FLOOD HAZARD MAPPING USING ON-SITE SURVEYED FLOOD MAP, HECRAS V.5 AND GIS TOOL: A CASE STUDY OF NAKHON RATCHASIMA MUNICIPALITY, THAILAND

Abstract

For a small flood affected area, satellite data normally provides physical properties of flood event with low accuracy information (location and boundary). Flood depth and flood duration cannot be identified from a snapshot of satellite image. Therefore, on-site surveying of historical flood properties and its impact are still essential, and this observed flood map is realistic and reliable information for future flood management. The objective of this study is to construct a flood hazard map from available observed flood map of the small flood affected area and use HEC-RAS V. 5 and GIS tool to formulate the flood hazard map for future scenarios. This method was applied for the municipality of Nakhon Ratchasima, Thailand. For a simulation, input physical parameters were generated by Hec-GeoRAS in ArcGIS based on DEM (5×5 m2). A range of calibrated Manning’s n in a main channel was obtained from fitting exercise with observed Rating curve. Land-use map was used to estimate the Manning’s n in floodplain depending upon the type of land cover. Simulated results were exported to ArcGIS to delineate water surface on floodplain. Then, the maximum discharge value at the observed station (M. 164) for return periods of 5, 10, 15, 25, 50, and 100 years were used as upstream input flood to simulate the flood map. It is found that, for the 2010 flooding event in the concerning area, the simulated flood hazard map subjected to the discharge of 50 years return period (217 m3/s) which is almost identical with the observed flood map from the surveying.