EFFECT OF LARGE DAMS ON SHORELINE RECESSION IN THE UPPER GULF OF THAILAND

The Chao Phraya River (CPY) Delta is the largest delta in Thailand. During the past 5,000 years, the delta front has prograded 4 meters annually on average. However, the CPY delta has undergone a significant shoreline change during the past six decades. Possible causes include: a rapid increase in local relative sea level, as a result of land subsidence; wave action combined with the loss of natural coastal protection produced by the destruction of mangrove shorelines due to aquiculture development (shrimp ponds); and reduction of sediment supply from the CPY River due to dam construction. However, there exists disagreement among various studies concerning the major causes of the ongoing shoreline retreat. This is due in part to the lack of supporting information, especially historical measurements of sedimentation rates, which can be used to quantify the influence of those factors on the severe and rapid shoreline retreat in this area. The purpose of this study was to determine the rate of sediment accumulation and sediment ages in the CPY delta using radiometric dating techniques, and to study the effect of construction of two large dams in the Chao Phraya River system. A sediment core taken from the east side of the CPY delta was used to study the sedimentation rate in the nearshore CPY delta. Based on the ²¹⁰Pb dating of the core, ages were calculated using the Constant Rate of Supply (CRS) model and linear regression trend analysis. The results of the analysis show that the average sediment accumulation rate from this site in the CPY delta over the past century is 10.7 mm/yr (0.84 gm/cm/yr). By considering the historical sediment deposition rates before and after the construction of large dams (the Bhumibol and Sirikit dams) in the Chao Phraya River system, the sediment deposition rates in the east side of the CPY Delta did not significantly change after the construction of dams. A possible slight increase in sedimentation may have been caused by the dramatic change in land use after 1954. It appears that the construction of large dams on the Chao Phraya River system is not related to the rapid shoreline retreat in the CPY delta during the past sixty years.