Author(s): Wei Jian; Diah Valentina Lestari; Edmond Yat-Man Lo

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Keywords: Compound flooding risk; River delta; Conditional probability; Non-stationary framework; EOF analysis

Abstract: The Chao Phraya River delta, home to the megacity of Bangkok, Thailand, faces escalating flooding risks with significant socio-economic implications. This study investigates the occurrences of compound flooding involving multiple flood drivers: river discharge, local precipitation, and coastal storm surge. We analyze the interdependence among these flood drivers through three distinct cases where each flood driver exceeds the annual 90th percentile. Our findings reveal a moderate correlation between discharge and precipitation, with concurrent extreme events being highly likely when either driver exceeds the 90th percentile thresholds. In contrast, storm surge shows greater independence from the other drivers, although elevated precipitation or discharge often coincide with storm surge extreme days. The non-stationary variability in these dependences highlights the intensifying compounding relationship between discharge and rainfall, although the rate remains within historical range. Further analysis of the compound extreme scenarios during reported flood years in the broader Chao Phraya River Basin reveals that prolonged periods of elevated discharge and rainfall anomalies are more frequent during flood years, as compared to non-flood years. Empirical orthogonal function (EOF) analysis further reveals that enhanced moisture availability and transport are key mechanisms driving flooding, with slightly intensified low-level convergence and localized upward motion further amplifying flood development. Our study improves the understanding of key mechanisms underlying compound flooding in the region and provides valuable insights into the evolving flood risk landscape.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P1750-cd

Year: 2025