Analytical Modelling of Flood Inundation Extent and Depth for the Benue Valley Floodplain, Benue State

Recurrent flooding of the Benue Valley floodplain in Benue State, North Central Nigeria, displaces hundreds of thousands of people and damages substantial agricultural infrastructure annually, yet detailed hydrodynamic flood inundation modelling for comprehensive risk mapping has not been conducted for this floodplain. This study develops and calibrates a two-dimensional hydrodynamic flood inundation model for the Benue Valley floodplain between Makurdi and Idah. HEC-RAS 2D hydrodynamic modelling was employed, with a digital terrain model derived from SRTM 30m data corrected with differential GPS survey data at critical locations. River discharge boundary conditions were derived from Makurdi gauge records (1980 to 2023). Model calibration used the 2012 and 2022 major flood events. Remote sensing inundation mapping from Sentinel-1 SAR imagery during the 2022 flood event was used for calibration validation. Model scenarios were run for 2-year, 10-year, 20-year, and 100-year return period floods. Results indicate that the 10-year flood inundates 1,280 km2 of the floodplain, affecting 312 communities. Maximum inundation depths reach 4.2 m at the confluence zones. Inundation extent shows strong sensitivity to upstream reservoir (Kainji and Jebba) operational release patterns. The study contributes the first calibrated 2D flood inundation model for the Benue Valley and generates flood risk maps for immediate application in emergency planning and land use control.

Keywords: flood inundation modelling, HEC-RAS 2D, Benue Valley floodplain, Benue State, flood risk mapping