Correlate Mapping of Impervious Surfaces as Flood Risk Assessment Strategy in Owerri, Southeastern Nigeria

Owerri is rapidly growing in population and built environment, with corresponding, high increase in pavements or paved surfaces. This study explores the integrated approach of Remote Sensing (RS) and Geographic Information systems (GIS) techniques in flood management with the goal of mapping areas vulnerable to flood hazard and, increase in paved surfaces. Digital elevation dataset from Shuttle Radar Topographical Mission (SRTM) were downloaded from USGS explorer. Also, Landsat 5 ETM of 1986, Landsat 7 ETM+ of 2000 and Landsat 8 ETM+ of 2016 imageries were obtained and subjected to supervised classification, using maximum likelihood classifier with ERDAS Imagine 2014. The derived map displayed the spatial and statistical variations of the classified Land Use and Land Cover (LULC) of 1986, 2000 and 2016. The result showed impervious surface rising from 1986 values of 31,625.93 Ha, to 47,979.09 Ha in 2000 and 50,297.33 Ha in 2016, implying approximately 31.2% in 1986, 47.31% in 2000 and 49.61 in 2016. The mean change in impervious surface from 1986 to 2000 was 16.1% compared with 2.3% between the periods. This implies the measure and spate of land conversion in Owerri between the periods showed upward swing. The mean percentage change from 1986 to 2016 revealed increases from 31.2% to 47.31% and to 49.61% for 1986, 2000 and 2016, respectively. Digital Elevation Model was developed with ArcGIS to identify flood prone areas within the study area. A flow accumulation model was created using the DEM before re-classification into high risk, moderate risk and low risk zones using contours and based on elevation. This was overlaid on the impervious layer of the area, to produce a vulnerability map showing locations at a particular level of risk, according to their proximity and extent of paved surface area. This confirms that changes in impervious surfaces, significantly, produce corresponding effect in flood vulnerability. This study recommended that adequate land use planning be enforced.