Watch and Upgrade or Deconstruct and Relocate: Derna Catastrophe Lessons Amid the Climate-change Era of Unpredictable Flash Floods

Abstract

Water structures' resilience to accelerated global warming impacts is attracting increased attention. Adaptation to unprecedented flash floods in semi-arid regions (e.g., Middle East and North Africa (MENA)) is becoming more challenging under the huge uncertainty of their frequency and intensity, lack of accurate hydrological studies, and water-resilient cities' absence in these wadi systems. Ten to twenty percent of Derna's population (∼90,000 people) were reported dead and missing along with more than 35,000 residents having been displaced (also, 737 completely collapsed, and 2859 partially collapsed houses) following two destruction waves after two-dams failures during unprecedented storm Daniel on the 11^th of September 2023. In this study, we deeply investigated the causes and consequences of this significant main dam (i.e., Bu Mansour) breach. We started by discussing the history of dams in the region and the Derna flash flood protection system design background. Then, we rebuilt the most probable scenarios and simulated the dam break analysis validated by ground truthing methods. No matter how well-maintained the main dam was, the unprecedented Daniel storm flood highly exceeded its design capacity. Aside from the widely reported poor maintenance, our investigation of the overflow shafts of the two dams raised a potentially fatal design issue with the main dam. We concluded the most probable dynamics of the two dams' failure through two waves of destruction confirmed by diverse field observations. Ironically, the study concluded that a similar flood induced by the storm could leave much less damage in the case of the absence of any dams (i.e., far less maximum depth and velocity occurrence and fewer flooded areas). Hence, there is an urgent need to reassess similar dams’ design criteria, upgrade, or simply deconstruct similar existing aging dams -including those with small annual storage-to-design capacity ratios- along with relocating residents and infrastructure away from the low-lying areas, all aimed at safeguarding human lives in similar ungauged wadi systems.