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.

Barrier islands are indicators of coastal resilience. Previous studies have proven that barrier islands are surprisingly resilient to extreme storm events. At present, little is known about barrier systems’ resilience to seismic events triggering tsunamis, co-seismic subsidence, and liquefaction. The objective of this study is, therefore, to investigate the morphological resilience of the barrier islands in responding to those secondary effects of seismic activity of the Sumatra–Andaman subduction zone and the Great Sumatran Fault system. Spatial analysis in Geographical Information Systems (GIS) was utilized to detect shoreline changes from the multi-source datasets of centennial time scale, including old topographic maps and satellite images from 1898 until 2017. Additionally, the earthquake and tsunami records and established conceptual models of storm effects to barrier systems, are corroborated to support possible forcing factors analysis. Two selected coastal sections possess different geomorphic settings are investigated: (1) Lambadeuk, the coast overlying the Sumatran Fault system, (2) Kuala Gigieng, located in between two segments of the Sumatran Fault System. Seven consecutive pairs of comparable old topographic maps and satellite images reveal remarkable morphological changes in the form of breaching, landward migrating, sinking, and complete disappearing in different periods of observation. While semi-protected embayed Lambadeuk is not resilient to repeated co-seismic land subsidence, the wave-dominated Kuala Gigieng coast is not resilient to the combination of tsunami and liquefaction events. The mega-tsunami triggered by the 2004 earthquake led to irreversible changes in the barrier islands on both coasts.

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