A visual guide to why heavy rain caused such devastation and killed so many people in the city of Derna.
Image source, AFPMore than 5,000 people are known to have died and thousands more are missing after devastating floods swept through the Libyan port city of Derna.
Entire neighbourhoods disappeared into the sea as a huge tsunami-like torrent of water swept through the city.
Whole families were washed away, according to a Libyan journalist who has been speaking to survivors in the city, and who described the situation as “beyond catastrophic”.
BBC Verify and the BBC’s visual journalism team have been analysing some of the reasons why the floods caused such catastrophic damage in Derna.
Record rainfall
The water was brought by Storm Daniel which reached Libya on Sunday.
The storm – a Mediterranean hurricane-like system known as a medicane – brought more than 400mm of rain to parts of the north-east coast in a 24-hour period.
That is an extraordinary deluge of water for a region which usually sees around 1.5mm of rain throughout the whole of September.
Libya’s National Meteorological Centre says it is a new rainfall record.
It’s too early to attribute with certainty the severity of this storm to rising global temperatures.
However, climate change is thought to be increasing the frequency of the strongest medicanes.
Prof Liz Stephens, an expert in climate risks and resilience at Reading University, says scientists are confident that climate change is supercharging the rainfall associated with such storms.
Two dams overwhelmed
The Wadi Derna river runs from Libya’s inland mountains, through the city of Derna and into the Mediterranean.
It is dry for much of the year, but the unusually heavy rain overwhelmed two crucial dams and destroyed several bridges.
Residents of the city, who had been ordered by the local authorities to stay in their homes, reported hearing a loud blast before the city was engulfed in water.
“The dams would have held back the water initially, with their failure potentially releasing all the water in one go.
“The debris caught up in the floodwaters would have added to the destructive power,” says Prof Stephens.
Your device may not support this visualisation
Experts who have spoken to BBC Verify say it’s too early to know whether the extreme rainfall was simply too much for the dams to handle, or whether the condition of the structures also played a role.
Based on their observations, the dams are likely to be made from dumped and compacted soil or rocks, which is not as strong as concrete.
“These dams are susceptible to overtopping [when water exceeds a dam’s capacity], and while concrete dams can survive overtopping, rockfill dams usually cannot,” says Exeter University’s Prof Dragan Savic, an expert in hydraulic engineering.
Your device may not support this visualisation
It appears that the upper dam failed first, according to structural engineer Andrew Barr.
He says the water then probably flowed down the rocky river valley towards the lower dam before overwhelming it, resulting in the sudden and catastrophic flooding of the city which lies trapped between mountains and the sea.
As rescue efforts in the city continue, Libyan journalist Johr Ali, who has spoken to survivors in the city, told the BBC: “People are hearing the cries of babies underground, they don’t know how to get to them.”
“People are using shovels to get the bodies from underneath the ground, they are using their own hands. They all say it’s like doomsday,” Mr Ali reported.
Image source, Getty Images
Image source, AFP
Produced by Chris Clayton, Mike Hills, Paul Sargeant, Tural Ahmedzade, Kady Wardell, Gerry Fletcher, Filipa Silverio and Erwan Rivault. Additional reporting: Mark Poynting, Peter Mwai and Jake Horton
