Development of an Original Integrated Catchment-Urban Hydrological Model for Flood Management in Nigerian Cities

Urban flooding in Nigerian cities involves complex interactions between rainfall-runoff processes in contributing rural catchments, sewer and drainage system hydraulics, and overland flow in the urban built environment, yet models capable of representing these interactions in the Nigerian context do not exist in validated form. This study develops an original integrated catchment-urban hydrological model framework for application to flood management in Nigerian cities. The model couples a semi-distributed rural catchment rainfall-runoff module (calibrated for Nigerian tropical soils, vegetation, and rainfall patterns) with a 1D/2D urban drainage and overland flow model using a dual drainage approach. Novel model components include a tropical urbanisation parameter that accounts for the rapid impervious surface development characteristic of Nigerian informal urban growth, and a climate-variable potential evapotranspiration module using locally calibrated Hargreaves-Samani coefficients. The integrated model is implemented in a Python-based open-source framework. Full calibration and validation are conducted for the Ogba-Isheri catchment in Lagos State using observed rainfall, flow gauge, and flood inundation records from 2015 to 2022. Calibration achieves Nash-Sutcliffe efficiency of 0.78 for flow and 0.74 for inundation extent. The model is applied to evaluate eight flood management scenarios for the 2030 Lagos urban development horizon. The study provides an original, freely accessible modelling tool for Nigerian urban flood management practice.

Keywords: integrated urban hydrological model, urban flooding, Lagos, catchment-urban coupling, flood management