Investigation of dam failures during Storm Daniel in Derna, Libya

The aim of this thesis is to investigate and simulate the catastrophic flood that struck the city of Derna, Libya, in September 2023, during the extreme Storm Daniel. The unprecedented rainfall depths recorded over a 24-hour period generated extreme dis charge rates in the Wadi Derna catchment, leading to the cascading failure of two earth fill dams (Bu Mansour and Al Bilad) and, ultimately, the complete inundation of the urban area, resulting in tragic human and material losses. Furthermore, this research seeks to comprehend the dynamics of the flood wave and depict the impacts of failure using advanced computational tools. Initially, Digital Elevation Models (DEM) were processed, and the watershed was delineated within a Geographic Information Systems (QGIS) environment. Subsequently, a detailed hydraulic model was developed using HEC-RAS 2D software, integrating the Rain-on-Grid methodology. Within the frame work of this study, five distinct simulation scenarios were executed and analyzed to investigate the system's behavior under varying conditions. Specifically, the influence of the breach formation rate (Linear vs. Sine Wave progression) was examined, the retention capacity of the structures was evaluated (based on initial reservoir levels), and the breach trigger mechanisms were analyzed (Water Surface Elevation vs. Time trig ger). The comparative assessment revealed that the extent of the inundation is primarily determined by the total released water volume and minimally by the breach formation rate. The loss of the downstream structure's retention capacity emerged as the most critical parameter. The worst-case scenario occurred when the breach initiation was time-based, resulting in the upstream dam (Bu Mansour) failing after having reached 100% of its storage capacity. Under these conditions, the sequence of the collapse was reversed, with the downstream dam failing first and the upstream dam following half an hour later. This sequence stripped the wadi bed of any retention capability and gen erated a characteristic "double peak" in the hydrograph, forming an initially misleading picture of the flood prior to the arrival of the sweeping main wave. The model's execu tion produced hazard maps (maximum depth, velocity, and momentum flux), which classify most of the city as a high-risk zone. In conclusion, this thesis highlights the necessity for continuous inspection and regular maintenance of earthfill infrastructures and establishes the hydraulic foundation for the future coupling of these results with population evacuation modeling software, aiming to substantially strengthen local Civil Protection plans.