Wetlands International

How mangroves help in reducing flooding and coastal erosion


Inhabitants of low lying delta areas are particularly exposed to flooding and erosion caused by storms and hurricanes. These pressures increase with climate change and sea level rise. Coastal wetlands, such as mangroves, can play a key role in damage mitigation during disasters, as well as in stabilising coastlines. They also contribute to aquaculture and fisheries. Integration of ecosystem-based coastal protection in climate change adaptation and disaster risk reduction policies and resulting measures to conserve these landscapes are essential if mangroves are to keep protecting us. Full article featured in Outreach Magazine.

Mangroves may help to mitigate disaster risk and damage in three ways, complementing other measures in a coastal defense strategy. First, mangroves can reduce storm surge levels by up to half a meter for each kilometer of mangrove that the storm surge passes through. Secondly, the height of wind and swell waves is reduced by 13 to 66 per cent within the first 100m of mangroves. These waves can be superimposed on top of storm surges, so their reduction can make a critical difference to storm impacts. Even during ‘everyday’ conditions, when such waves may be small, they still contribute to coastal erosion if their energy is not dissipated by a dense tangle of mangrove roots and branches. Their devastating impact is felt for example in parts of Central Java, where the coastline retreated hundreds of meters in a decade due to a mix of mangrove deforestation, aquaculture development, soil subsidence and sea level rise. Thirdly, mangroves can help to stabilise sediments, both through their active growth and deposition of organic matter and by capturing sediments.

To perform these functions effectively and to constitute a resilient mangrove, the mangrove green belt should be wide enough, and contain enough diversity of species and age groups. Relatively wide mangrove belts are still present in many countries, including large parts of South America, the USA, West and Central Africa, Bangladesh, India, Southeast Asia and Australia, although in many of these places they are severely threatened by deforestation and unsustainable land-use practices, such as the conversion of mangrove forest to fish and shrimp ponds.

Often the automatic response to erosion is to construct hard engineered structures, such as breakwaters, which limit sediment input even more, thereby aggravating erosion. In order to stop the erosion process and regain a stable coastline, the loss of sediment needs to be reversed. The best way to do this is through ‘building with nature’, using engineering techniques in combination with natural processes. For example, by placing well-designed permeable structures made of local materials such as bamboo, twigs or other brushwood in front of the coastline.

These structures mimic the function of mangrove roots and branches as they dissipate waves and trap sediment. Once the sediment is sufficiently stable and enough elevation has been gained mangroves will naturally recolonise and once again protect the coast. In the Netherlands this approach has been applied with salt marshes for a century and the governments of Indonesia and Vietnam are now testing it for mangrove-mud coastlines. ‘Building with nature’ solutions are often cheaper than conventional solutions and become stronger over time as the ecosystems mature. Local ownership and participation is of course key to success.

Acknowledging that mangroves are crucial in the face of climate change, a major concern of country delegates has been that mangroves may be lost as sea levels rise. The latest evidence suggests however that in many areas mangroves may be able to keep pace with rising sea levels when the conditions are right. They do so by capturing sediments and organic matter, thus building up soils (and storing carbon in the process). This highlights the need to maintain and restore healthy mangroves and their sediment supplies. In many areas sediment supply and hydrology have been severely disturbed because of upstream dams and coastal infrastructure. In areas where mangrove soils may not be able to keep pace with sea level rise, space should be allowed for mangroves to move landward.

Careful mangrove management, in combination with soft or hard engineering measures, will maintain or restore immense value and help to secure a safer future for many coastal populations. Article featured in Outreach Magazine.

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