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Reconstructing the 1-in-100-yr Flood in Northern Thailand

Reconstructing the 1-in-100-yr Flood in Northern Thailand. Ren é e Kidson University of Cambridge. Synopsis:. Hydraulic modelling and proxy dating identifies the 1-in-100 yr flood for the Mae Chaem river, Thailand

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Reconstructing the 1-in-100-yr Flood in Northern Thailand

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  1. Reconstructing the 1-in-100-yr Flood in Northern Thailand Renée Kidson University of Cambridge

  2. Synopsis: • Hydraulic modelling and proxy dating identifies the 1-in-100 yr flood for the Mae Chaem river, Thailand • Flood Frequency Models fitted to 50 yr gauging record: estimate 1-in-100 yr Q • Compare with independent reconstruction • Power-Law model produces closest estimate

  3. Methods: • Hydraulic modelling • HEC-RAS; 8 km, 50 cross sections • Calibration of Manning’s n: • PSIs from several gauged high-flow events • PSI dating proxies: • C14 dating • Dendrochronology • Oral history • Written historical evidence • Anecdotal • Climatic

  4. Methods (cont) • Flood Frequency Models: • Fitted to 48 yr gauged record (1953-2001) • Types: • Log- Normal (LN) • Gumbel EV 1 • Log Pearson III (LP3) • Power-Law

  5. Study Site • Northern Thailand: South West (Indian) monsoon climate • Tropical cyclone incursion from South China Sea • Steep topography: orographic rainfall enhancement – prone to extreme events

  6. 1. Hydraulic Model Calibration

  7. 2001 Flood: Q (known) = 794 cms n (inferred) = 0.072

  8. PSIs: 2 palaeofloods • Types: • Elevated fluvial sand deposits • Wood debris in gorge caves

  9. Palaeoflood 1: n (assumed) = 0.072 Q (inferred) = 980 cms

  10. Palaeoflood 1 • Anecdotal evidence: Palaeoflood 1 = 1960 flood • 1960 flood = 1030 cms • Further Manning’s n calibration • n = 0.068

  11. Palaeoflood 2: n (assumed) = 0.068 Q (inferred) = 2420 cms

  12. 2. Proxy Dating of Palaeoflood 2

  13. C14 Dating • Teak log in cave • Problematic for late Holocene specimens • Heartwood & Sapwood dating • Dendrochronological count to cross-check • 161 tree rings • Heartwood: 1685-1726 • Sapwood: 1831-1880 • Palaeoflood 2 postdates 1880

  14. Other Proxies • Written historical records: • Bombay Burmah Trading Corporation purchased forest lease 1905 • Western artefacts in cave: Palaeoflood 2 postdates 1905 • Slackwater Deposit • Charcoal C14 date: 1889-1908

  15. Climatic Proxy • Contemporary Site Climate record: • Rainfall Anomaly correlation with All India Rainfall Index

  16. All India Rainfall Index 1876-2000 1961 1917

  17. 3. Flood Frequency Analysis

  18. Reconstructed discharge: 2420 m3s-1

  19. Power-Law ? • Malamud et al. 1996 The 1993 Mississippi River Flood: a One Hundred or a One Thousand year event? • Theoretical basis: fractal statistics • Well understood for other phenomena e.g. earthquakes • Empirical evidence: essentiality of palaeoflood reconstructions • Alila and Mtiraoui (2002): ‘the selection of the most plausible distribution for flood frequency analysis should be based on hydrological reasoning as opposed to the sole application of the traditional statistical goodness-of-fit tests’.

  20. Implications • Flood Risk Perception • Gumbel EV1 = official Thai distribution • Global Warming scenarios: • recommend incorporation of Power-Law estimates as an upper bound for FFA

  21. Conclusions • Palaeoflood discharge estimate • Hydraulic modelling • High confidence: calibration opportunities (several gauged flood events) • Reliably dated (several proxies) as 1-in-100 yr flood • Compared with FFA models based on gauged record • Power-Law model produced closest estimate to reconstructed event

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