Abstract:
Urban growth has been on the increase in developing countries and this unplanned growth increases flood risk in cities due to local changes in hydrological and hydro-meteorological conditions that increase flood hazard given that impervious surfaces increase surface water runoffs and reduce the soil infiltration capacity. This study aims to simulate the impact of urbanization on flood risk under historical and future landscape scenarios in Abeokuta Watershed. This was done by assessing the historical Land Use /Land Cover changes (LULC); simulating future urban growth; simulating runoff under different land use scenarios; examining the impact of land cover changes on runoff. Methodologically, the post-classification change detection method was applied to examine the extent and nature of historical LULC changes using remotely sensed data. Future land use scenarios were estimated by employing the use of Dinamica EGO model to project urban growth for the year 2027 and 2036. Hydrologic changes due to urban growth were assessed using HEC-HMS model in which Soil Conservation Service Curve Number (SCS-CN) was used for evaluating run-off characteristics for historical and future land use scenarios (1984, 2000, 2018,2036). Results from this study showed steady urban growth from 1984 to 2018, with Urban and Bare surfaces covering 54% of the Watershed in 2018, compared to 38% in 2000 and 30% in 1984 and this is projected to increase to about 65% in 2027 and 79% in 2036. The most significant land cover change is the conversions of land from Forests and Vegetation to urban land. This study revealed that the impact of this urban growth was historically significant with land cover change between 1984 and 2018 causing a 39.1% peak discharge incremental and 51% increase of runoff volume and the projected land cover change is expected to bring about 60% increase in peak discharge and 70.4% increase in runoff depth. Although the increment in peak flow discharge is mostly largely attributed to storm size as shown in the annual rainfall-runoff simulation with the
year 2000 having a higher annual peak flow (420.9m3/s) than 2018 (287.2 m3/s). However, under uniform storm events for the different land use scenarios; urbanization is revealed to have a considerable impact on peak flow in some sub-basins, which could have severe implications for flash flooding in these sub-basins. The study concludes that urbanization has a considerable impact on rainfall runoff characteristics and could also increase flood risk
