The role of mangroves in protecting our coasts against natural hazards such as storms, tsunamis and coastal erosion has been widely promoted. But the supposed coastal protection services of mangroves have also been subject to debate. The Nature Conservancy and Wetlands International together with the University of Cambridge set out to map the current state of knowledge about the role of mangroves in coastal defence and put the different findings and views in perspective. The conclusion is that mangroves can indeed reduce the risk from a large number of hazards. This practical guidebook summarises the findings of the reviews and provides practical management recommendations to coastal zone managers and policymakers. These are the key messages:
Section 1. Is my shore at risk?
- Coastal managers need to understand risk in terms of hazard, exposure and vulnerability prior to determining what role mangroves can play
- The importance of mangroves in coastal defence and disaster risk reduction will depend both on the site characteristics and the local hazard context.
Section 2. The role of mangroves in coastal risk reduction
- Wind and swell waves are rapidly reduced as they pass through mangroves, lessening wave damage during storms.
- Wide mangrove belts, ideally thousands of meters across, can be effective in reducing the flooding impacts of storm surges occurring during major storms (also called cyclones, typhoons or hurricanes). This can significantly reduce flood extent in low lying areas. Narrower mangrove belts, hundreds of meters wide, will still be able to reduce wind speed, the impact of waves on top of the surge and flooding impact to some degree.
- Wide areas of mangroves can reduce tsunami heights, helping to reduce loss of life and damage to property in areas behind mangroves.
- The dense roots of mangroves help to bind and build soils. The above-ground roots slow down water flows, encourage deposition of sediments and reduce erosion.
- Over time mangroves can actively build up soils, increasing the thickness of the mangrove soil, which may be critical as sea level rise accelerates.
Section 3. Managing mangroves for coastal defence
- Mangroves don’t always provide a stand-alone solution; they may need to be combined with other risk reduction measures to achieve a desired level of protection. If they are integrated appropriately, mangroves can contribute to risk reduction in almost every coastal setting, ranging from rural to urban and from natural to heavily degraded landscapes.
- For mangroves to optimally contribute to risk reduction, their conservation needs to be incorporated into broader coastal zone management planning: they need to be protected and restored, allowing wise use where possible.
- Mangroves, and their coastal risk reduction function, can recover in most places where appropriate ecological and social conditions are present or restored.
Section 4. Recognizing the multiple values of mangroves
- Mangroves are among the most valuable ecosystems in the world. Decision makers, and the public, need to take full account of the many benefits that mangroves provide, and consider the true costs that may incur from mangrove loss.