Estimated Hydraulic Simulation of The Collapse of The Derna Valley Dams (2023); Numerical Analysis of The Causes of Disaster and The Inevitability of Collapse Under Extreme Climatic Conditions

Abstract

This study analyzes the causes of the collapse of the Wadi Derna dams, which occurred on Monday, 11 September 2023, and explains the factors that magnified the disaster. Using data compiled from previous researches on local dams behavior and hydraulic conditions, the study approximates hydraulic simulation models to identify the drivers of failure and to interpret the mechanisms involved. It also evaluates whether preventive measures could have averted—or substantially mitigated—the catastrophe. The hydraulic simulations indicate that cyclone rainfall exceeded by more than fourfold the precipitation level required to trigger flooding in the wadi. As a result, the generated runoff volume increased to more than eight times the dam’s storage capacity. Field investigations and direct observations support the identification of five key elements responsible for severe human and material losses: (i) high flow velocity combined with a short travel time to the city center, leaving minimal opportunity for survival; (ii) flood-wave height that inundated buildings and people inside; (iii) flow impact force capable of displacing infrastructure from foundations and driving it toward the sea; (iv) heavy sediment transport, with deposition estimated to cover approximately one third of the city; and (v) synergistic interactions among these factors. Model results show that flow velocities arising from the upper catchment (≈765 m above mean sea level) reached about 22 m/s (≈79 km/h). The post-failure wave height was estimated at roughly 25 m, with the wave reaching the city center within approximately 10 minutes. The associated impact force was estimated at about 1190.2 tonnes, and sediment loads at approximately 28,641,600 tonnes of clay and silt. The study concludes that failure cannot be explained solely by maintenance negligence or monitoring limitations. Even under optimal construction and condition, the dam system would likely not have withstood a cyclone delivering unprecedented rainfall; at best, the failure process might have been delayed by a few additional hours rather than fully prevented.