Formulating a warning threshold for coastal compound flooding: A copula-based approach.

• Compound-flooding events, triggered by heavy rainfall coupled with storm tides, are a significant concern in coastal urban areas. • A copula-based analysis is applied to develop an easy-to-use empirical formula for quickly assessing the threat of compound flooding. • The compound-flooding formula has been proved to be more reliable overall than the conventional rainfall-based thresholds for coastal urban areas. To calculate warning thresholds for compound-flooding events triggered by heavy rainfall coupled with storm tides in Taiwan's coastal urban areas, we applied copula-based analysis to observation data collected from 2001 till 2022 for Taipei City and New Taipei City and developed an empirical formula that accounts for both the capacity of the drainage infrastructure, which partially depends on the coastal sea level and varies over time, and the amount of precipitation. Compared against observation data from flood detectors, our predictions exhibited an accuracy of 85.2 % and 78.8 % for Taipei City and New Taipei City, respectively, thus improving upon the 62.8 % and 68.5 % success rates for thresholds estimated using only the hourly accumulated rainfall. These promising preliminary results suggest that reliable flood warnings for tidal-basin regions can be expedited by employing our formula and inputting rainfall and sea-level values from ensemble typhoon and storm-surge forecasts. [ABSTRACT FROM AUTHOR]