Flood hazard mapping in the whole Awash River Basin by integrating flood generating factors that is drainage density, elevation, land use, soil type, rainfall, and slope using GIS and AHP multi-criteria analysis. Inundated area mapping along the Awash River network with a particular return period from peak discharge of some selected gauging stations using the HEC-GeoRAS/ HEC-RAS model.
Flood Inundation Area and Hazard Assessment Mapping of the Awash River Basin in Ethiopia using GIS and HEC-GeoRAS/ HEC-RAS model
Floods are one of the most common hazards in the world and human activities have been frequently changing floods behavior over time. Some areas in the Awash River Basin have been frequently flooded causing loss of life, property destruction, and crop damage due to short-time intense rainfall in the main rainy season (June-September). Flood generating factors, i.e. slope, elevation, rainfall, drainage density, land use, and soil type were rated and combined to delineate flood hazard zones using a multi-criteria evaluation technique in a GIS environment. The weight of each flood generating factor was computed by pair wise comparison for a final weighted overlay analysis of all factors to generate the flood hazard map. The flood hazard map indicates that 2103.34, 35406.63, 59271.09, 162827.96, and 1491.66 km2 corresponds with very high, high, moderate, low, and very low flood hazard, respectively. The low-lying downstream parts of the Awash River Basin are in the high to very high flood hazard zone. The flooded areas along the Awash River have been mapped based on the 5% exccedance highest flows for different return periods using the HEC-RAS model, GIS for spatial data processing and HEC-GeoRAS for interfacing between HEC-RAS and GIS. The areas along the Awash River that area simulated to be inundated for 10, 25, 50 and 100 year return periods are areas near the Lake Yardi, the lower plains around Dubti town down to Lake Abe and north of the Awash town in the vicinity of Melka Worer. The inundated areas for a 100 year return period was rather high relative to other return periods. The flooded areas were high particularly from Dubti down to Lake Abe for all return periods. The flooded areas along the Awash River are 117, 107, 84, 68, and 38 km2 for 100, 50, 25, 10, and 5 year return periods, respectively when using 5% highest data from the Adaitu gauging station. Inundated areas in the upper and middle part of Awash River Basin are low as compared to the downstream part. Proper land use management, afforestation along the Awash River Basin, conservation of the existing forest by involving the local community is significant to reduce the adverse effects of flooding particularly in the low-lying flood prone areas.