Anatomy of a foreseeable disaster: Lessons from the 2023 dam-breaching flood in Derna, Libya.

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.