Wave attenuation in mangrove forests: an investigation through field and theoretical studies

Brinkman, Richard Michael (2006) Wave attenuation in mangrove forests: an investigation through field and theoretical studies. PhD thesis, James Cook University.

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Abstract

Mangroves are woody forests that exist at the confluence of the marine and terrestrial environments. These forests are highly biologically productive and play a key role in supporting coastal food chains, and trapping and stabilizing coastal sediments. Mangroves are also known to dissipate significant amounts of wave energy over relatively short distance, which has significance in the area of coastal protection. However, understanding the quantitative effects of mangrove vegetation in reducing surface wave energy has, until now, received very limited research interest. This study presents a field and theoretical investigation of the attenuation of random, wind–induced surface wave energy in mangrove forests. Field observations of wave processes in mangrove forests were undertaken at three study sites with different wave energy regimes. At each site, an array of wave gauges was placed along a transect aligned with the dominant wave direction to measure changes in wave characteristics as waves propagated shoreward through the mangroves. Mean rates of attenuation of total wave energy and significant wave height observed at the three sites averaged 1.5% m⁻¹, and 1.1 % m⁻¹ respectively. Attenuation rates are found to be frequency related, with preferred attenuation of shorter period waves. Field data also indicate an increase in the energy transmitted into the forest with increased water depth.

Two theoretical approaches were developed to investigate and model the attenuation of wave energy for waves propagating through mangrove forests: In the first theoretical approach, the water depth in the mangrove forest was assumed constant, and the wave motion was described by a set of amplification factors for individual spectral components. The second approach was developed for a mangrove forest with arbitrary bathymetry, and in this case the wave motion within the forest was determined by solving the mild slope equation with dissipation. Both approaches investigate energy dissipation in the frequency domain by treating the mangrove forest as a random media porous to wave energy, with certain characteristics determined using the geometry of mangrove vegetation. Also, both cases employ modified drag co-efficient to introduce the dependence of the drag coefficient on the spatial density of the vegetation. Theoretical results from the model with constant water depth show good qualitative agreement with the key wave propagation features identified in the field data, with predicted rates of wave energy attenuation controlled primarily by prescribed vegetation characteristics. Results from the model with arbitrary bathymetry demonstrate that the model was able to reproduce observed rates of wave energy attenuation. Wave energy attenuation was shown to depend strongly on the spatial density of the mangrove vegetation and its structural arrangements, and on the spectral characteristics of the incident waves. Wave energy attenuation was also found to be a function of water depth, with increased energy transmitted into the forest with increased water depth, due to the non-uniform vertical structure typical of mangrove vegetation.

The results of this study demonstrate that it is possible to numerically model the dominant energy dissipation processes and thus predict attenuation of surface wave height within mangrove forests. The ability to predict the attenuation of surface wave energy due mangroves has relevance in the field of co[a]stal protection. A model developed in this thesis is applied to assess the theoretical performance of a temporary coastal protection measure using bamboo, and evaluate its cost effectiveness compared to other accepted low-cost measures for attenuating wave energy. The proposed design of a bamboo wave attenuation structure is significantly cheaper than other published designs, for an equivalent level of energy attenuation.

Item ID: 1351
Item Type: Thesis (PhD)
Keywords: Wave energy attenuation, Random wind–induced surface wave energy, Mangrove forests, Field data, Mean rates of attenuation, Total wave energy, Significant wave height, Numerical models, Constant water depth, Arbitrary bathymetry, Spatial density of mangrove vegetation, Spectral characteristics of incident waves, Non-uniform vertical structure, Bamboo wave attenuation structure, Theoretical performance, Cost effectiveness
Date Deposited: 24 Jan 2007
FoR Codes: 02 PHYSICAL SCIENCES @ 50%
01 MATHEMATICAL SCIENCES @ 50%
SEO Codes: 96 ENVIRONMENT > 9609 Land and Water Management > 960903 Coastal and Estuarine Water Management @ 100%
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