Reduction of Wind and Swell Waves by Mangroves

Author:

A. McIvor, I. Möller, T. Spencer, and M. Spalding

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 defense 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.

Main Results and Conclusions:

• Mangroves are important for coastal protection in a number of ways: “Mangrove vegetation causes wave attenuation because it acts as an obstacle for the oscillatory water flow in the waves (Box 2), creating drag: as the water flows around the mangrove vegetation, it has to change direction and do work against the friction of the mangrove surface”(6).
• WAPROMAN & SWAN model systems were used to study wave attenuation through mangroves: “Numerical and statistical models of wave attenuation in mangroves have been developed to facilitate better understanding and prediction of the behaviour of waves in mangroves”(16)…” Both the WAPROMAN and adapted SWAN models are based on quantifying the work done by the movement of water on plant stems”(17).
  • Case Study #1: “Narayan et al. (2010) use the modified SWAN model of Burger (2005) and Suzuki et al. (2011) to estimate wave attenuation at Dhamra port behind Kanika Sands mangrove island, Orissa, India, for cyclone-induced wind waves of varying return periods (Figure 10; see Narayan (2009) for a more detailed description of the study) (20)… They concluded that an extension of the vegetation on the northern side of the island would decrease wave height at the port”(21).
• Factors of wave attenuation that were important to this study are as follows: “Wave height reduction within a mangrove forest depends on the width of the forest, mangrove tree morphology relative to water depth, topography and wave height. Mangrove species with aerial roots are more effective at attenuating waves in shallow water, when the waves encounter the roots; species without aerial roots are more able to attenuate waves when the water level reaches the branches”(24).
• These two models (WAPROMAN & SWAN) and can help with conservation efforts: “By changing the forest widths and configuration in the model, engineers can plan how to manage and restore mangroves as part of an integrated coastal defense strategy”(24-25).
• In conclusion: “While more research is needed, existing knowledge is sufficient to substantiate the claim that mangroves attenuate wind and swell waves. Appropriate management of mangrove areas could increase wave attenuation. This might include the protection of mangrove areas in key settings, or lead to the restoration or planting of mangroves in degraded and deforested settings, where local conditions have been shown to support the establishment of mangrove seedlings. To achieve the highest level of protection from wind and swell waves, a dense mangrove forest, including species with aerial roots, is recommended. The width of mangrove belt required will depend on the height of waves against which protection is needed and the density of mangrove vegetation through which the waves will pass”(25).

Works Cited:

Burger, B. (2005) Wave attenuation in mangrove forests: numerical modelling of wave attenuation by implementation of a physical description of vegetation in SWAN. Masters thesis submitted to the Dep’t of Civil Engineering and Geosciences, Delft University of Technology. URL: http://repository.tudelft.nl/view/ir/uuid%3A0e4c6450-fe5d-4693-9ca9-58da&#8230;.

Narayan, S. (2009) The effectiveness of mangroves in attenuating cyclone-induced waves. Masters thesis submitted to the Dep’t of Civil Engineering and Geosciences, Delft University of Technology. URL: http://repository.tudelft.nl/view/ir/uuid%3A6ece41e5-3609-45b5-902e-11b4&#8230;.

Narayan, S., Suzuki, T., Stive, M.J.F., Verhagen, H.J., Ursem, W.N.J. and Ranasinghe, R. (2010) On the effectiveness of mangroves in attenuating cyclone-induced waves. Proceedings of the International Conference on Coastal Engineering 32 (no page numbers). URL: http://journals.tdl.org/ICCE/article/view/1250.

Suzuki, T., Zijlema, M., Burger, B., Meijer, M.C. and Narayan, S. (2012) Wave dissipation by vegetation with layer schematization in SWAN. Coastal Engineering 59(1), 64-71.