Modelling flood hazards impacted by ungauged river in urbanised area using HEC-RAS and GIS

Abstract

Ilorin City is located downstream of Asa, Oyun and Awun River Basins. The Asa River is a major river that traverses the city and divides the metropolitan area into east and west, almost equally. The river often overflows its banks to inundate adjacent communities, influencing severe economic damage and impact on human lives. However, efforts to mitigate this have majorly been focused on dredging the Asa River channel which has not solved the problem. For an accurate spatial and temporal understanding of the risks of floods and their potential hazards, it is important to estimate floods using river hydrology. The objective of this study is to model steady flow of the rivers using flow data and to map flood-prone areas in Ilorin using HEC-RAS integrated with GIS. Using the HEC-GeoRAS extension in the GIS environment, the geometric data of the rivers were obtained from the 30 m resolution digital elevation model (DEM) and input into HEC-RAS applying Manning Co-efficient values of 0.04, 0.045, and 0.04. For each river, flow data (Q) was given as the upstream boundary condition while a normal depth of 0.001 was assigned for the downstream condition to model a steady flow and inundation extents. The result of the HEC-RAS model has shown the flood-prone areas along the river channels delineated. The floodplain map produced reveals the spatial distribution and extent of the high flood-risk areas in the Ilorin metropolis. The total flooded area covers approximately 60.95 km2 (18%) majorly along the river channels. This study has demonstrated that integration of hydraulic modelling using HEC-RAS and GIS process is capable of producing an inundation flood map with good accuracy that will aid in suggesting effective measures to mitigate the impact of flooding.