An analysis of the factors responsible for the shoreline retreat of the Chao Phraya Delta (Thailand)

Deltas are inherently low-lying structures and thus subject to large threats due to sea level rise, erosion and other coastal processes. The shorelines in many deltas around the world are now retreating and most cases appear to result from a decreasing sediment supply as a consequence of upstream darn construction. We present here results of an investigation of riverine sediment fluxes, coastal retreat, and coastal sediment accumulation in the Chao Phraya River and Delta (Thailand). This deltaic shoreline has one of the highest rates of shoreline retreat in the world. Surprisingly, our results show that in spite of the construction of two large storage dams, one on the Ping River (Bhumibol Dam, 1964) and the other on the Nan River (Sirikit Dam, 1972) that merge to form the Chao Phraya, sediment accumulation in the delta was actually higher over the last several decades than prior to darn construction. The recent higher rates of sediment accumulation, based on Pb-210 dating, appear to be the result of increased sediment supply in the lower reaches of the river relating to expansion of aquaculture and other activities in the delta beginning in the 1970s. We also show that mangrove removal, in order to further develop shrimp farming, charcoal production, and other pursuits, was not responsible for most of the shoreline erosion. Rather, subsidence, mainly induced by groundwater withdrawal, together with worldwide sea level rise appears to be the main factor affecting the very rapid shoreline retreat of the Chao Phraya Delta. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

Deltas are facing threats such as sea level rise and erosion, with many retreating shorelines attributed to decreased sediment supply from upstream dam construction. Despite the presence of two large dams in the Chao Phraya River and Delta, sediment accumulation has actually increased in recent decades, mainly due to expansion of activities in the delta since the 1970s. Shoreline erosion in the Chao Phraya Delta is primarily caused by subsidence induced by groundwater withdrawal and global sea level rise, rather than mangrove removal for development purposes.