FUOYE Journal of Engineering and Technology, Volume 3, Issue 2, September 2018 ISSN: 2579-0625 (Online), 2579-0617 (Paper) FUOYEJET © 2018 12 engineering.fuoye.edu.ng/journal Mapping and Evaluation of Flood Risk Areas along Asa River using Remote Sensing and GIS Techniques *Kamoru A. Adeniran, Yinka A. Ottawale and Matthew S. Ogunshina Department of Agricultural and Biosystems Engineering, University of Ilorin, Ilorin Nigeria kadeniran_2003@yahoo.com|otayinkus@yahoo.com|segunogunshina@gmail.com Abstract-Flooding in Ilorin city has become a yearly occurrence. Mapping and evaluation of flood risk areas along Asa River in Ilorin metropolis was carried out using the Geographic Information System (GIS) and remote sensing. The technique used includes conversion of Digital Terrain Model (DTM) to Triangulated Irregular Network (TIN) format. The geometric data was obtained from TIN through the use of United States Army Corps of Engineers, Hydrologic Engineering Centre, Geo River Analysis System (USACE HEC-geoRAS) in GIS. The obtained geometric data, Manning’s roughness coefficient (n), Expansion and contraction coefficient values and steady flow data of the River were used in HEC-RAS. The n values of 0.035, 0.016 and 0.02 were used for the channel, 0.045, 0.016 and 0.03 were used for the overbank and 0.2 was used for the bridges. Contraction and expansion co efficient value of 0.1 and 0.3 were used for channel and 0.3 and 0.5 were used for the bridges. Gumbel equation was used to estimate the flow for return period of 10, 50 and 100 years and the values of 155.13, 213.44 and 221.43 m 3 /s were obtained respectively. Delineated map was then compared with TIN terrain model to generate inundation map. The map revealed that some areas in Ilorin such as Coca-Cola Road, Baba Ode, Unity road, Obo Road, Taiwo-Isale, Amilengbe, Isale Koko, Mubo Phase 1, Mubo Phase 11, Royal Valley and Akerebiata prone to flood disasters. Estimated maximum top width for inundated area along the river ranges from 900.74 to 2375.11m. Keywords-GIS, River Asa, DEM, Flood risk, HEC-RAS, Ground slope ——————————  —————————— . 1 INTRODUCTION lood is defined as a state of high water level along a river channel or on coast that leads to inundation of land which is not normally submerged (Panda, 2014). The devastating impacts of flood on human’s lives and properties is becoming more worrisome in the world especially in developing countries where little effort can be made to control it. The occurrences of these phenomenon is often and could be due to heavy rainfall (from climate change), dam failure, river overtopping their defenses, blockage of river channels, improper planning of the settlement and lack of proper control measure to curb the occurrences (Eguaroje et al., 2012). In a large area of land, the financial implications, lesser labour requirements couples with less time of operation were among other factors that gave Geographical Information System (GIS) the edge over conventional hydrological monitoring systems in mapping and assessments of flood risk. For this reason, Klemas (2015) opined that conventional hydrologic monitoring systems have limited use in flood forecasting and mapping flood area. Causes of flood has been reported by several researchers among which is Olabode et.al, (2014) who traced the causes to heavy rainfall, overflow of drainage channels due to its blockages and emergency release of water from dams. While Ogunlela & Adelodun (2014) noted the occurrence of heavy rain in the upstream usually leads to the unsteadiness in a river as well as breaches in the embankment system, Macchi & Tiepolo (2014) observed that sea level rise, rapid population growth and urbanization are key factor responsible for flood in all part of the world. Shiru et al. (2015) reported that flooding in Ilorin city has become a yearly occurrence. * Corresponding Author Increasing population which results in competition for space and more generation of wastes, improper drainage design and insufficient drainage systems, paving of surfaces, refuse dumping in drainages and water ways are amongst the causes of flooding in the city. In 1997, Kwara recorded flood in Lafiagi, Patigi, Kpada and Gbogbondogi and this was traced back to heavy rainfall which resulted in serious flooding and the displacement of over 4,000 people in the Patigi, Edu and Moro Districts of Kwara State (Ogunbodede & Sunmola, 2014). During rainy seasons in Ilorin, precipitation often falls as rain leading to changes in Asa river flows. The river often overruns its bank and resulted into recurrent flooding of the roads in Wahab Folawiyo Road popularly known as Unity and its environs, movements of vehicles and humans becomes impeded, offices and business activities and residential houses are often affected by the flood (Kolawole et al., 2011). Advent of computer technology in hydraulic modeling provide greater flexibility for engineers to execute engineering tasks such as floodplain mapping through the use of Digital Elevation Models (DEM) in the form of Triangular Irregular Networks (TIN) which has flexibility of affording engineers the ability to create geometric representations in a cost effective manner (Solaimani, 2009). There are several methods for hydraulic modeling process for floodplain mapping. One of them is the Hydrologic Engineering Centre River Analysis System (HEC-RAS), (USACE, 2006).HEC RAS support sediment transport computation, water transport analysis, steady and unsteady flow, water surface profile calculation in natural and constructed channels (USACE, 2002). The objective of the study was to analyze water elevation profile of Asa River in Ilorin metropolis and to identify the areas prone to flood disasters. F