Hydrological and hydraulic modelling for flood control planning in the Mokwa watershed, Nigeria

Flooding is a recurrent hazard in many data-scarce African watersheds, where limited stream-gauge information constrains event reconstruction and flood-control planning. This study developed an integrated GIS-HEC-HMS-HEC-RAS workflow to characterize the May 2025 flood response of the Mokwa watershed, Niger State, Nigeria. A 30 m SRTM digital elevation model, Esri Sentinel-2 10 m land-cover data, FAO soil information and hourly ERA5 precipitation were used to delineate a 206.6 km² watershed and parameterize three hydrologically connected sub-basins. ERA5 precipitation was extracted from the grid cell containing the watershed centroid and applied uniformly to all sub-basins. HEC-HMS 4.13 employed the SCS Curve Number loss method, SCS Unit Hydrograph transform and lag routing. The baseline simulation produced an outlet peak discharge of 21.8 m³/s on 29 May 2025 at 17:10 and an event runoff depth of 5.36 mm. Sensitivity tests showed that + 10% and + 20% rainfall increased outlet peak discharge to 29.8 and 38.6 m³/s, respectively; changing sub-basin Curve Numbers by -5 and + 5 produced outlet peaks of 11.3 and 34.8 m³/s, while ± 20% changes in basin lag times shifted the peak magnitude to 26.0 and 18.8 m³/s. The routed ReachMO1 hydrograph, with a peak of 20.5 m³/s, was used as the upstream inflow to the HEC-RAS 2D model. The hydraulic simulation produced a maximum water depth of 3.68 m, maximum velocity of 1.95 m/s, and water-surface elevations of approximately 42.08–58.06 m. The maximum inundation envelope covered 11.77 km²; within the 9.85 km² classified depth area, 37.42% was low hazard, 55.03% moderate, 7.35% high and 0.20% very high. Because no observed discharge or flood-stage series were available, the model was not formally calibrated or validated and the results are interpreted as an event-based, sensitivity-tested assessment for an ungauged watershed. The workflow nevertheless provides a reproducible basis for preliminary flood assessment and flood-control planning in data-scarce settings.