Compound, Cascading, and Multihazard Perils: Lessons Learnt from Hurricanes Helene and Milton

The 2024 hurricane season presented unique challenges in hydrological and risk modeling with the consecutive landfalls of Hurricanes Helene and Milton in Florida, USA. This study investigates the compounded, cascading, and multihazard perils associated with these events, focusing on the interplay of antecedent conditions, vulnerability, and exposure.

One of the factors considered was the influence of antecedent soil moisture and river storages on hydrological modeling. Hurricane Helene, which made landfall in early September, saturated the soil and filled river systems to near capacity. These conditions significantly altered the hydrological response to Hurricane Milton, which struck just two weeks later. Hydrological models had to account for the already saturated soils and high river levels, which exacerbated flooding and runoff, leading locally to more extensive inundation than would have been predicted for Hurricane Milton in isolation.

Another point of focus is the impact on vulnerability, particularly the presence of debris from Hurricane Helene affecting the region's resilience. Debris obstructed drainage systems, increased the potential for secondary flooding, and complicated emergency response efforts. Additionally, the weakened infrastructure and partially damaged buildings from the first hurricane heightened the susceptibility of the population to the subsequent event, resulting in higher overall damage and more prolonged recovery periods.

Finally, the study examines the effect on exposure, including the "build-back-better" phenomenon observed in even previously to the aftermath of Hurricane Helene. While some structures were rebuilt to higher standards, providing increased resilience against Hurricane Milton, many areas remained in a state of recovery, with temporary shelters and makeshift repairs that were less able to withstand the impact of the second hurricane. This mixed state of exposure created a complex landscape for risk assessment and emergency planning.

Overall, the lessons learnt from Hurricanes Helene and Milton underscore the importance of incorporating antecedent conditions into hydrological models, considering the cumulative impacts on vulnerability, and recognizing the dynamic nature of exposure in multihazard scenarios. These insights are crucial for improving predictive models and enhancing resilience strategies in regions prone to sequential natural disasters.