Environment2 min
Libya's Derna dam collapse: what the ICOLD assessment found
A joint review by ICOLD, UNESCO and the World Bank shows how a freak storm, thin rainfall records and a spillway design flaw combined in Derna.
When Storm Daniel hit Libya around 9 September 2023, two dams above the coastal city of Derna failed. The result was one of the deadliest dam disasters on record. World Bank water specialist Marcus Wishart walked delegates through the lessons at ICOLD's annual meeting in New Delhi.
Libya is not a country people link with big dams. It is arid and short of water, yet it has 18 large dams, mostly along the north coast. Fourteen are listed in ICOLD's World Register of Dams. Many sit on wadis, seasonal riverbeds that run dry for much of the year. They protect towns from floods, feed farms and help refill underground aquifers.
Derna and Abu Mansour were built in the 1970s to do exactly that. Derna dam stood about 1 km upstream of the city. It was a 40-metre clay-core embankment with a morning glory spillway rated at 350 cubic metres per second. Abu Mansour, roughly 11 km further up, was taller at 73 metres, with a 476 sq km catchment and a spillway rated at 170 cubic metres per second. Wishart noted that Abu Mansour goes by several names, which muddied early reports after the collapse.
The storm itself was extreme. Rainfall ran up to 200 times above the long-term average for the period. Wishart said heavy rain of this kind is now up to 50 times more likely than in a climate 1.2°C cooler. He also pointed to a second question: the dams were designed with a limited rainfall and river-flow record, so the original assumptions may have been too narrow.
The human toll is still being counted. Early assessments recorded more than 4,000 deaths, 10,000 people unaccounted for and 34,000 displaced, with over 250,000 people affected. Newer estimates put deaths between 5,900 and 20,000. That would make this the second-deadliest dam failure in recorded history. Losses reached US$1.65 billion, or 3.6% of GDP. More than 7% of the country's housing was damaged or destroyed, and early reconstruction costs came to US$1.8 billion.
ICOLD, UNESCO and the World Bank then worked together on an assessment. Wishart said that was hard, because public reports conflicted, including material from the original design firm. He also stressed how volatile Libya's water system is. Total storage is around 380–390 million cubic metres, and three dams hold more than half of it. Yet the long-term average annual flow is only about 60 million cubic metres. One key factor in the disaster, he said, was the design of the dams' morning glory spillways.
The takeaway reaches well beyond Libya. Ageing dams in dry regions may face storms far bigger than their designers planned for, and a warming climate raises that risk. Read the full account of the assessment in the original NS Energy report.
