Survey of Water Quality of the Chao Phraya River Flood in Thailand in 2011 Focusing on Flow and Status of Floodwater

A massive flood struck the Chao Phraya River Basin in Thailand from August to December in 2011. The total flood volume was estimated to be 15 billion m3, and the area of damaged agricultural land was as large as the Kanto Plain. To understand the planar and temporal characteristics of water quality of floodwater along with the flow, the authors conducted a field survey of the flood from Chai Nat, Ayutthaya to Bangkok in November and December. The results show that some indices were higher in the southern area from Ayutthaya and that the effect of the first flash was observed in an area where inundation had started a few days earlier in Bangkok. E. coli was detected in the Chao Phraya River in inundated Ayutthaya in November, suggesting that human excrement flowed into floodwater. However, it was not detected in December there, suggesting that the inflow of human excrement stopped owing to the extinction of the inundated area.