Geomorphological consequences of the Derna dams collapse, erasing a century of urban sprawl

The catastrophic failure of two dams occurred between September 10th and 11th, 2023, resulting in 30 million cubic meters of water and debris destroying a large part of the city of Derna, along the northern coastline of Northeastern Libya. The flood caused over 10,000 fatalities and displaced thousands of people from the city. The dams failed due to heavy rainfall caused by Storm Daniel, which impacted large parts of the Mediterranean basin. Here, we present the geomorphological context of this calamitous event and its effects on Derna's urban sprawl over the last century. Old maps and aerial photographs have allowed the reconstruction of Derna's settlement evolution over the past century. The city of Derna is located in a coastal area on a large alluvial fan formed by the Wadi Derna. Water flows intermittently here during heavy rains. The watershed is extensive (575 square kilometers), but the river is relatively short, with minimal runoff times. Over the years, two dams (Abu Mansur and Derna dams) upstream of the city were constructed to control soil erosion and prevent flooding. The heavy rains impacting the Cyrenaica coastline resulted in a flash flood along the wadis flowing from the mountains and crossing the city of Derna. The destruction was closely connected to the urban sprawl, primarily affecting areas of the city built in the last 70 years. The most recent urban expansion of Derna took place in the lower parts of the alluvial fan. Additionally, high-resolution remote-sensing data allows for the assessment of the geomorphological effects of the event and the quantification of the volumes involved through DEM of Difference analysis. This work is funded by Italian Ministry of University and Research through PRIN project “GEOTRes – Geoheritage threating and resilience: mapping the impact of geomorphic and human processes in sensitive”.