Application of Analytical Hierarchy Process (AHP) and GIS Spatial Modelling for Flood Vulnerability Assessment in the Bonsa Catchment, Southwestern Ghana
Flash floods remain a major environmental hazard in Ghana, yet vulnerability assessments in rapidly developing municipalities receive limited attention compared to major urban centres such as Accra. This study applies an integrated Analytical Hierarchy Process (AHP) and GIS-based spatial modelling to delineate flood vulnerability zones within the Bonsa catchment in southwestern Ghana. Eight thematic layers of elevation, slope, drainage density, distance from drainage, landcover, geology, rainfall and population were standardised and weighted using AHP to reflect their relative influence on flood generation. The model achieved a Consistency Index (CI) of 0.121 and a Consistency Ratio (CR) of 0.086, confirming the reliability of expert judgements. Weighted overlay analysis produced five vulnerability classes: very high (0.9 km2; 0.09%), high (113.2 km2; 10.7%), moderate (587.8 km2; 55.6%), low (352 km2; 33.4%), and very low (1.94 km2; 0.18%). Field-based historical flood points aligned strongly with the very high and high vulnerability zones, validating the model outputs. The most influential factors were rainfall intensity, slope, and elevation, while population distribution enhanced the spatial precision of vulnerability delineation. The drainage network exhibited a dendritic pattern with moderate drainage density, and land cover analysis revealed extensive built-up areas (57.4 km2) and mined-out zones (48.1 km2), both contributing to increased runoff. The study demonstrates that integrating AHP with GIS provides a cost-effective and robust framework for flood vulnerability assessment in developing municipalities. The resulting flood vulnerability map offers a valuable decision-support tool for watershed management, disaster preparedness, and sustainable spatial planning in the Bonsa catchment.
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