Flooding vulnerability in Algiers (Algeria): an analytic hierarchy process

Flooding is the second most serious hazard in Algeria, after earthquakes. According to the Civil Protection Services census, the department of Algiers experienced 174 floods between 1946 and 2021, several devastating floods. This study examines Algiers' vulnerability to flooding using the Hierarchical Multicriteria Analysis method, which aggregates eight criteria: altitude distribution, slopes, drainage density, topographic wetness index, land cover and land use, normalized difference vegetation index, soil types, and precipitations. With an 88% accuracy rate determined by the receiver operating characteristic method, the vulnerability map provides an essential tool for better risk and land-use management. The municipalities most vulnerable to flooding are located in the Mitidja (east of Algiers) and Algiers' Western Sahel. These areas cover 209 km(2) (10% of Algiers' urbanized area) and are primarily located in the watersheds of El Harrach, El Hamiz, and Reghaia, while 26% of urbanized areas are moderately vulnerable. However, these areas require special attention because of their high population density. In 2021, urbanization in Algiers reached 44.35%, with major wadis beds recording nearly half (similar to 49.4%) of their watershed area as urbanized. These percentages underscore the urgency of addressing the impact of urban expansion on heightened flood risks. Our findings emphasize the need for a more rigorous implementation of land-use planning regulations, as well as prevention, adaptation, and mitigation measures.