Reduction of Wind and Swell Waves by Mangroves

Abstract

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.