedlings. The report further suggests that dense mangrove forest, including species with aerial roots, will offer the highest level of protection from wind and swell waves.

Wetlands International Indonesia is currently working in Nusa Tenggara and Java on the building of green mangrove buffers to protect coastal areas. Together with local communities already more than half a million trees have been planted. “Mangroves are the best and cheapest way to protect coastal areas from waves. We need to protect and restore natural infrastructure like this all over Indonesia” urges Nyoman Suryadiputra.

Further research

With confirmation that mangroves can attenuate wind and swell waves, Anna McIvor says there is still a pressing need to better understand the roles that ecosystems can play in defending coasts against e.g. against storm surges, tsunamis, sea level rise and erosion. This first technical report launches a series of technical papers from The Nature Conservancy and Wetlands International addressing these topics.

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