The Effects of Thailand's Great Flood of 2011 on River Sediment Discharage in the Upper Chao Phraya River Basin

Severe flooding which occurred during the 2011 monsoon season in Thailand was the heaviest flooding in the past 50 years. The rainfall over the northern part of Thailand, especially during July-August 2011, was higher than 150% of normal rainfall. During the flooding period, the flow rates in four major rivers (the Ping, Wang, Yom and Nan River) in the upper Chao Phraya River basin increased between 1.1 and 5.3 times of average discharge. The strength of river flow plays an important role in sediment processes. However, these processes are very complex and area specific. This study aims to examine the river sediment discharge in four major rivers in Thailand, which are the main sources of sediment of the Chao Phraya estuary system, during Thailand's great flood of 2011. Field surveys were carried out in four major rivers of the upper Chao Phraya River Basin; the Ping, Wang, Yom and Nan River, in October 2011 (during the great flood) and October 2012 (one year after the flood). Measurements included river cross sections, flow velocities, suspended sediment concentration, and bed load transport in each river. Analysis of this data indicated that total sediment transport rates in four main rivers during the flooding of 2011 were 1.3-29 times higher than during the same period in 2012. Moreover, it was found that the flooding of 2011 significantly affected the proportions between suspended sediment transport and bed sediment transport in each river though in different ways. The percentages of suspended sediment transport of the Ping, and Wang rivers obviously increased during the flooding 2011. The significant rising of the bed sediment transport percentages in the Wang and Nan rivers occurred as a result of the flood of 2011. However, the flood had unremarkable effects on the sediment discharge in the Yom river. Nevertheless, it was also found that the bed load movement of these rivers varied between 5% and 26% of their suspended sediment discharges.