Wave Attenuation in Mangrove Forests; Field Data Obtained in Trang, Thailand

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.

Cross-shore gradients of physical disturbance in mangroves: implications for seedling establishment

“…The transition from mudflat to the pneumatophores 5414 T. Balke et al: Cross-shore gradients of physical disturbance in mangroves of the Avicennia/Sonneratia zone was very abrupt. The Rhizophora zone at all transects was characterised by monospecific stands of mature trees with the characteristic stilt roots (see Horstman et al, 2012). Whereas the pioneer zone at transect B was dominated by mature trees (see , transects A, C and D were dominated by younger trees, smaller than approximately 5 m in height.…”

“…dominated zone. The tidal amplitude near the field sites is between 1 and 3 m, and wave measurements by Horstman et al (2012) near transect B and D showed that typical incident wave heights at the mudflat fronting the forest did not exceed 10 cm with wave periods of 3-5 s during the measurement campaign. During a stormy period wave heights of up to 30 cm were measured near transect B .…”

“…Wave height and vegetation density measurements were carried out close to transect B and transect D during the same field campaign by Horstman et al (2012) and confirmed the limited energy attenuation at the open forest fringe. This was explained by the combination of a low vegetation density (< 5 ‰, above 10cm) above the dense pneumatophores (17-23 ‰, at 5 cm), with inundation heights of up to 2 m .…”

“…2, left photo). Erosion was applied stepwise by 3 mm each run; thereafter the seedlings were exposed to typical waves (measured in the field, Horstman et al, 2012) with 8 cm wave height (0.5 Hz) in the flume tank for 150 seconds. If the seedling did not topple, the erosion treatment was repeated and the seedling put back in to the flume for the next wave run.…”

Effect of Mimic Vegetation with Different Stiffness on Regular Wave Propagation and Turbulence

“…Tschirky and Hall [23] and Lima et al [24] performed experiments that indicated that an increase in plant density enhanced the wave dissipation effect. However, Mazda et al [25] and Horstman et al [26] found that when the water depth in the mangrove was more than the height of the aerial roots, an increase in water depth reduced the wave dissipation effect, and when the water depth increased to the height of the mangrove leaves, the wave dissipation effect increased. Cruise and Muslesh [27] used a rigid pole to simulate the emerged portion of rigid vegetation and studied the effect of plant diameter and arrangement on water depth and velocity.…”