An Evaluation of the Impact of Urban Growth on Runoff in Abeokuta, Southwestern Nigeria

Abstract

Abeokuta is one of the urban areas in Nigeria with high cases of runoff fatalitiesin recent times, indicating the need for a proper understanding of prominentrunoff-generating mechanisms, as well as the causative factors. Consequently,this paper, which is focused on flood-prone settlements, is aimed at providinginformation on the impact of change in urban land use on runoff in the studyarea. The specific objectives were to determine the change in average runoff interms of the Soil Conservation Service Curve Number (SCN-CN), and to assessthe impact of urban growth on runoff in the area. The SCN-CN was derivedfrom the 30m spatial Shuttle Radar Topography Mission Digital ElevationModel (SRTM DEM). At the same time, land cover change was estimated usingLandsat TM+ from 2000 and Landsat 8 OLI from 2018. Data were analysedusing the Hydrologic Engineering Centre’s Hydrologic Modelling System(HEC-HMS) and ArcGIS (version 10.1). The results showed a 14% increase(from 39% in 2000 to 53% in 2018) in urban areas of selected catchments; anda relative increase in average CN from 76.9 units in 2000 to 79.9 units in 2018,suggesting an increase in runoff potential relative to the increase inurban/impermeable space in the catchment. Annual discharge depth increasedfrom 891.84mm to 956.9mm, while peak discharge increased from 161.9m3/s to196.2m3/s. Runoff in the study area tends to exhibit spatial variability that issimilar to the pattern exhibited by built-up areas across the study area,suggesting that the development of built-up areas can explain runoffexacerbation in part of the area. The use of SCN-CN and satellite images makesthe approach reproducible, and the mixed methods of geographic informationsystem and hydrological model revealed the spatial variability typically hiddenin stand-alone hydrological models. The paper recommends further studieswith the use of less coarse datasets, as well as the implementation of policiesthat focus on sustainable urban growth in the region, and other cities.