Libya floods: Why did the Derna dams fail?

Collapse of 50-year-old dam near city caused deadly domino effect, experts say

The collapse of two dams on Monday unleashed a torrent of mud and water into the eastern Libyan city of Derna, killing thousands. More than 10,000 are feared missing in the flooding caused by Storm Daniel.

As authorities continue to carry out relief efforts, attention has turned to a 70-metre-tall dam built in the wadi, or dried river bed, upstream from the city.

The collapse of the dam led to the destruction of a second dam downstream.

The initial dam was built along the river in the 1970s by a Yugoslavian company to control seasonal floods, News Libya reported.

A 2016 webpage from the company Hidrotehnika-Hidroenergetika, now based in Serbia, describes the structure as an “embankment dam” built with rocks and clay. It said it was completed in 1977.

Derna deputy mayor Ahmed Madroud said on Wednesday that the structures had not been maintained since 2002. Public anger has risen amid rumours on social media that funds were allocated for its maintenance.

But experts have told The National that it is too early to say what exactly was behind the failure of the dam, especially as the city was hit hard in the civil war that followed the fall of Muammar Qaddafi.

Many point to the years of conflict that has led to widespread neglect of infrastructure.

"Dams in Libya and especially in the east of the country, haven't received maintenance in years and management bodies haven't provided adequate financial support and means to support the dams, which has led to adverse effects," says Malak Altaeb, a fellow at the Centre for Climate and Security, who studied at the University of Tripoli.

"Instead of protecting from flooding, the dams ended up flooding the city," she tells The National.

Climate shock

Storm Daniel caused a deluge of rain to fall on Libya, while the region also faced a summer heatwave.

Parched ground worsens flash flooding because the earth cannot quickly absorb water.

“Rainfall exceeded 100mm in just three days, where the average monthly rainfall in the whole of September is under 1.5mm,” said Reach, a data service for humanitarian emergencies.

Andy Hughes, an engineer with the Dams & Reservoirs engineering services company in the UK, said the second dam was not able to hold back the torrent after the first dam was destroyed.

“Clearly, it is a devastating tragedy with significant loss of life. It also appears to be a cascade failure – one dam causing the failure of the next dam down, like dominoes,” he says.

He said the flood probably caused “overtopping of the dam, leading to erosion and failure, which then released its contents on the next dam down".

Embankment dams, unlike watertight concrete arch or buttress dams, have been criticised as being vulnerable to extreme weather, especially when the reservoir behind the dam fills up and “overtops” the structure.

Embankment dams have a core wall, but this can become weakened as water erodes the rock and earth around it.

Torrent of mud

“If you look at the video of the destroyed dam, you can see a reinforced concrete wall, part of which is still standing but most of it has been shorn off,” said Tilman Roschinski, an expert on hydrology and geology at the Plan4Risk consultancy.

“We don’t know at this stage what the wall was designed to withstand, but it raises questions, especially on the maintenance issue, such as the quality of the concrete which can seriously deteriorate over years when exposed to water.”

Mr Tilman said that if the last flood in the area was in the early 1960s, as Libyan media has reported, fine particles of mud and silt will have built up in the area around the dam since then.

It may seem like an academic point, but he emphasised that such material, “which would normally have been washed away” by natural flooding over the decades, would have mixed with "the enormous velocity of the floodwater, greatly increasing the pressure on the dam, and increasing its destructive force".

Such muddy water can have twice the density of normal floodwater, up to 2g per cubic centimetre, he said. That equates to a kind of liquified soil with the density of sinking sand.

Mr Hughes said this called into question whether the dam designers could have anticipated a flood of this magnitude.

“Different countries have different standards for spillway design and in this country [the UK] we would design for the probable maximum flood to give the maximum protection to those downstream," he said.

“Designs of this type, if made of sand as is normally the case with a ‘wadi type’ dam, will sometimes involve the armouring of the dam to resist overtopping because the dam is above a community.”

Mr Hughes said the dam in Wadi Derna might “not have been designed to cater for the failure of an upstream dam".

A fragile state

“The Derna floods follow a pattern similar to the August 2020 Beirut port explosion and Turkey’s earthquake in March of this year – a government unable to respond to, or prevent, predictable disasters because its capacity is hollowed out by corruption, indifference and incompetence,” said Keith Mines, a former US diplomat and expert on post conflict recovery at the US Institute of Peace.

Mr Mines, who recently wrote a book on post-conflict recovery, Why Nation Building Matters, said some blame for the disaster in Libya has to lie with the international community. He referred to the wider failure to bring warring parties together and focus on rebuilding the country.

“Investing in the hard work of bringing about political settlements that create viable, self-functioning, independent and popular democratic nations that can facilitate state functions carrying out flood control, earthquake relief, and the reduction of dangerous stockpiles of munitions remains one of the fundamental pillars of global stability,” he said.