Mangroves for coastal defence: Guidelines for coastal managers & policy makers
The role of mangroves in protecting our coasts against natural hazards such as storms, tsunamis and coastal erosion has been widely acknowledged. Even so, the level of protection provided by mangroves remains subject to debate. Numerous mangrove restoration projects were instigated after the 2004 East Asian tsunami in the belief that replacing lost mangroves would reduce future risk, but others raised concerns that not all of these projects were well conceived, and that some might create greater risks by inducing a false sense of security. Can mangroves reduce waves and storm surges? How will they influence the forces of a tsunami? Do they actually contribute to stabilizing coasts and build-up of soils? Can they keep up with sea level rise? A rich scientific literature exists describing many of these processes, but careful scrutiny is needed to determine what is known or unknown, and what remains uncertain.
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. An extensive review process yielded three technical reports that describe the extent to which mangroves reduce wind and swell waves, storm surges and erosion and how they build up soils in response to rising sea levels. The conclusion is that mangroves can indeed reduce risk from a large number of hazards, including waves, strom surges, tsunamis, soil erosion and local level sea rise.
This practical guidebook* summarises the findings of the reviews and provides practical management recommendations to coastal zone managers and policymakers. It helps the reader to assess the risk context in a target area, to define hazard-specific mangrove management interventions and to incorporate these in risk reduction strategies, climate change adaptation protocols and broader coastal development planning. Case studies provide practical examples of mangrove management approaches and references to background information, practical tools for risk assessment and mangrove management are provided throughout the book.
*download from right-hand column.
The key messages from the four sections of the report:
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.
The benefits offered by mangrove forests include timber & fuel production, productive fishing grounds, carbon storage, enhanced tourism and recreation, and water purification.
- Mangroves for coastal defence: Guidelines for coastal managers & policy makers
- Related article: To plant or not to plant? Stopping malpractices in using mangroves to increase coastal resilience
These guidelines are mainly based on 3 reviews, together summarizing hundreds of scientific papers on the role of mangroves in coastal risk reduction:
- McIvor, AL., Möller I., Spencer, T. and Spalding, M. 2012a. Reduction of wind and swell waves by mangroves. Natural Coastal Protection Series: Report 1. Cambridge Coastal Research Unit Working Paper 40. Published by The Nature Conservancy and Wetlands International.
- McIvor AL., Spencer, T., Möller, I. and Spalding, M. 2012b. Storm surge reduction by mangroves. Natural Coastal Protection Series: Report 2. Cambridge Coastal Research Unit Working Paper 41. Published by The Nature Conservancy and Wetlands International.
- McIvor, AL., Spencer, T., Möller I, and Spalding, M. 2013. The response of mangrove soil surface elevation to sea level rise. Natural Coastal Protection Series: Report 3. Cambridge Coastal Research Unit Working Paper 42. Published by The Nature Conservancy and Wetlands International.
This publication was made in the framework of the project Mangrove Capital: `Capturing Mangrove Values in Land Use Planning and Production Systems ́ and with support from the Ministry of Foreign Affairs the Netherlands/ DGIS.
This work forms part of the Nature Conservancy’s Mapping Ocean Wealth project, a collaborative work to quantify the value of coastal and marine ecosystem services at global to local scales – www.nature.org/oceanwealth
This work is supported through a lead gift from the Lyda Hill Foundation.
Spalding M, McIvor A, Tonneijck FH, Tol S and van Eijk P (2014) Mangroves for coastal defence. Guidelines for coastal managers & policy makers. Wetlands International and The Nature Conservancy. 42 p