Application of geographical information systems for flood risk mapping

Abstract

The objective o f this study was to assess the risk o f different flo o d scenarios fo r Ratnapura town due to flash floods and estimate the flo o d inundation areas according to the ward wise o f the town area. The data and information o f the study gathered from different ways such as digital contour data (1:50,000), ward wise base map o f the municipal area (1:5000), andflood flow data (1977 -2003), at the Ratnapura gauging station. Using the Gumble method, floodfrequency analysis was performed to assess the flo o d return period and the probability o f floods. The GIS data analysis was done using ILWIS 3.2 functions. After geometric corrections o f digital contour data, Digital Elevation Model (DEM) was created. Results o f the flo o d frequency analysis, flo o d scenario maps were produced. Ward wise extents o f flo o d inundation areas were also calculated. Results revealed that the highest floods would occur every 2000 years and the inundation area would be 1115.0 ha with a probability o f 0.5%, followed by in every 100 years the inundation area would be 1086.2 hectares with a probability o f 1%, in every 50 years the inundation area would be 974.2 hectares with a probability o f 2% and so on and so forth. The lowest floods would occur every year and inundation area would be 65.9 hectares with 99% probability