The effects of Thailand׳s Great Flood of 2011 on river sediment discharge in the upper Chao Phraya River basin, Thailand

Abstract

Severe flooding that occurred during the 2011 monsoon season in Thailand was the heaviest flooding in the past 50yr. The rainfall over the northern part of Thailand, especially during July–August 2011, was 150% higher than average. During the flooding period, river flows of the four major Chao Phraya River tributaries (Ping, Wang, Yom, and Nan rivers) increased in the range of 1.4–5 times the average discharge. This study examined the river sediment discharge of the four major rivers in the upper Chao Phraya River basin in Thailand. The four rivers are considered the main sources of sediment supply to the Chao Phraya Estuary. River surveys of the Ping, Wang, Yom, and Nan rivers were carried out in October 2011 (during the Great Flood) and October 2012 (one year after the flood). Survey data included river cross sections, flow velocities, suspended sediment concentrations, and bed load transport in each river. Analyses of these data indicated that total sediment transport rates for the four main rivers during the flooding of 2011 were 2.3–5.6 times higher than the average sediment discharge over 60yr. The flood of 2011 significantly affected the sediment characteristics including the proportions of suspended and bed sediment loads in each river though in different ways. The rates of sediment transport per unit discharge for the Ping and Wang rivers dramatically increased during the 2011 flood, but the flooding had minimal effects on the sediment characteristics in the Yom and Nan rivers. The amount of total sediment discharge in each river caused by the 2011 flooding varied between 0.3 and 1.6Mt. Additionally, the bed load transport in these rivers varied between ~0% and 26% of the suspended sediment discharge.