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In Alexandria, one of the globe’s most ancient cities, the incidence of building collapses has surged from about one per year to 40 annually over the past decade. (Photo/iStock)
Science/Technology
Coastal erosion endangers this historical metropolis — along with others closer to home
Research from USC on the diminishing coastlines of Alexandria, Egypt, provides nature-based strategies for safeguarding coastal cities worldwide, including those in California.
A recent USC investigation demonstrates a significant rise in building collapses in the ancient Egyptian port city of Alexandria, directly associated with increasing sea levels and infiltration of seawater.
Previously a seldom-seen event, building collapses in Alexandria — recognized as one of the planet’s oldest cities, often referred to as the “bride of the Mediterranean” due to its grandeur — have surged from roughly one per year to a concerning 40 annually over the last decade, according to the scholars.
“The actual toll of this devastation goes well beyond mere bricks and mortar. We are witnessing the slow vanishing of historic coastal cities, with Alexandria sounding the alarm. What once appeared to be distant climate-related dangers is now a pressing reality,” stated Essam Heggy, a water specialist at the USC Viterbi School of Engineering and the primary author of the study.
Alexandria is located on the southern shore of the Mediterranean Sea. Its historical status as the cradle of civilization continues to shape us today. (Google Map)
“For centuries, Alexandria’s architecture stood as a testament to resilient engineering, withstanding earthquakes, storm surges, tsunamis, and more. But now, rising oceans and more powerful storms — driven by climate change — are reversing in decades what took millennia of human creativity to establish,” remarked Sara Fouad, a landscape architect at the Technical University of Munich (TUM) and the leading author of the study.
Coastal erosion: Sinking cities and rising oceans
Even minor elevations in sea levels — just a few centimeters — can inflict severe impacts, Heggy expressed, endangering even cities as historically durable as Alexandria, which has endured centuries of earthquakes, invasions, and fires, as well as a contemporary city like Los Angeles, where flash floods and mudslides are now hindering recovery efforts from the recent wildfires.
Published in Earth’s Future, an AGU journal, the investigation aligns with alarming findings from NASA and NOAA indicating that areas of California — such as the San Francisco Bay Area, Central Valley, and coastal Southern California — are experiencing subsidence. These slight elevation shifts can notably amplify flood hazards and saltwater intrusion, the scientists caution.
Similar to Alexandria, California’s coastal urban centers confront escalating dangers from saltwater intrusion, which
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undermines infrastructure, diminishes water resources, and increases the cost of living.
“Our research disputes the widespread belief that concerns arise only with a one-meter sea level rise,” Heggy stated. “What we are presenting is that coastlines worldwide, particularly Mediterranean coastlines akin to California’s, are already undergoing transformations leading to building failures at an alarming rate.”
Monitoring coastal erosion in Alexandria, Egypt
The researchers utilized a three-part strategy to evaluate the impact of shoreline alterations on Alexandria’s structures.
Initially, they developed a comprehensive digital map employing geographic information system technology to pinpoint the sites of collapsed buildings across six districts in the city’s historical urban area, which is one of its most densely settled regions. This map documents critical information about each structure, including its precise location, dimensions, construction materials, age, foundation depth, and number of stories.
The data, gathered from site inspections, government publications, news archives, and announcements from private construction firms, spans from 2001 to 2021, encompassing both fully and partially collapsed edifices.
Next, they merged satellite imagery with historical maps from 1887, 1959, and 2001 to track the movement of shorelines and enhance their understanding of how sections of Alexandria’s 50-mile coastline have shifted tens of meters inland in the last twenty years. By determining the rate of shoreline retreat over the past century, the researchers investigated how the receding coastline is elevating groundwater levels, bringing them into contact with coastal building foundations.
Ultimately, the team evaluated chemical “signatures” known as isotopes in soil samples to study the ramifications of seawater intrusion. They analyzed specific isotopes, such as B7, in each sample to ascertain the soil’s mechanical characteristics. Elevated B7 levels suggest stronger, more stable soil, while diminished levels indicate erosion.
“Our analysis of isotopes indicates that buildings are failing from the ground upwards, with seawater intrusion eroding foundations and compromising the soil. It is not the structures themselves that are at risk, but rather the land beneath them,” mentioned Ibrahim H. Saleh, a soil radiation scientist at Alexandria University and co-author of the study.
“Our findings illustrate that coastal buildings face the risk of collapse even without direct encroachment from seawater, contrary to popular belief,” Heggy added.
A nature-based strategy for safeguarding coastal cities
To address coastal erosion and seawater intrusion, the researchers advocate for a nature-based strategy: establishing sand dunes and vegetative barriers along the coastline to impede the advance of seawater, thereby preventing seawater intrusion from elevating groundwater levels to building foundations. This sustainable and cost-effective method can be implemented in numerous densely populated coastal regions around the globe, according to Steffen Nijhuis, a landscape-oriented urbanist from Delft University of Technology in the Netherlands and co-author of the study.
“Safeguarding the rich architectural features of Mediterranean historic cities serves as a compelling reminder of the essential role landscape transformation plays in fostering climate-resilient societies,” declared Udo Weilacher, a landscape architect at TUM and study co-author.
“Historic cities like Alexandria, which symbolize the foundation of cultural exchange, innovation, and history, are vital for preserving our collective human heritage,” Heggy stated. “As climate change accelerates sea level rise and coastal erosion, protecting these cities transcends mere building preservation; it’s about safeguarding our identity.”
About the research: Co-authors of the research include Sara S. Fouad and Udo Weilacher from the Technical University of Munich in Germany; Oula Amrouni and Abderraouf Hzami from the National Institute of Marine Sciences and Technologies at the University of Carthage in Tunisia; Steffen Nijhuis from Delft University of Technology in the Netherlands; Nesma Mohamed and Ibrahim H. Saleh from the Institute of Graduate Studies and Research at Alexandria University in Egypt; Seifeddine Jomaa from the Helmholtz Centre for Environmental Research – UFZ in Magdeburg, Germany; and Yasser Elsheshtawy from the School of Architecture, Planning and Preservation at Columbia University.
This research received funding from the Zumberge Research and Innovation Fund at the University of Southern California, allocated to the Arid Climates and Water Research Center (AWARE). Additional support for this work was provided by the NASA Jet Propulsion Laboratory, the Alexander von Humboldt Foundation, and the Technical University of Munich.