Mangroves for coastal defence. How to reduce waves naturally..

Mangroves can contribute to risk reduction in many coastal settings, according to a report by Wetlands International and The Nature Conservancy, offering guidelines for policymakers. ‘In some places this role of mangroves can be enhanced by combining the protection benefits with other risk reduction measures, making them part of a wider coastal defence and disaster risk reduction strategy. ‘

In their report, researchers explain that on every coastline waves and currents create change, sometimes bringing sediments to the coast, but sometimes causing erosion and the loss of land. Where mangroves occur they generally reduce erosion and enhance sedimentation. The mangrove vegetation reduces wave energy and slows the flow of water over the soil surface, reducing the water’s capacity to dislodge sediments and carry them out of the mangrove area.  At the same time the slower water flows can allow already suspended sediments to settle out from the water, resulting in increased deposition of sediment.

Typical mangrove soils are rich in organic matter produced by the mangroves themselves, including living roots but also dead leaves and woody materials. The dense network of fine roots helps to Armour the soil from erosive forces and to trap and bind soil particles together. When mangroves are lost, the patterns of sediment movements can change dramatically – where mud and soil were stable or even gradually building up, they may begin to erode so that land disappears into the sea.

Wetlands’ guidance for  coastal managers:  

• Maintain wide mangrove belts. To significantly reduce everyday waves you need a belt of hundreds of meters wide – the more the better. A few trees won’t help very much.

• The more obstacles the better, so mangroves of various species of different age and size are most valuable in reducing wave heights. In a dense mangrove forest packed with aerial roots and low branches, a wave may be reduced to half its height after 100m passage through the mangroves, while in a more open forest, it might take 500m of mangroves to reduce such a wave.

• Even relatively young or small mangrove forests can reduce the height of common wind and swell waves. Hence, reforested areas become effective at reducing waves in just a few years.

• Protection of adjacent ecosystems is important as these can also help attenuate waves. They include sandbanks, sea grasses and coral reefs, as well as landward dunes and salt marshes.

You can read the full report here: