Journal of Water Resource and Hydraulic Engineering Sept. 2014, Vol. 3 Iss. 3, PP. 48-59 - 48 - Flood Frequency Analysis and Inundation Mapping of Lower Ogun River Basin Shakirudeen Odunuga 1 and Saheed A. Raji 2 1 Department of Geography, University of Lagos, Akoka-Yaba, Lagos. Nigeria. 2 Laboratory for Remote Sensing and Geographic Information Systems (GIS), Department of Geography, University of Lagos, Akoka-Yaba, Lagos. Nigeria Email: 1 sodunuga@unilag.edu.ng, 1 odushak@gmail.com, 2 rsa_seun@yahoo.co.uk. Abstract- Flood frequency analysis and inundation mapping of lower Ogun River basin was implemented using the Gumbel probability distribution method. This method was however tested with Log Pearson Type III to ascertain the best fitting statistical measure for hydrological fluxes using Chi Square. Basin delineation data was extracted from the topographic maps of Ilaro SE 1, 2, 3 & 4 which covers the study area while gauge height data at Adiyan intake was used as the principal data for flood frequency assessment and inundation modelling based on (Tr) return periods 2, 5, 10, 25, 50 and 100 years. The results showed modelled water level values of 2.22 metres, 2.24 metres, 2.28 metres, 2.38 metres, 2.55 metres, and 2.90 metres for each of the return periods. Consequent upon this, the inundated area is projected to increase to almost 30% of the area from an initial 23% for most of the scenarios. These findings provide clearer picture for the pattern of hydrological fluxes of the lower Ogun River Basin. Sustainable planning and developmental measures that consider the modelled pattern of hydrological fluxes of the study area were recommended for decision making on urban areas. Keywords- Flood estimation; Flood modelling, Floodplain; Gumbel distribution; Log Pearson Type III and Return period. I. INTRODUCTION Globally, flooding is regarded as the most ubiquitous natural disaster that affects people in different dimensions [1]. The 20 th century witnessed an unprecedented rate of flood occurrence within most drainage basins in the world. Specifically, the yearly flood evaluation showed that in the past fifty years, flood incidences have increased by almost 10-fold from about 20 in 1945 to almost 190 in 2005 [2]. It has also been estimated that about a third of the total landscape of the earth is flood-prone, consequently affecting close to 82% of the world’s population [3]. Similarly, the United Nations [4] has stated that about 170,000 flood-related deaths were recorded worldwide within a period of two decades, further between 1980 and 2000. More specifically, the World Meteorological Organisation (WMO) claimed that between 1970 and 2012, about 89% of the reported global disasters were due to flooding and storms, resulting in deaths of millions and huge economic losses [5]. These statistics portray the fact that flooding is a global environmental issue that requires consistent and continuous assessment through a scientifically suitable methodology. In Nigeria, flood occurrence is connected to different anthropogenic activities and occurs within its spatial dimensionalities. This dimensionality is connected to a location where river flooding, urban flooding and coastal flooding with concomitant losses in lives and properties are the common types. River flooding, coupled with urban development, however, intensifies the rate of flooding in the lower section of the Ogun River basin in southwest Nigeria. A key issue considered in the choice of this study area is the paucity of information concerning the probability of flood occurrence and the flood extent in the area. This necessitates the estimation of flood frequency assessment and inundation mapping. Flood frequency analysis is the universal method used for the estimation of recurrent interval of any hydrological event [6, 22]. Flood frequency analysis is pivotally essential for the management of flood with respect to design, planning and operations through the use of fundamental knowledge of flood characteristics in terms of peak flow, volume and duration. [7] state that flood estimation via flood frequency analysis provides a basic standard for flood risk evaluation especially in areas that are close to floodplains [8]. More precisely, [6] affirms that return periods associated with annual maximum flood peaks are modelled using flood frequency analysis. This gives room for prediction of such extreme events (especially river runoff) since they are unpredictable and difficult to estimate spatiotemporally. Flood frequency analysis has been used extensively by hydrological scientists and hydrological engineers [9]. Scientists and researchers find flood frequency analysis a useful tool for the estimation of the contribution of precipitation to stream flow and river runoff to flood occurrence. As a hydrological tool, flood frequency analysis is useful for the estimation of design flood that forms a pivot for designing hydrological structures. Such structures are commonly designed based on the statistical flood history of the entire drainage area [10]. [11] affirm that the choice of a method of statistically estimating flood scenarios using any of the available methods should be based on the following; 1. Variability of the documented records concerning water level and stream flow; and 2. Suitability of the method to the geographical area in view.