EXTREME FLOOD HISTORY OF OB LUANG GORGE, THAILAND. CLIMATIC OR LAND USE CHANGE?

Differentiating the effects of climatic and land use change in the hydrological record is challenging. Palaeoflood hydrology, the study of past extreme floods, offers a useful investigative approach providing insights to this challenge, and the range and sophistication of tools available to the hydrologist in the form of data collection and statistical analysis possibilities, has improved. This paper analyses some extreme flood evidence of Ob Luang Gorge, Mae Chaem River, Thailand. Aside from being a unique contribution from a poorly-studied geographic region, the Mae Chaem catchment offers the potential to test hypotheses regarding the interaction of monsoonal climate and land use change through its considerable palaeoflood evidence. A variety of Palaeoflood Stage Indicators (PSIs), including debris deposits in caves and tributary mouths, were dated and an hydraulic model constructed to simulate the flood events that formed these deposits. Dendrochronological specimens available as PSIs were used to extend the instrumented climatic record. This was analysed using database-mechanistic, spectral, fractal and wavelet techniques to detect monsoonal shifts. This analysis was then integrated with the PSI hydraulic model, and an ‘exceedance threshold’ approach taken to assessing flood frequency and likelihood. Confidence and Uncertainty are discussed, and inferences and interpretation of flood evidence and history are made.