More 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". 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. The upper dam had a storage capacity of 1.5 million cubic metres of water, whilst the lower dam could hold 22.5 million cubic metres. Each cubic metre of water weighs about one tonne (1,000kg), so 1.5 million cubic metres of water would weigh 1.5 million tonnes. Combine that weight with moving downhill, and it can produce enormous power. Witnesses have said that the waters were nearly three metres in places. It is estimated that six inches (20cm) of fast moving flood-water is enough to knock someone off their feet, and 2ft (60cm) is enough to float a car. So it is no surprise that whole buildings were taken out in the floods. https://www.bbc.com/news/world-africa-66799518