Sets of beach ridge plains located as far as ten kilometres inland at the Chumphon estuary area, southern peninsular Thailand, provide evidence of an ancient sea-level change. Relict coastal landforms including former beach ridge plains, old lagoons, swales, and former tidal flats between beach ridges are found inland at elevations of 1-5 m above the present sea level. Ancient beach ridges here comprise three sets and all are inferred to have been deposited during the Holocene. The orientation of the inner beach ridge plains confirms their deposition by south to north longshore currents, whereas the middle and outer ridges were probably formed by north-to-south currents. Grain size analysis shows that all ancient beach ridge deposits are similar and are composed of fine-to medium-grained sand with high sphericity. Likewise, the compositions are similar, with quartz as a major component and feldspar and ferromagnesian minerals as minor components, reflecting the same sediment provenance. Marine fossils found in the former tidal deposit indicate intertidal and mangrove environments. Optical stimulating luminescence dating of three sets of beach ridge plains indicate that deposition occurred 8900-5600, 5900-2700, and 3800-1600 years ago for inner, middle, and outer beach ridge plains, respectively.
Abstract:The Khorat Plateau from northeast Thailand, the upstream part of the Mun River flows through clastic sedimentary rocks. A massive amount of sand was transported. We aimed to understand the evolution of fluvial system and to discuss the advantages of two shallow geophysical methods for describing subsurface morphology of modern and paleo-channels. We applied Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to characterize the lateral, vertical morphological and sedimentary structures of paleo-channels, floodplain and recent point bars. Both methods were interpreted together with on-sites boreholes to describe the physical properties of subsurface sediments. As a result, we concluded that four radar reflection patterns including reflection free, shingled, inclined and hummocky reflections were appropriated to apply as criteria to characterize lateral accretion, the meandering rivers with channel-filled sequence and floodplain were detected from ERT profiles. The changes in resistivity correspond well with differences in particle size and show relationship with ERT lithological classes. Clay, silt, sand, loam and bedrock were classified by the resistivity data. Geometry of paleo-channel embayment and lithological differences can be detected by ERT, whereas GPR provides detail subsurface facies for describing point bar sand deposit better than ERT.
The Chao Phraya River flows in the largest river basin of Thailand and represents one of the important agricultural and industrial areas in Southeast Asia. The Ping River is one major upstream branch flowing down slope southwardly, joining the Chao Phraya River in the low-lying central plain and ending its course at the Gulf of Thailand. Surprisingly, the overflow occurs frequently and rapidly at the Lower Ping River where channel slope is high, and in particular area, sand-choked is extensively observed, even in normal rainfall condition. In contrary, at the downstream part, the erosion of river bank and shoreline around the mouth of Chao Phraya River has been spatially increasing in place where there should be a massive sediment supply to form a delta. Here we use Landsat imageries taken in 1987, 1997, 2007 and 2017 to analyze geomorphological changes of rivers. Results show that both rivers have undergone the rapid decreasing of water storage capacity and increasing of sand bar areas in river embayment. The total emerged sand bar area in the Lower Ping River increases from 1987 to 2017 up to 28.8 km2. The excessive trapped bed sediments deposition along the upper reaches is responsible for the shallower of river embankment leading to rapid overflow during flooding. At the Chao Phraya River mouth, a total of 18.8 km2 of the coastal area has been eroded from 1987 to 2017.This is caused by the reducing of sediment supply leading to non-equilibrium in the deltaic zone of the upper Gulf of Thailand. There are several possibility implications from this study involving construction of weir, in-channel sand mining, reservoir sedimentation and coastal erosion management.
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