Coastal erosion and mangrove progradation of Southern Thailand

Approximately 60% of the southern Thai coastline used to be occupied by mangroves according to the first mangrove forest assessment in 1961. During the past three decades, these mangrove areas have been reduced to about 50% with less than 10% left on the east coast. Coastal erosion and accretion occur irregularly along the coast but an intensification of erosion has been noticed during the past decade. This study assessed the relationship between mangrove presence and changes in coastal area. Mangrove colonization rates were assessed using in situ transects and remote sensing time series. Both methods led to comparable estimates ranging between 5 and 40 m y−1. Quantitative data on changes of coastal segments along southern Thai coastlines as well as available possible factors responsible for these changes were compiled. Overall, net erosion prevailed (1.3 ± 0.4 m y−1). The Gulf of Thailand coastline in the East of the country was found to be most dynamic: change occurred along more coastal segments than in the West (43% vs. 16%). Rates of erosion and accretion were also higher, 3.6 versus 2.9 m y−1 and 2.6 versus 1.5 m y−1, respectively. Total area losses accounted for 0.91 km2 y−1 for the Gulf coast and 0.25 km2 y−1 for the West. Coasts with and without mangroves behaved differently: in the presence of mangroves less erosion was observed whilst expansion occurred at particular coastal types with mangrove existence, i.e. river mouths and sheltered bays. Possible underlying causes were examined using multivariate analysis. Eroded areas were found to increase with increased area of shrimp farms, increased fetch to the prevailing monsoon, and when dams reduced riverine inputs. Notably, however, in areas where erosion prevailed, the presence of mangroves reduced these erosion rates. Mangrove loss was found to be higher in the presence of shrimp farms and in areas where mangrove forests used to be extensive in the past.