Derna Was Warned: The Hour-by-Hour Story of a Preventable Disaster

LIBYA AT THE CLIMATE CROSSROADS  ·  PART 4 OF 6

Derna Was Warned: The Hour-by-Hour Story of a Preventable Disaster

Cracks were reported in the 1980s. Repairs were urged in 2003. The weather centre warned three days ahead. On 11 September 2023, both dams above Derna failed anyway — and floods have kept killing people across Libya ever since.

At 00:30 on the night of 11 September 2023, water began pouring over the top of the Abu Mansour dam, twelve kilometres above the city of Derna.

Most of the city was asleep. Residents had been told to stay indoors. Few had been told to leave.

At 02:40, the dam began to break.

What happened in the next half hour would become one of the deadliest weather disasters in modern North African history — and one of the most thoroughly predicted.

The scale of the loss

Official counts recorded more than 4,300 deaths, with thousands more missing. Independent scientists estimate the true toll may be far higher — between 11,000 and 24,000 people, up to a fifth of the city.

An assessment by the World Bank, the European Union and the United Nations found that the storm affected about 1.5 million people — one in five Libyans — and damaged or destroyed more than 18,500 homes, 7% of all housing in the country.

How the dams failed

The Derna valley is a steep, narrow limestone gorge running down from the Green Mountain to the sea. In the 1970s, two earth dams were built across it to protect the city from floods: a large one upstream, Abu Mansour, and a smaller one just a kilometre above the city.

Both had funnel-shaped overflow outlets that could release water only at a limited rate. The upper dam's outlet could pass about 170 cubic metres per second.

On the night of Storm Daniel, water poured into its reservoir at about 4,000 cubic metres per second — more than twenty times what it could release.

A scientific reconstruction published in 2025, built from computer models and eyewitness accounts, pieced together what followed:

How the Derna dams failed, hour by hour (night of 10–11 September 2023)

▼  water runs off steep, partly bare slopes

▼  a surge races down the gorge

▼  a wall of water and debris

The same study reached a conclusion that matters enormously: Derna would have flooded even without the dams — but their collapse made the flood far deadlier. It made the flooded area nearly half as large again, and the water about seven times deeper in many streets.

And a third dam — planned, but never built — would have held back much of the water.

Forty years of warnings

This is the part of the story that turns tragedy into scandal.^,

• 1986: A major storm damages the dams. A government study later cited by the Attorney General finds "cracks and fissures".
• 1998: Cracks are reported again. Around 2002, maintenance reportedly stops.
• 2003: A Swiss engineering review concludes both dams need strengthening — and recommends building a third dam. The full report has never been made public.
• 2007: A $30 million repair contract is signed with a Turkish company. Almost no work is done.
• 2008: A diplomatic row between Libya and Switzerland drives the Swiss engineers supervising the project out of the country.
• 2011: The revolution freezes everything.
• 2012–2013: More than $2 million is set aside for maintenance. A later state audit finds "no work was done".
• 2022: A Libyan civil engineer, Abdelwanees Ashoor, publishes a study warning that in a major flood "the consequences will be disastrous".
• About 7 September 2023: The national weather centre warns of the storm, three days ahead.
• 11 September 2023: Both dams fail.

Derna was not a natural disaster made worse by human error. It was a failure of government, triggered by the weather.

In 2024, twelve officials were convicted, with sentences of up to 27 years. But accountability after the fact does not bring anyone back.

It keeps happening

If Derna were a one-off, it would be a terrible chapter in Libya's history. It is not. The same weaknesses keep killing people in smaller floods that rarely make international news:

• Ghat, 2019: Floods put much of this southern town under up to two metres of water and drove about 2,500 people from their homes.
• Zliten, 2024: Underground water rose to the surface and flooded a whole neighbourhood. Walls cracked; about 50 families had to move. Nobody could say exactly why, because no one monitors the groundwater.
• The north-west, December 2024: Up to 155 mm of rain in two days. Houses and hospitals flooded. Six people died, including two rescuers. The weather centre had warned two days earlier.
• The west and south, September 2026: Four days of floods from Mizda to Murzuq. A two-year-old girl died when her family's home collapsed. At least 79 people were injured, and a bridge was swept away.

Look at the pattern. Each time, a warning existed. Each time, there was no clear system to turn that warning into action — no one with the authority to close the road, open the shelter, send the text message and move people out.

Why the warnings don't reach people

• No one is in charge. Libya has no single national civil-protection authority. During emergencies, rival authorities in the west and east have issued conflicting instructions.
• Towns have duties but no money. A 2012 law makes municipalities responsible for civil protection, but they get almost no budget for drains, pumps or rescue equipment.
• The last mile is missing. Only about 15% of people in risk areas are covered by early warnings.
• Homes stand in the water's path. Unplanned building since 2011 has pushed houses into dry riverbeds and flood plains.

Turning grief into protection

Derna cost Libya about $1.65 billion in damage and losses — equal to 3.6% of the country's entire yearly economic output. Rebuilding needs are estimated at $1.8 billion.

The world now has a fund for exactly this kind of harm — the Loss and Damage fund — and Libya can apply. Its first funding window offers grants of usually $5–20 million per request. To win support, Libya needs evidence: a national register recording, after every disaster, who died, who lost their home, what land and services were destroyed.

Beyond money, three changes would save lives:

1. A national emergency agency, created by law, with the power to order evacuations anywhere in the country, above political divisions.
2. A ban on building in flood paths, and a dam-safety law requiring regular independent inspections and emergency plans for everyone living downstream.
3. Replanting the mountains. Restoring the slopes of the Green Mountain — and protecting the Nafusa Mountains from the same fate — slows rainwater, lets it soak in, and reduces the force of floods.

And one more, for trust: publish the 2003 dam study and the repair contracts. Derna's families deserve to know.

1. ^

   Nature Communications, “How extreme rainfall and failing dams unleashed the Derna flood disaster” (2025).

2. ^

   World Bank, European Union and United Nations, Libya Storm Daniel Post-Disaster Needs Assessment (2024).

3. ^

   Nature Communications, “How extreme rainfall and failing dams unleashed the Derna flood disaster” (2025).

4. ^

   Nature Communications, “How extreme rainfall and failing dams unleashed the Derna flood disaster” (2025).

5. ^

   Associated Press, “Warnings that Libya’s Derna dams might burst went unheeded” (2023).

6. ^

   The Sentry, Corruption Threatens Libya’s Derna Again (2024).

7. ^

   Libya Herald, reporting on the Ghat floods (June 2019).

8. ^

   World Meteorological Organization, “The north-western regions of Libya affected by severe weather event on 5 & 6 December 2024” (2024).

9. ^

   Xinhua, “Child killed, 2 others injured as home collapses amid floods in S. Libya” (September 2026).

10. ^

    State of Libya, National Adaptation Plan (2025).

11. ^

    State of Libya, First Nationally Determined Contribution under the Paris Agreement (2026).

12. ^

    Fund for responding to Loss and Damage, Barbados Implementation Modalities (2025–2026).