More than 11 000 people are now believed to have died in floods which hit the city of Derna on the northeast coast of Libya on Sunday 10 September 2023, with a possibility the number of dead may rise as high as 20 000. The floods occurred when high rainfall associated with Storm Daniel deposited about 400 mm of rain on the city over a 24 hour period (compared to an average of 1.5 mm for the entire month of September), which caused two dams on the Wadi Derna, a seasonal river which flows through the city and gives it its name, two collapse. The upper of these dams, which collapsed first, held about 1.5 million cubic metres of water (i.e. 1.5 million tonnes in mass). The failure of this dam deposited most of this water into the lake behind the second dam, which already held a further 22.5 million cubic metres (22.5 million tonnes) of water, causing that dam to fail as well, and sending a three metre high surge of water through the centre of the city with sufficient force to knock down multi-story buildings in its path.
Both of the dams which failed are believed to have been of the rockfill type, which are more prone to failure when over-topped than solid concrete dams. Rockfill dams have an impermeable, water-resilient face on their upstream side, usually made from concrete, clay, or a similar material, which retains a lake behind it, but behind this is a supporting mass of permeable rock, which supports the dam, but which can be washed away once water starts to come over the top of the dam. The dams were installed in the 1970s, during the reign of the former Libyan leader, Muammar Gadaffi, and were initially subject to regular maintenance work. However, this appears to have become erratic in nature following Gadaffi's assassination in 2011, and the civil war in Libya which followed this event. Accusations are now being made by academics and opposition politicians in Libya that this lack of maintenance was a major contributing failure to the collapse, and the government promising an enquiry and possible prosecutions.
As well as the huge number of immediate deaths caused by the flooding, the city of Derma, which before the flood had a population of about 90 000, is facing a huge ongoing humanitarian crisis, with about 30 000 people made homeless by the event and effectively no safe sources of water left in the city, due to the huge number of bodies present. Relief workers from a dozen nations have now reached the city, but the scale of the task ahead is daunting, with some agencies arguing the city should be completely evacuated as a safety measure, although with much of Libya already close to a humanitarian crisis, it is difficult to see where people would go, unless neighbouring states are willing to accept them.
Most storms form due to heating of air over the sea. As the air is heated the air pressure drops and the air rises, causing new air to rush in from outside the forming storm zone. If this zone is sufficiently large, then it will be influenced by the Coriolis Effect, which loosely speaking means the winds closer to the equator will be faster than those further away, causing the storm to rotate, clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
Whilst the high winds associated these storms is extremely dangerous, the real danger from such storms is often the flooding. Each millibar drop in air pressure can lead to a 1 cm rise in sea level, and large storms can be accompanied by storm surges several meters high. This tends to be accompanied by high levels of rainfall, caused by water picked up by the storm while still at sea, which can lead to flooding, swollen rivers and landslides; which occur when waterlogged soils on hill slopes lose their cohesion and slump downwards, over whatever happens to be in their path.
Such storms are common in tropical regions, but less so on the more temperate Mediterranean. However, this summer much of the eastern Mediterranean has suffered a prolonged heat wave, with temperatures regularly exceeding 40°C, and heat related deaths recorded across much of southern Europe, as well as a series of wildfires which have swept across Greece and other countries as vegetation has dried out. These events have been caused by a combination of an anticyclone (high pressure system) which formed over the Sahara then migrated northwards, intensified by an El Niño system over the southern Pacific, which tends to drive temperatures upwards in the Northern Hemisphere, both of which are considered to have been intensified by global warming.
Storm Daniel is the largest and deadliest tropical-cyclone-like storm ever recorded in the Mediterranean Region, as well as the deadliest storm globally since 2008, when Cyclone Nargis killed almost 140 000 people in Myanmar. The storm began as a low pressure system over the Ionian Sea, which then swept over the Balkan Peninsula on 4 September 2023, causing widespread flooding in Greece, Bulgaria, and Turkey, before tuning south and crossing the Mediterranean, making landfall close to Benghazi on the Libyan coast on 10 September, and then rapidly dissipating.
See also...
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
