Key Factors Influencing the Severity of Fluvial Flood Hazard from Tropical Cyclones

Knowledge of the key drivers of the severity of river flooding from tropical cyclones (TCs) is vital for emergency preparedness and disaster risk reduction activities. This global study examines landfalling TCs in the decade from 2010 to 2019 to identify those characteristics that influence whether a storm has an increased flood hazard. The highest positive correlations are found between flood severity and the total precipitation associated with the TC. Significant negative correlations are found between flood severity and the translation speed of the TC, indicating that slower-moving storms that rain over an area for longer tend to have higher flood severity. Larger and more intense TCs increase the likelihood of having a larger area affected by severe flooding but not its duration or magnitude, and it is found that the fluvial flood hazard can be severe in all intensity categories of TC, including those of tropical storm strength. Catchment characteristics such as antecedent soil moisture and slope also play a role in modulating flood severity, and severe flooding is more likely in cases in which multiple drivers are present. The improved knowledge of the key drivers of fluvial flooding in TCs can help to inform research priorities to help with flood early warning, such as increasing the focus on translation speed in model evaluation and impact-based forecasting. Significance Statement Knowing ahead of landfall which TCs are likely to lead to significant river flooding will help those responsible for emergency planning make appropriate decisions to minimize loss of life and property. We compare 280 TCs and find that the cases with slow-moving, large, and intense cyclones, affecting areas with wet antecedent conditions, have the highest likelihood of experiencing widespread flooding. Slower-moving storms also have an increased risk of longer and more extreme floods. Our results show the importance of considering aspects such as the speed of forward movement along the whole flood early warning chain, from model evaluation and development, through to warning design and communication, to better inform forecast-based action prior to TC landfall.