Sara S. Fouad, Essam Heggy, Oula Amrouni, Abderraouf Hzami, Steffen Nijhuis, Nesma Mohamed, Ibrahim H. Saleh, Seifeddine Jomaa, Yasser Elsheshtawy, Udo Weilacher
First published: 12 February 2025
Abstract
The low-lying, arid coastal regions of the Southern Mediterranean Basin, extending over 4,600 km, face daunting sea level rise and hydroclimatic changes due to shifting weather patterns. The impact of these factors on coastal urban buildings and infrastructure must be better understood. Alexandria, a historic and densely populated port city in Egypt representative of several coastal towns in the Southern Mediterranean, has experienced over 280 building collapses along its shorelines over the past two decades, and the root causes are still under investigation. We examine the decadal changes in coastal and hydroclimatic drivers along the city’s coastline using photogrammetric satellite images from 1974 to 2021. We explore the interconnectivity between shoreline retreat, ground subsidence, and building collapses. Our results suggest that collapses are correlated with severe coastal erosion driven by sediment imbalances resulting from decades of inefficient landscape management and urban expansion along the city’s waterfront. This severe erosion, combined with sea level rise, increases seawater intrusion, raising groundwater levels in coastal aquifers. Degrading ground stability and accelerating corrosion in building foundations ultimately culminating in collapses. We identified a coastal area of high vulnerability with over 7,000 buildings at risk, surpassing any other vulnerable zone in the Mediterranean Basin. We propose cost-effective and nature-based techniques for coastal landscape adaptation to alleviate these dangers in Alexandria and other Southern Mediterranean cities facing similar climatic challenges.
Key Points
- Collapses of coastal buildings increased ten-fold over the last 20 years in the historic Mediterranean port city of Alexandria, threatening 7,000 buildings
- Collapses correlate with areas undergoing chronic and severe shoreline erosion and sea level rise, accelerating seawater intrusion in coastal aquifers
- Seawater intrusion uplifts the coastal groundwater levels to reach building foundations, accelerating their corrosion and collapse
Plain Language Summary
We examine the reasons for the rise in structural failures of buildings along the 70 km coastline of the ancient port city of Alexandria in the Southern Mediterranean. The city is internationally recognized for its significant susceptibility to sea level rise. Severe shoreline retreat due to coastal erosion and rising sea levels significantly accelerates seawater intrusion in coastal aquifers, compromising ground mechanical qualities and degrading building foundations. This ultimately results in their failure. Over 7,000 structures adjacent to the old city coastline are at risk of collapse, rendering it the most climate-vulnerable metropolitan sector in the Mediterranean region. Comparable tendencies are noted in other coastal cities in emerging nations undergoing recent hydroclimatic alterations and where mitigation plans have yet to be enacted. We propose cost-effective and nature-based techniques for coastal landscape adaptation to alleviate these dangers in Alexandria and other cities in emerging nations facing similar climatic challenges.
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