Evolution of Mangrove Muddy Coast in the Western Coast of the Upper Gulf of Thailand Over the Past Six Decades

This study analyzed the shoreline change processes along the Phetchaburi intertidal mudflats with mangroves located on the western portion of the Upper Gulf of Thailand during the period 1953–2017. Historical shoreline positions along the coast were derived from the aerial photographs taken in 1953, 1967, 1976, 1994, 2002 and the satellite imagery in 2006, 2009, 2014, and 2017 using the Geographic Information System (ArcGIS) software. All imagery data were geo-referenced into Universal Transverse Mercator projection with the World Geodetic System 1983 (WGS1983) to eliminate distortion from the aerial photographs and satellite imagery. The shoreline positions along the study area were then digitized for each time period. The outer boundary of the mangrove forest was mainly used as a proxy to define shoreline positions of the muddy beach. The Digital Shoreline Analysis System (DSAS) version 4.4 was used to analyze the changes of shoreline positions in the study area. Results from this study revealed that about 26% of the 30 km Phetchaburi muddy shoreline in the mid portion of the study area had continuously retreated over the past sixty years with an average rate of −6.5 ± 1.8 m/y. The remaining, which was mainly located in the uppermost portion (near the major river mouths) and the lowermost portion (near a natural sandspit) had advanced seaward with the maximum and average rates of 30.0 ± 1.8 and 8.6 ± 1.8 m/y, respectively. Unlike mangrove muddy coasts in other regions, the results of this study suggested that the conversion of mangrove forest to agriculture and shrimp farming had not affected the rate of shoreline accretion of the study area. The variation of shoreline advanced rates likely reflected the changes in river sediment supply from the Phetchaburi River basin and in coastal sediment transport processes of the Phetchaburi coastal system. Meanwhile, the local wave and current conditions and the adjacent shoreline stabilization appeared to be the major factors dominating the shoreline recession of the Phetchaburi coast during the past six decades.