Inundation Mapping using Hydraulic model and Geographic Information System Case Study: Tar River basin during Hurricane Floyd

Floods are one of the most common natural hazards in United States has and they result in annual average of six billions dollars in losses and 160 deaths. In order to reduce the effects of natural hazards, emergency responders, policy makers and general public need accessible, simple and clear forecast information. Previous studies have shown that hydraulic models and Geographic Information System (GIS) are efficient tools to develop inundation maps. The National Weather Service recently began providing static inundation maps at selected river forecast points on the internet through the Advanced Hydrologic Prediction Service website. In this study we used; a 1-D hydraulic model based on coupling the Hydrologic Engineering Centers River Analysis System (HEC-RAS); GIS through an ArcMAP 9.2 extension, HEC-GeoRAS; digital terrain models developed from LIDAR technology and Aerial images.

The objective of this project is to evaluate a HEC-RAS model from a section of the Tar River basin at Greenville, North Carolina, during the Hurricane Floyd event; comparing computed water surface elevation with observed high water marks (HWM) and evaluating the resultant flood extent maps. After running the HEC-RAS model we exported the computed surface water elevation (SWE) to HEC-GeoRAS. We then produced the correspondent flood extent maps and compared the results with the HWM and found that the SWE has been underestimated by 1-2 feet. This error is due to the uncertainties associated with the digital data and GIS analysis. We conclude that HEC-RAS models are valuable tools for inundation mapping. If the hydraulic model is georeferenced, it covers the entire floodplain and high resolution elevation data is used for the flood extent calculations