Influences Of Geologic Conditions And Anthropogenic Activities On Changes Of Geomorphology And Sediment Load Characteristics Of The Ping And Chao Phraya Rivers

Abstract

The Chao Phraya River flows into the largest river basin of Thailand. The Ping River is one of its the major upstream branches flowing down slope southwardly, joining the Chao Phraya River in the central plain and ends at the Gulf of Thailand. The sand-choked is extensively observed and the flood overflow occurs frequently and rapidly at the Lower Ping River. In contrary, the Chao Phraya River, the erosion of river bank and shoreline around the river mouth has been spatially increasing in place. The Landsat imageries taken in 1987, 1997, 2007 and 2017 were used to analyze geomorphological changes of both rivers. Results show that the total emerged sand bar area in the Lower Ping River had increased 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. In contrary, at the Chao Phraya River mouth, a total of 18.8 km2 of the coastal area had been eroded. Along the Lower Ping River, the bedload transport rates significantly decline toward the downstream. Most of bedload has been trapped above Bhumibol Dam. Then the bedload has been resupplied again by tributaries downstream and trapped within the succession of weirs. The combination of high supplied bedload from tributaries and low and suppressed discharge by dams and weirs accelerates growth rate of sandbars. However, within the succession of weirs severe bank collapses can occur locally as rapid growth of sandbars abruptly change the direction and increase flow velocity. Both anthropologic, and geologic factors play important roles in changing hydrosedimentary conditions of the Lower Ping River. The anthropologic factors include, river regulations, high deforestation rate, and intense in-channel sand mining. Whereas geologic factors are the underlying lithology and degree of weathering and erosion. The high rate of bedload budget links to highly weathered granite outcrops in the mountainous regions. Using Digital Shoreline Analysis System (DSAS) software and Satellite images from Google Earth can locates significant changes of river accretion and erosion along the riverbanks. The Integrated GPR-Electrical Resistivity Survey (IGRS) technique has revealed the internal structure and determined the thickness of a sandbar on the Lower Ping River. The sandbar thickness detected from this study is 10-12 m. There are several possibility implications from this study involving construction of weir, in-channel sand mining, reservoir sedimentation and coastal erosion management.