In September 2023, torrential rain from Storm Daniel put overwhelming pressure on the Derna dams, making their eventual catastrophic collapse inevitable.
The storm began as a low-pressure system in the Black Sea on September 4 and tracked inland across the Balkan Peninsula, bringing unprecedented rainfall, particularly in Greece’s Thessaly region on September 5 to 6, as well as Bulgaria and Turkey. The system then traversed the warm Mediterranean Sea, where due to favorable weather and sea conditions, it transitioned into a medicane — a portmanteau of “Mediterranean” and “hurricane”. The medicane made landfall in Benghazi, Libya, on September 10, with tropical storm force winds of 70 to 80 kilometers per hour, and record-breaking rainfall (Figure 1).
Source : NASA
Storm Daniel's development was aided by an “Omega blocking” pattern. This meteorological phenomenon occurs when a high-pressure system is flanked by two low-pressure systems, creating a configuration that resembles the Greek letter omega (Ω ). Such a setup typically disrupts the usual west-to-east progression of weather systems. In Daniel's case, this Omega block effectively anchored the storm, providing conditions conducive to its development. The storm’s intensification was also fueled by unusually high sea surface temperatures in the Mediterranean Sea.
Storm Daniel brought exceptional rainfall to Greece (Figure 2) with Zagora in Thessaly receiving 1,092 millimeters in under 24 hours, a staggering 138% of the average annual rainfall for this region.
Source : Alamy
The extreme weather significantly disrupted transportation, damaging major routes across Greece. Agricultural losses were considerable; 750 square kilometers of farmland flooded and more than 200,000 livestock perished, leading to problems with waterborne diseases.
Greek Prime Minister Kyriakos Mitsotakis estimated direct economic damages at €2.5 billion (US $2.8 billion), while academics projected a broader economic impact of up to €5 billion (US $5.6 billion).[4] According to the Hellenic Association of Insurance Companies (HAIC), insurance claims are expected to be around €400 million (US $440 million), marking Greece's largest payout for flood and storm damages since HAIC's records began in 1993 — but far below the estimated economic loss. This reflects a substantial protection gap, which is unsurprising as Greece's insurance penetration stands at just 2.4% of GDP, well below the European Union average of 7.5%.
Storm Daniel struck Greece a few weeks after the country grappled with devastating wildfires. In response to these consecutive natural disasters, the Greek Prime Minister proposed tax breaks for individuals purchasing insurance. Additionally, the government is actively considering mandating natural catastrophe insurance coverage in the future.[5]
Despite a state of emergency being declared in Libya before the storm's landfall, the fractured nature of the country led to a breakdown in the transmission and messaging of warnings.
The storm made landfall near Benghazi on September 10; the most destructive aspect was its heavy rainfall. While the totals were lower than those in Greece, large areas of the country saw record-breaking precipitation. Al Bayda accumulated 414 millimeters over 24 hours, 76% of the annual average.[6] However, while heavy rainfall ravaged the region, the true tragedy unfolded in Derna. Here residents were told to avoid the beach and advised to shelter at home, presumably in case of a storm surge.[7]
Two aging dams constructed in the 1970s played a critical role in the catastrophe. The structures had been built in a steep-sided canyon, the Wadi Derna. They were designed to supply water and help reduce the incidence of flooding.[8] As well as highlighting maintenance issues, a 2022 report suggested that the designers may have underestimated the potential discharge from the catchment that could be generated during extreme events.[9],[10] In the early hours of September 11, catastrophic failure occurred and the upper dam collapsed, unleashing a torrent that overwhelmed the 75-meter-tall lower dam, situated only 100 meters upstream of the nearest houses.A quarter of Derna, built on the Wadi's historic delta, was obliterated (Figure 3).
Source : Alamy
Rescue efforts faced delays due to storm damage and security concerns, leaving at least 4,300 confirmed dead and 8,000 to 10,000 still missing.[11],[12] Economic damages in Libya are projected to be several billion U.S. dollars. When combined with the losses in Greece, Storm Daniel will far exceed the previous record for a Mediterranean storm set by Tropical Storm Rolf in 2011, which inflicted approximately $2 billion in damages (in 2023 U.S. dollars).
The tragedy exposes glaring vulnerabilities. Unreliable governance during the Gaddafi era was compounded by conflict in the years following his death. Together these hampered both preventive measures and emergency response. Poor communication and an incomplete understanding of the storm's potential led to inadequate warnings, leaving some residents vulnerable in flood-prone areas.
The storm's impact on infrastructure, including the closure of the port and damage to roads, impeded the initiation of recovery efforts, including aid delivery. Even months after the storm, over 40,000 remain internally displaced within Libya. [12] International aid organizations such as the Red Cross are actively working to provide support for 200,000 people affected by the disaster.
Natural disasters are often exacerbated by human actions, and the tale of Derna is more poignant because it was a disaster waiting to happen. This is what can happen when a society is complacent and doesn’t heed warnings. Societies are dependent on engineered infrastructure, which requires inspection and maintenance, but a significant proportion of global infrastructure is operating at or beyond its design life. Governance challenges vary across nations, with some facing additional strains from conflict as is the case in Libya. Many governments have also made difficult choices over expenditure since the 2007 – 2008 banking crisis and the 2019 – 2020 global pandemic. When these issues intersect with natural hazards that exceed the design criteria of structures, which is becoming increasingly likely due to climate change, there may be more tough times ahead. But there is also an opportunity to avert future disasters, contingent on our willingness to acknowledge and act upon these warnings.
The Mediterranean region was severely impacted by Storm Daniel, an intense weather system that unleashed unprecedented rainfall and resulted in significant flooding across Bulgaria, Greece, Turkey and Libya. With economic damages estimated at multiple billion U.S. dollars and at least 4,300 fatalities, it is both the costliest and the deadliest Mediterranean storm in recorded history.
