Mapping flood inundation areas using GIS and HEC-RAS model at Fetam River, Upper Abbay Basin, Ethiopia

Flood occurs due to rapid population growth, land degradation, and climate change, and causes harmful damages to human beings and properties. This can be minimized by giving attention to flood risk measures. One of these measures is inundating flood hazard areas. Therefore, this study was conducted to map flood inundation areas along the Fetam River using GIS and HEC_RAS. Flood inundation mapping is used to define the zones which are more susceptible to flood along the river when the release of a stream surpasses the bank-full stage along the river. Using the past data on river banks and the release of earlier floods besides topographic data, maps were made to illustrate areas predictable to be covered with the flood for different releases. The flooded areas on Fetam River have been represented depending on 5% incidence peak flows for different reoccurrence eras using the HEC-RAS model, GIS for spatial data handling, and HEC-GeoRAS for interfacing among HEC-RAS and GIS. These critical floods were damaging the areas around Fetam River, which is hazardous to social and economic growth due to loss of lives and destruction of properties. Built-up areas and agricultural fields are located along the river banks and are highly susceptible to flooding. The inundated areas were particularly high from Abaya Kalo, Achigi, and Denbun kebeles for all return periods. These flooded areas along the side of the Fetam River are 27.31, 24.85, 20.47, 17.34, and 13.78 km2 for 100, 50, 25, 10, and 5 years return periods, respectively. Generally, this study discovered that flooded areas in the upstream and middle parts of Fetam River are high as related to the downstream parts.