Alexandria, a city that has withstood centuries of earthquakes, wars, and conquests, is facing a threat from rising sea levels and sea water intrusion that is slowly eroding its foundation. In the last decade, the number of building collapses has increased from one per year to a staggering 40, according to a study by researchers from the University of Southern California and Alexandria University.
“The true cost of this loss extends far beyond bricks and mortar. We are witnessing the gradual disappearance of historic coastal cities, with Alexandria sounding the alarm,” says Essam Heggy, Water Scientist, USC Viterbi School of Engineering, and corresponding Author of the study.
Rising sea levels are not just about water creeping up the shoreline but also seeping underground, weakening the soil that holds up buildings, a process the researchers have tracked through isotope analysis, revealing that seawater is pushing groundwater upward, eroding the base of Alexandria’s structures.
“Our isotope analysis revealed that buildings are collapsing from the bottom up, as seawater intrusion erodes foundations and weakens the soil,” says Ibrahim H. Saleh, Soil Radiation Scientist at Alexandria University, the study’ s co-Author.
The scientists used a mix of digital mapping, historical shoreline records, and soil testing to confirm this crisis. According to evidence from the satellite images, parts of the coastline have receded by tens of meters over the last century, forcing the ocean closer to densely populated areas.
According to Sara Fouad, a Landscape Architect from the Technical University of Munich and the study’s first author, despite the resilient engineering, rising seas and intensifying storms, fueled by climate change, are undoing in decades what took a millennium of human ingenuity to create.
The collapse of buildings, for many Alexandrians, is more than a financial disaster. ‘This house belonged to my grandfather, and now it is gone,’ says one resident who decries the loss of family legacies. Rising repair costs, insurance challenges, and economic instability have left thousands at risk.
Thousands of kilometers away, Mombasa, Kenya’s historic coastal city, faces similar threats. Mombasa’s Old Town, a UNESCO-listed site, has seen increased flooding due to rising sea levels. As in Alexandria, seawater intrusion threatens the very foundations of its Swahili architecture.
“What we’re showing here is that coastlines globally, especially Mediterranean coastlines similar to California’s, are already changing and causing building collapses at an unprecedented rate,” says Essam Heggy. In California, coastal cities like San Francisco and Los Angeles are grappling with land subsidence and saltwater intrusion, compounding flood risks.
To help mitigate the compounding crisis, the experts propose a nature-based solution using dunes and vegetation barriers, which would serve as a buffer against encroaching seawater and help stabilize groundwater levels. Moreover, similar projects have been tested in the Netherlands, where green infrastructure has helped mitigate coastal flooding.
“A nature-based solution, such as sand dunes and vegetation barriers, offers a sustainable and cost-effective way to combat coastal erosion,” says Steffen Nijhuis, a Landscape-Based Urbanist from Delft University of Technology and co-Author of the study.
The urgency of Alexandria’s fate, if left unchecked, could be a preview for the impending catastrophe on other historic cities. Mombasa, Venice, and Bangkok, all coastal cities with deep cultural roots, are at risk of losing their heritage to rising seas. Consequently, addressing this crisis demands global attention and timely intervention.
“Historic cities like Alexandria, which represent the cradle of cultural exchange, innovation and history, are crucial for safeguarding our shared human heritage, ” emphasizes Udo Weilacher, Landscape Architect at the Technical University of Munich and the study’s co-author.
