APPLICATION OF REMOTE SENSING AND GEOGRAPHIC INFORMATION SYSTEM FOR FLOOD RISK ASSESSMENT IN ONDO STATE, NIGERIA.

Abstract

The aim of this project was to perform flood risk assessment based on hazard and vulnerability analysis using remote sensing and Geographic Information System techniques. The objectives were to produce land cover Maps, analyse rainfall intensity, produce flood risk assessment maps and to suggest proactive measures for flood disaster reduction in five (5) local government areas of Ondo State, namely Akure North, Akure South, Ondo West, Eseodo, and Ilaje. In achieving the stated objectives, a field survey was conducted in the study areas during which measurements were taken using Global Positioning System (GPS). Dataset used for the study include GPS measurements, Landsat Enhanced Thematic Mapper (Landsat 7 ETM+ of 2002 and 2007), Digital Elevation model, rainfall intensity from Tropical Rainfall Micrometer Measurement (1998 to 2007) and soil samples. ILWIS 3.3 and ArcGIS software were used for the analysis of the data. Method of analysis includes: Criteria maps; landcover, Normalized Difference Vegetation Index (NDVI), soil analysis map, rainfall intensity, terrain model (slope angle, flow accumulation map) were used to generate the flood risk assessment map using spatial multi-criteria evaluation (SMCE) operation in ILWIS 3.3. Land cover map were derived based on supervised classification and maximum likelihood algorithm. The risk levels were categorized into very high risk, high risk, moderate risk, low risk and very low risk levels. The classification (6 classes) for the different study areas showed significant changes in the land cover over the period under study .This indicates various degrees of land degradation, deforestation that have occurred in the study areas making it susceptible to flooding. Results showed that the risk level varies within the various towns in the local government areas. Majorly built-up areas were at high and moderate risks while vegetated areas were observed to be significantly at low risk. For instance in Ondo Town and its environs,very high risk and high risk areas increased from 2% to 2.3% and 11.5% to 13% respective from 2002 to 2007. In Igbekebo very high risk areas reduced from 22.3% in 2002 to 16.5% in 2007 while high risk areas increased from 43.1% to 45.1% during the same period. In Araromi the Ilaje Local Govt. Area it was observed that, the area is at very risk level, the very high risk was 10% in 2002 and 14.4% in 2007. High risk area reduced from 35.5% in 2002 to 33.5% in 2007. Moderate risk area also reduced from 31.6% to 29.8% between 2002 and 2007 while very low risk area increased from 4.9% in 2002 to 5.5% in 2007. In Akure north and south Local Govt. Areas very high risk areas increased from 0.9% to 9.3% between 2002 and 2007, high risk area also increased from 13.9% to 57.0% between 2002 and 2007. Moderate risk areas reduced from 61.6% in 2002 to 33.1% in 2007. Low risk areas reduced from 22.5% to 0.4% in 2002 and 2007 while very low risk areas also reduced from 1.1% to 0.1% between 2002 and 2007 respectively. For rainfall intensity, it was observed that Akure North ,Akure South and Ondo West Local government areas had the same pattern of rainfall intensity, they had their highest rainfall intensity during the second peak i.e month of September while the coaster Local governments i.e Araromi,Igbokoda and Igbekebo in Ilaje and Ese Odo Local governments had their highest peak of rainfall intensity during the first peak of rainfall i.e during the months of June and July respectively. Therefore, Government is encouraged to produce flood risk maps of each local government in the State and make it available for the Town Planners, construction Engineers, policy makers, land surveyors and to the general public.