Geophysical Contribution to Urban Flood Risk Assessment in Ibadan, South West Nigeria

Urban flooding in Ibadan, Oyo State, is recurrent and destructive, with significant property loss and occasional fatalities documented in recent years, yet the contribution of subsurface hydrogeological conditions to surface flooding patterns has not been systematically assessed through geophysical methods. This professional study integrates electrical resistivity mapping, HVSR ambient noise measurements, and surface drainage analysis to evaluate the subsurface factors contributing to urban flood risk in four flood-prone districts of Ibadan. ERT profiles were acquired at 20 locations in flood-prone districts to characterise the depth to the water table and the capacity of the vadose zone to absorb storm water. HVSR measurements at 35 sites provided sediment thickness estimates for comparison with flood inundation extent during recent high-rainfall events. GIS analysis integrated geophysical results with 30-year rainfall records, DEM-derived drainage catchment boundaries, and flood damage reports. Areas where the water table is within 1 metre of the surface at the onset of the rainy season, as indicated by low near-surface resistivity values, show 3.5 times higher flood inundation frequency than areas with deeper water tables. Sediment amplification factors from HVSR data are not significantly correlated with flooding frequency in this setting, confirming that surface hydrology rather than seismic amplification governs the primary flood risk factor. The geophysical flood risk classification provides a first-of-its-kind subsurface data layer for Ibadan's Metropolitan Master Plan revision. Keywords: urban flooding, electrical resistivity, water table, Ibadan, flood risk