Analytical Study of the Application of GIS for Flood Risk Mapping in Anambra State

Flood risk mapping using geographic information systems provides spatial intelligence for land use planning, emergency response, and infrastructure investment prioritisation, but comprehensive GIS-based flood risk maps have not been systematically developed for most local government areas in Anambra State, South East Nigeria. This study develops and validates a GIS-based flood risk mapping methodology for Anambra State and applies it to high-risk areas. A multi-criteria GIS analysis methodology was adopted, integrating topographic data (SRTM DEM), rainfall intensity data, land use and land cover classification (Sentinel-2 imagery), soil permeability, drainage density, and proximity to water bodies as risk indicators. The Analytical Hierarchy Process was used to weight risk factor layers. The composite risk map was validated against documented flood event records from Anambra State Emergency Management Agency (2015 to 2023). Accuracy assessment used the Kappa coefficient and producer-user accuracy metrics. Results indicate that the flood risk model achieves 81 percent overall accuracy in classifying historical flood-affected areas. High and very high risk zones cover 22 percent of Anambra State territory, predominantly in the floodplain areas of Onitsha, Ogbaru, and Anambra East LGAs. Land use change (conversion of wetlands to residential) since 2010 has increased the medium-risk zone area by 34 percent. The validated risk maps are proposed for adoption by the Anambra State Ministry of Environment and Physical Planning for development control.

Keywords: GIS flood risk mapping, Anambra State, multi-criteria analysis, flood hazard, South East Nigeria