Flood Vulnerability Assessment in a Semi-Arid Watershed: The Case of Oued Mgharouel (Northeastern Algeria)

Rapid and uncontrolled urban expansion in the Oued Mgharouel watershed has significantly increased the exposure of populations and infrastructure to flood risk. This study adopts a multidimensional vulnerability assessment framework integrating three key dimensions: social vulnerability (measured through population density), physical and structural vulnerability (evaluated through building quality, geotechnical soil characteristics, and illegal construction), and functional vulnerability (based on the exposure of critical infrastructure and essential services). Social vulnerability mapping reveals that the downstream areas of the watershed are the most exposed, with moderate to very high population density levels, while the built-up area represents only 8.65% of the total watershed surface. Structural analysis identifies areas of strong to very strong vulnerability, particularly in zones characterized by poor geotechnical conditions, active landslides, low-lying alluvial terraces adjacent to the watercourse, and anarchic constructions using non-resistant materials. The global vulnerability map, derived from the combination of social and structural layers under ArcMap, highlights that the downstream sectors, notably the Avocats neighborhood and the 7th Km of Boussouf, face the highest flood risk levels, confirmed by field surveys documenting recurrent overflows, culvert damage, and drainage network failures along RN 05. Key aggravating anthropogenic factors include soil impermeabilization, absence of vegetation cover, illegal encroachments on the watercourse banks, neglected drainage infrastructure, and the absence of a risk culture among local residents. The findings underscore the urgent need for integrated land-use planning, early warning systems, and preventive risk governance strategies in this flood-prone peri-urban watershed.