September 12, 2023

expert reaction to Storm Daniel and Libya flooding

Scientists respond to news that Eastern Libya has been hit by severe flooding following Storm Daniel.

Prof Hannah Cloke, Professor of Hydrology, University of Reading, said:

“To be effective, flood forecasting systems need good data on forecast rainfall and river levels, a network of well maintained measuring instruments on the ground, and a clear plan to get people out of harm’s way.  The tragic death toll in Libya from catastrophic flooding that has decimated a city shows what can happen if any parts of this chain are not in place or don’t work properly.

“It seems likely that the worst of the flooding has been caused by a dam failure, although it is possible that very sudden, very heavy rain has caused flash floods on their own.

“This part of North Africa is characterised by a climate where rivers often run dry for much of the year.  Dams are built both to collect water for drinking and agriculture, and to protect settlements downstream from flash floods.  Dams are massive pieces of infrastructure, costing huge amounts of money and effort to build, and require regular maintenance once they are in use.  Even in more politically stable and richer countries, we have seen the effects of poor dam maintenance in causing risks to people from dam failures.  We don’t yet know the full cause of this dam failure, but we do know that Libya has far from the ideal economic or political conditions to monitor and maintain this kind of infrastructure, or provide adequate flood early warning systems.”

Dr Kiran Tota-Maharaj, Reader in Civil & Environmental Engineering (Water and Environmental Engineering), Aston University, said:

“Storm Daniel has the characteristics of a tropical depression, approximately 170 millimetres (6.7 inches) of rainfall occurred fell in Libya.  Torrential rains of between 150 – 240 mm caused flash floods in several cities, including Al-Bayda, which recorded the highest rainfall rate of 414.1 mm.

“Flash floods, which is considered as one of the worst weather-related natural disasters are highly unpredictable following brief spells of heavy rain.  This region in Libya is subjected to flash floods, where floods from the mountains causing heavy damage to hydraulic structures and features of Dams.  These floods are made up of sudden, unexpected and heavy rains or a strong surge of water, which usually hit the steep sloped mountainous catchments and have inundated many regions in Libya.  The sweeping flash floods also led to the death of many residents and great losses of property.  Entire neighbourhoods in Derna disappeared, along with their residents swept away by water after two ageing dams collapsed making the situation catastrophic and out of control, the city of Derna is surrounded by mountains, so the flash flooding occurred quite rapidly, taking over with surface-water levels rising as high as 3 metres (10 feet).  Engineers have previously issued warnings about the risks of these dams bursting and the urgent need to strengthen their defenses, which unfortunately didn’t occur.  Early Warning Systems (EWS), which are effective ways to reduce the risks of flash floods have not been properly implemented.  When EWS are issued before a flash flood event, additional time is created to take action and save lives and infrastructure.  The unexpected arrival of a flash flood in Libya, in combination with its force, limited understanding of the risks and small space-time scales provide explicit challenges for the development and implementation of an EWS system for flash floods.

“Thousands of people’s lives have been sadly lost after the massive flood ripped through the city of Derna as a result from the heave storm conditions and excessive rainfall.  There have been several areas severely affected by widespread flooding, damage to infrastructure, and loss of life.  The disastrous flooding event is likely the cause of the two dams’ collapses, making thousands of residents of the valley and the city of Derna, Libya vulnerable as a result of the storm.  Entire neighbourhoods of Derna by the bank of the swollen river had been ravaged and washed away.”

Dr Liz Stephens, Professor in Climate Risks and Resilience, University of Reading, said:

“Much of the devastation from Storm Daniel in eastern Libya appears to be caused by catastrophic flooding from the failure of one or more dams following exceptional rainfall totals.  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.

“Medicanes such as Storm Daniel are relatively rare, and tend to occur more frequently in the western portion of the Mediterranean Sea than the arid Libyan coastline.  It is more difficult to understand the potential for catastrophic extreme events in an arid climate, where even moderate rainfall events are few and far between.  This makes it a challenge to design and build resilient infrastructure.

“Climate change is thought to be increasing the intensity of the strongest medicanes, and we are confident that climate change is supercharging the rainfall associated with such storms.  It would be interesting to evaluate whether, like 40 degree temperatures in the UK, the rainfall totals in eastern Libya would have been physically implausible without climate change.  However, this is a complex question that would have to take into account any changes in storm track as well as the rainfall totals.”

Dr Carola Koenig, Reader and Vice Dean International in the College of Engineering, Design and Physical Sciences, Brunel University London, said:

“High precipitation on its own won’t cause a dam to fail.  If, however, the dam is not managed well, e.g. structurally impaired or the spillway is not well managed or insufficient, this could cause dam failure as the latter would cause a rise in water pressure and hence increase forces acting on the dam wall.

“The dam break will probably have caused a high number of deaths downstream, but this is likely to be overlapped with other flood deaths.”

Dr Kevin Collins, Senior Lecturer Environment & Systems, The Open University, said:

“The dam collapse has caused catastrophic loss of life and damage to infrastructure.  The collapse was caused by the pressure of floodwaters on the dam after intense rain from a hurricane-like storm which forms in the Mediterranean called a medicane.  It is not really possible to attribute one event – however catastrophic – to climate change, but climate change does increase the likelihood of this kind of extreme weather.

