The biggest threat to deserts? Floods

A new study by researchers from the USC Viterbi School of Engineering, in collaboration with researchers from the Institut de Physique du Globe de Paris at Université Paris Cité, has found that increased soil erosion in coastal areas due to desertification is exacerbating the impacts of flooding on port cities in the Middle East and North Africa.

The researchers focused their observations on the devastating 2023 floods in the city of Derna, Libya, which claimed the lives of more than 11,300 people, and showed how increased soil erosion contributed significantly to the catastrophic toll of these unusual desert floods.

The research, published in Nature Communicationswas published nearly a year after the deadly flood of September 10, 2023. The co-authors believe their work highlights the alarming vulnerability facing drylands given the increasing frequency of extreme weather events due to climate change and the urgent need for advanced Earth observation programs to monitor and characterize these areas.

Over the past decade, the North African Sahara, an area larger than the continental United States, has faced a dangerous combination of conditions: increasingly arid conditions, interspersed with intense coastal rains.

The causes of these changes are as follows: increasing desertification has led to more intense droughts and rainstorms have increased in frequency in the region due to the rise in sea water temperature in the eastern Mediterranean due to global warming.

The study’s corresponding author, Essam Heggy, who is a research scientist in the Microwave, Sensor and Imaging Systems Laboratory (MiXIL) in the Ming Hsieh Department of Electrical and Computer Engineering and a co-principal investigator at the USC Viterbi Center for Arid and Aquatic Areas Research and Exploration (AWARE), says that together, these two extreme conditions increase soil erosion and generate deadly mudslides that are difficult to control with the region’s aging dams.

While some researchers believe that droughts are the deadliest threat to the Sahara, Heggy warns that this is not the case. His paper, “Assessing Erosion from Flash Floods after Storm Daniel in Libya,” provides evidence.

A year ago, in the fall of 2023, Storm Daniel, also known as “Medicane Daniel,” struck the eastern coast of Libya, causing unprecedented flash flooding that killed more than 11,300 people and caused widespread infrastructure damage. (Flash floods of this nature have not been seen on the continent in more than 100 years, according to Yale Climate Connections.)

The authors explain that Africa’s deadliest flood in a century, which occurred in the desert, occurred due to a combination of factors: unusually high rainfall, the collapse of two flood control dams and the failure of the city’s “blue” or water infrastructure to regulate the extreme event.

They suggest that sediment loading, resulting from surface erosion, increased the density of flowing water and exacerbated the catastrophic impact of flash floods in the coastal cities of Derna and Susah, where 66% of urban areas in Derna and 48% of those in Susah suffered moderate to high damage.

Using a series of Sentinel-1A orbital C-band synthetic aperture radar images, the researchers measured changes in the coherence of the returned signal, which informed changes in surface textural properties before and after the storm hit. (These differences serve as a proxy for flood erosion mapping and infrastructure damage assessment.)

The researchers demonstrated that the flow of the rivers was heavily loaded and thickened by soil erosion, which increased the destructive nature of the flow. This contributed to the failure of two dams that were supposed to protect the city and residents of Derna.

According to Heggy, existing runoff models are useful for estimating the extent of flooding. However, they cannot assess surface erosion in deserts, which can have devastating consequences, as seen in Derna.

Radar satellites help overcome this limitation, Heggy says. “Improving monitoring of arid watersheds using advanced radar satellites will be essential to mitigate these devastating risks in several regions of the Sahara, the Arabian Peninsula and other deserts.”

Jonathan Normand, a visiting graduate student at USC’s AWARE Center and first author of the study, said, “Today, you can post to social media from the middle of a desert thanks to the hundreds of communications satellites orbiting the Earth. Yet researchers have only a limited number of satellites to capture the complexities of Earth dynamics and surface processes in deserts.”

“The sequence of events that occurred in Libya is one that can occur in many populated areas of North Africa and the Arabian Peninsula,” Heggy and colleagues warn in a parallel study conducted with researchers from Morocco and Spain.

There are additional risk factors, too, Heggy notes: storms are getting stronger and cities are getting more crowded and less organized in terms of policies aimed at reducing development and increasing disaster preparedness.

“The deadly floods in Derna show that regional policymakers in the Middle East and North Africa are still not listening enough to science, even though the last two climate change conferences were held in the region. The deadliest enemy we face is our own belief that these extremes are one-off events that will not recur. Climate models tell us they will hit even harder.”