Abstract
Mangroves thrive in sheltered intertidal areas in the tropics and sub-tropics. Due to this position at the interface between land and sea, mangroves play an important role in the attenuation of waves. Dissipation of wave energy in mangrove forests is an interesting feature from the viewpoint of coastal protection. Nevertheless, field data are sparse and modeling attempts reveal the need for additional data. This paper presents the results of an extensive field campaign, lasting 6 months, in mangroves along the Andaman coast of Trang Province in southern Thailand. Wave attenuation has been studied along two contrasting transects with different elevation and vegetation characteristics and different orientations towards the Andaman Sea. Along the Kantang transect, which is mostly exposed to swell waves, vegetation densities increased from 4.5 to 9.3 volume-†° along the transect and on average 63% of the incident wave energy was attenuated over a distance of 246 m. Along the Palian transect, mostly exposed to sea waves instead, vegetation increased from 4.3 to 19 volume-†° and 72% of the incident wave energy was attenuated over this 98 m transect. It was found that standardized wave attenuation correlates well with incident wave energy, when attenuation is analyzed per vegetation zone. Energy reduction rates of these zones, defined by the gradient of the correlations between the standardized wave attenuation and incident wave energy, are found to increase significantly with vegetation density. Consistently, wave reduction rates, expressing the gradient of the correlation between wave height reduction and incident wave heights, are found to be 0.001-0.014 for the study sites and also show a significant and increasing trend with vegetation densities.
Highlights
Mangroves form a key-ecosystem in the intertidal area of tropical and sub-tropical coastlines
A significant difference exists between the findings for the plots in the Avicennia and Rhizophora zones
For the Rhizophora plots, the near bed vegetation density is of the same order, but shows some more spreading for the different plots (15-39‰)
Summary
Mangroves form a key-ecosystem in the intertidal area of tropical and sub-tropical coastlines. Mangrove vegetation consists of salt tolerant trees and shrubs, usually thriving between mean sea level and high water spring level They are able to resist the hydrodynamic forces faced at sheltered coastlines such as estuaries and lagoons (Augustinus, 1995). Mangroves are even said to have had a positive contribution to the safety of coastal areas during the Indian Ocean tsunami in 2004 (Danielsen et al, 2005) To these physical benefits to tropical coastal areas, mangroves are an outstanding habitat for numerous species of fish, insects, birds, crustaceans, reptiles and mammals. Despite this key-function of mangroves in the intertidal area, they are in rapid decline. There is an urgent need to unravel the physical contribution of mangroves to coastal safety in order to increase awareness of the need for mangrove preservation
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