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.
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.
Mangrove work by Wetlands InternationaI: http://www.wetlands.org/mangroves
Mangrove work by TNC: http://www.naturalcoastalprotection.org and http://coastalresilience.org
Read the article on Mongabay 'Mangroves protect coastal areas against storm damage'
Read the blog from the IUCN Conference 'A big YES to mangroves, but where and how?'
Communications and Advocacy Officer Wetlands International
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Lead author, TNC
McIvor, A.L., Möller, I., Spencer, T. and Spalding. M. (2012) 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. 27 pages. ISSN 2050-7941.
Download the report
Coastal populations are particularly vulnerable to the impacts of extreme events such as storms and hurricanes, and these pressures may be exacerbated through the influence of climate change and sea level rise. Coastal ecosystems such as mangrove forests are increasingly being promoted and used as a tool in coastal defence strategies. There remains, however, a pressing need to better understand the roles that ecosystems can play in defending coasts. This report focuses on mangrove forests and the role they can play in reducing wind and swell waves. While mangrove forests are usually found on shores with little incoming wave energy, they may receive larger waves during storms, hurricanes and periods of high winds. Large wind and swell waves can cause flooding and damage to coastal infrastructure. By reducing wave energy and height, mangroves can potentially reduce associated damage.
All evidence suggests 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. The highest rate of wave height reduction per unit distance occurs near the mangrove edge, as waves begin their passage through the mangroves.
A number of characteristics of mangroves affect the rate of reduction of wave height with distance, most notably the physical structure of the trees. Waves are most rapidly reduced when they pass through a greater density of obstacles. Mangroves with aerial roots will attenuate waves in shallow water more rapidly than those without. At greater water depths, waves may pass above aerial roots, but the lower branches can perform a similar function. The slope of the shore and the height of the waves also affect wave reduction rates through mangroves.
To understand the level of protection provided by mangroves, and to plan how to increase it, the passage of waves through mangroves has been modelled numerically using both a standard wave model used by coastal engineers called SWAN (Simulating WAves Nearshore) (Suzuki et al., 2011), as well as a model developed specifically for waves in mangroves called WAPROMAN (WAve PROpagation in MANgrove Forest) (Vo-Luong and Massel, 2008). These models are able to predict typical levels of wave attenuation given a knowledge of the mangrove characteristics, the wave parameters and the local bathymetry and topography. A statistical model has also been developed to explore the relationship between some standard forest measurements (tree height, tree density and canopy closure) and wave attenuation with distance (Bao, 2011). This model has been able to predict wave reduction within the Vietnamese mangroves where it was developed, and could be used to determine the width of mangrove belt needed to deliver a predefined level of protection from waves.
While there is a general confirmation that mangroves can attenuate wind and swell waves, research has focused on small waves (wave height < 70 cm), and there is a need to measure the attenuation of larger wind and swell waves associated with greater water depths, which may occur during storms and cyclones. More datasets are also needed to test the wider validity of the existing wave models under different wave conditions and in areas with different types of mangrove forest and different topographies.