The deadliest flood in Africa within the past 100 years didn’t happen in the continent’s rain-soaked equatorial jungles or along the mighty Nile, Congo, and Niger Rivers. It happened in Libya, along its dry northern coast.
When the Mediterranean cyclone Storm Daniel slunk down from Greece to strike Libya’s northern coast in September 2023, it unleashed flash floods that took thousands of lives and left some 45,000 people displaced.
Now, new research shows that enhanced soil erosion in Libya loaded Storm Daniel’s floodwaters with high amounts of sediments, increasing their destructive power.
“The deadliest flood happened in the driest country in all Africa, the only nation that has no rivers at all.”
By analyzing satellite radar images, researchers mapped storm damage and tracked sediments washed out by the storm. The data show that 66% and 48% of the cities of Derna and Susah, respectively, were moderately or severely damaged.
“The deadliest flood happened in the driest country in all Africa, the only nation that has no rivers at all,” said the study’s coauthor, remote sensing scientist Essam Heggy of the University of Southern California and the NASA Jet Propulsion Laboratory.
The results, published in Nature Communications, point to a looming threat in the Mediterranean: increasingly catastrophic floods in arid regions driven by intensifying cycles of drought and deluge.
Drought and Deluge
Storm Daniel spun up over Greece’s Ionian Sea in early September 2023, fueled by lingering high sea temperatures after a sweltering summer in southern Europe. Storms like Daniel, sometimes called medicanes, are expected to get more intense as the Mediterranean warms.
Rain fell in torrents over southeastern Europe and Turkey, flooding towns and taking dozens of lives, before the storm moved south into Libya. There, disaster struck after two dams collapsed upstream of Derna. Water rushed down Wadi Derna, a dry riverbed, directly into the city.
Heggy and his colleague, graduate student Jonathan Normand, wanted to understand how soil erosion might have contributed to the damage wrought by Daniel’s catastrophic floods. The researchers compared radar images taken by satellites before and after the flood to assess soil erosion and damage to structures that would have been invisible to optical satellites.
They studied the broader watershed rather than just urban areas at its outlet, allowing them to identify where the storm eroded soils and track how floodwaters transported sediments.
The radar analysis revealed that flood erosion occurred in 22%–26% of rangelands, 11% of bare soils, and 11% of urban areas within the studied area of Wadi Derna’s watershed. Material washed into wadis, or dry riverbeds, forming streams of sediment-laden water. Loaded with soil and rocky material, these streams wrought more destruction than they would have otherwise, Heggy said. Sediment-laden streams contributed to the collapse of dams upstream of Derna, according to the researchers.
The result highlights the perhaps unintuitive connection between drought and flooding. Throughout 2021, 2022, and 2023, serious droughts and heat waves plagued the Mediterranean. Dry soils are less cohesive than wet ones. Plant roots hold soils together, but parched plants die off in drought. So when the rains finally come after a long dry spell, soils are more easily sloughed off the surface and entrained in floodwaters. The combined effect of drought and deluge can be deadly.
Drowning in the Desert
“What Storm Daniel told us is that the Mediterranean area is fragile.”
“What Storm Daniel told us is that the Mediterranean area is fragile and is not ready. Because Storm Daniel didn’t impact only Libya,” said physical geographer Paolo Tarolli of the Università Degli Studi di Padova in Italy who was not involved in the study. The new study highlights the importance of studying flood hazards in the Mediterranean beyond just Europe, which has historically received most of researchers’ attention, he added.
Intensifying waves of drought and extreme rain are causing problems across the region. Northeastern Italy suffered its most severe drought in 200 years in 2022, followed by two record-breaking floods within just 15 days of each other in May 2023. In Greece, where Storm Daniel first made landfall, the storm did substantial damage to agricultural lands, Tarolli said. He added that although cities are often the focus of flood adaptation efforts, severe floods in the countryside are serious too, especially because they can threaten food security.
The study is an alarm bell, Heggy said. He hopes it will draw attention to the need for climate adaptation in arid regions across the Mediterranean and beyond. “We need to act at the Mediterranean scale,” Tarolli said. “We need to include North African countries at the table of discussion.”
Adapting to the Mediterranean’s new normal could take many forms. Heggy pointed to the importance of better monitoring and analysis of storms and flooding across the region. Existing flood management infrastructure, including dams like the one that collapsed in Derna, needs to be assessed and maintained, he added. Tarolli and his colleagues have suggested establishing a pan-Mediterranean disaster risk management fund.
“We could have avoided it,” Heggy said of the destruction in Libya. Without monitoring and mitigation systems, floods and droughts in deserts will be more and more deadly, he added.
—Elise Cutts (@elisecutts), Science Writer
Citation: Cutts, E. (2024), Torrents of sediment-laden water worsened disastrous Libyan floods, Eos, 105, https://doi.org/10.1029/2024EO240470. Published on 25 October 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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