Assessing the Role of Geology in Flood Amplification in Ibadan Metropolis, Oyo State

Recurrent and increasingly severe flood events in Ibadan Metropolis, Oyo State, South West Nigeria, have caused loss of life and extensive property damage, with research debate centred on the relative contributions of urbanisation, climate variability, and geological substrate to flood amplification. This study assesses the geological factors contributing to flood hazard in Ibadan Metropolis, specifically evaluating the influence of lithology, weathering depth, soil permeability, and drainage basin morphometry on peak discharge and inundation extent. Geological mapping of the study area distinguished areas of shallow basement with low infiltration capacity from thick lateritic profiles with higher water retention potential. Hydrogeological testing at selected sites provided saturated hydraulic conductivity values for dominant soil units. Digital elevation model-derived basin morphometric indices were computed for twelve sub-catchments, including bifurcation ratio, drainage density, form factor, and circularity ratio, which were statistically correlated with observed inundation frequency from satellite flood extent mapping. Hydrological modelling using the HEC-HMS rainfall-runoff model was parameterised using geological and land cover data, with simulations run for rainfall events of varying return periods. The study demonstrates that sub-catchments underlain by shallow weathered basement generate peak flows approximately forty percent higher than comparable catchments on deep lateritic terrain for the same rainfall input. These findings are forwarded to the Oyo State Ministry of Environment and Natural Resources. Keywords: flood hazard, geology, Ibadan Metropolis, basin morphometry, runoff.