Evaluation of Drainage Network Efficiency in Onitsha Urban Area Using GIS and Hydrological Modelling

Urban flooding in Onitsha, Anambra State, is a recurrent problem associated with the city's dense commercial activity, inadequate drainage infrastructure, and susceptibility to Niger River backwater flooding. This study evaluates drainage network efficiency in Onitsha Urban Area using GIS and terrain-based hydrological modelling. High-resolution topographic data from differential GPS surveys are used to generate a detailed Digital Elevation Model of the study area, supplemented by SRTM data for the broader catchment. HEC-HMS hydrological modelling is applied to simulate surface runoff responses to design storm events with return periods of two, five, and ten years, using rainfall data from the Anambra State Meteorological Service. GIS network analysis is applied to assess the hydraulic capacity of mapped drains against computed peak runoff volumes. Results indicate that over seventy percent of mapped drain segments are hydraulically inadequate for the five-year design storm, with identified surcharge locations coinciding with observed flood hotspots in the Fegge and Inland Town districts. The study recommends specific drain upgrading works and provides spatial prioritisation for investment based on both hydraulic deficiency severity and population exposure in flood-prone areas. Outputs are submitted to the Onitsha North Local Government Area and the Anambra State Ministry of Works. Keywords: urban drainage, hydrological modelling, GIS, Onitsha, flood management.