Flood generation during the SW monsoon season in northern Thailand

Abstract: This paper analyses the annual floods for the city of Chiang Mai in northern Thailand using a relatively long dataset (1921–2009) to study the mechanisms behind flood generation in this region. Four floods of different magnitudes were chosen for the empirical analysis. Daily rainfall, flow and baseflow data are analysed together with information about tropical storm occurrence–frequency and El Nino ̃ Southern Oscillation (ENSO) events to identify the occurrence and causes of floods. We found that floods are caused by a variety of factors but the most extreme floods are often caused by a combination of wet catchment conditions and heavy rainfall due to monsoonal effects or tropical storms. Tropical storms play a variable role in that their occurrence in the earlier part of the monsoon season may only wet up the catchment and have no part to play in the peak-flow generation. However, heavy rainfall due to tropical storms during the later part of the monsoon season may result in extreme floods. Typhoon Damrey (2005) is such an example as a 100 year flood event occurred after the passage of this typhoon across northern Thailand. Flood forecasting and management therefore need more high-resolution information about rainfall patterns and tropical storm activity. Seasonal flooding is a regular feature of th