“It is also important to recognise that the storm itself is not just the single cause of the loss of life.  It is also partly a function of Libya’s limited ability to forecast weather impacts; limited warning and evacuation systems; and planning and design standards for infrastructure and cities.  As our climate changes, understanding, planning for and adapting to these more extreme types of events needs to be done by individuals, businesses, and communities in all countries.”

Dr Leslie Mabon, Lecturer in Environmental Systems, The Open University, said:

“The events that are unfolding in Libya are a reminder that there is no such thing as a natural disaster.  It is of course true that climate change can make weather extremes more frequent, unpredictable and violent, in ways that can exceed the ability of our existing infrastructure and systems to cope.  But at the same time, social, political, and economic factors all determine who and where is at greatest risk of harm when these extreme events happen.

“The complex political situation and history of protracted conflict in Libya pose challenges for developing risk communication and hazard assessment strategies, coordinating rescue operations, and also potentially for maintenance of critical infrastructure such as dams.  This is why it is vital that we view events such as those in Libya through the lens of disaster risk creation.  This means looking at both climate change and also the social and political factors – both internally and globally – that make people in countries such as Libya so vulnerable when extremes hit.”

Prof Suzanne Gray, of the University of Reading’s Department of Meteorology, said:

“Storm Daniel is a long-lived Mediterranean cyclone that has been active for more than a week since it formed as a low-pressure weather system around the 4th September.

“Intense Mediterranean cyclones with hurricane-like characteristics are termed medicanes (i.e. Mediterranean hurricane).  These systems can even have the cloudless “eye” visible in satellite imagery that is characteristic of hurricanes.

“Medicanes are relatively rare, about 1-3 per year, but can have devastating impacts on landfall due to their associated flooding, storm surge and strong winds.  Their genesis is mostly in the western Mediterranean and in the region extending between the Ionian Sea and the North-African coast.

“Fluxes of heat and moisture from the Mediterranean sea are usually considered to be important in medicane development and these fluxes are enhanced by warm sea surface temperatures.

“Medicane Ianos, one of the most devastating recent medicanes, had peak winds of 86 knots (99 mph) on landfall in Greece in September 2020, equivalent to a category two hurricane.  It caused infrastructure damage and four fatalities.

“The IPCC 6th assessment report concludes that there is consistent evidence that the frequency of medicanes decreases with climate warming, but the strongest medicanes become stronger.”

Response from the UK National Climate Impacts meeting, which is ongoing.  In particular from Prof Dann Mitchell (University of Bristol), Dr Segolene Berthou (Met Office), and Prof Len Shaffrey (University of Reading):

“The flooding we have seen in Libya is particularly extreme.  Often these kinds of extremes occur on the northern side of the Med as storms move northward, although there are examples, for instance in Morocco.  We expect that as the climate warms we have more moisture in the atmosphere, and we see, in general, more water can precipitate out.  We see a direct link between higher precipitation and flooding.  On top of this background increase, we have the local weather patterns.  For this particular event it was caused by a persistent high pressure blocking pattern which is currently dissipating, and both observations and modelling are not clear that this part of the event will increase in the future.  Local sea surface temperatures in the Eastern Atlantic and Eastern Med are 2-3C higher than usual, and are likely to have caused rainfall to be more intense, through more moisture in the air.  We expect climate change to increase ocean temperatures, but there is also a complex interplay between the atmospheric flow, the jet stream, which is projected to move more northward.  As such, we expect climate change to reduce the numbers of cyclones in the Med, but increase their intensity.  Significant damaging events from these are when infrastructure tipping points are reaching, for instance the extra strain seen on the dam in Lybia.”

Prof Lizzie Kendon, Professor of Climate Science at the University of Bristol Cabot Institute for the Environment, said:

“We expect the intensity of heavy rainfall to increase as the world warms.  This will not be realised as a smooth trend, and we should expect the occurrence of extreme events unprecedented in the observational record.  Storm Daniel is illustrative of the type of devastating flooding event we may expect increasingly in the future, but such events can occur just due to the natural variability of the climate – as they did in the past.  Therefore, care is needed before linking any specific extreme event to climate change.”

Dr Karsten Haustein, Climate Scientist and Meteorologist, Leipzig University, said:

“While no formal attribution of the role of climate change in making storm Daniel more intense has been conducted yet, it is safe to say that the Mediterranean sea surface temperatures (SSTs) have been considerably above average throughout summer.  This is certainly true for the region where ‘Daniel’ could form and wreak havoc over Greece and now Libya.  While SSTs are below average now that the storm has cooled them down, in an analogue scenario (same storm track but without human-induced climate change) the SSTs would still be colder.  But the warmer water does not only fuel those storms in terms of rainfall intensity, it also makes them more ferocious, i.e. the fact that ‘Daniel’ could form into a Medicane (low pressure system with features of a tropical cyclone or hurricane) is likely a result of warmer SSTs and hence man-made climate change as well.  With up to 400mm of rainfall in a short span of time in Libya, the infrastructure could probably not cope, leading to the collapse of the dam.”

Declared interests

Dr Kevin Collins: “No conflicts of interest to declare.”

Dr Leslie Mabon: “No conflicts of interest to declare.”

Prof Lizzie Kendon: “No conflicts of interest.”

Dr Karsten Haustein: “No conflict of interests.”

For all other experts, no reply to our request for DOIs was received.