Abstract:

Was the catastrophic flooding in Derna, Libya—one of the deadliest hydrometeorological disasters on record—an inevitable outcome of rare weather conditions, or did the design of the infrastructure fail to account for probable risks? On 10 to 11 September 2023, Storm Daniel, a Mediterranean tropical-like cyclone, caused heavy rainfall that led to the collapse of two dams and more than 5000 casualties in Derna. Using a combination of atmospheric reanalysis, satellite data, and hydrologic modeling, we overcame key limitations typical of data-scarce, high-variability regions and revealed that despite the catastrophic impact, the return periods of the rainfall and flood were only a few decades. Hydraulic simulations revealed that the dam failures amplified the damage nearly 20-fold compared to a dam-free scenario. With extensive and timely implications, our findings underscore the importance of uncertainty-aware risk assessment and highlight the value of distributed flood prevention and early warning systems in mitigating risks in vulnerable regions.