Flood Frequency Analysis of Big Akaki River, Awash Basin, Ethiopia

Abstract

Flood is one of the most dangerous naturally occurring catastrophic hazards now days displacing and killing people and destroying properties. During high flow, the Big Akaki River flooded the vicinity of the area and causes loss of cultivated land and life. Flood water inundates the floodplain areas and cause vast damages to life and property. The main objective of this paper is to analyze the inundation area along the Big Akaki River basin and to assess the flood hazards of basin. The annual maximum series of discharge or flow data for 15 years (1990 to 2004) at one flow gauging station, located at Akaki Bridge, were each fitted with four probability distribution models i.e.; Log-normal, normal, Gumbel and log Pearson type III and subjected to four specific measures of errors in prediction i.e., RMSE, RRMSE, CC and MAE in order to select the best probability distribution model that fits the observed flow data at the stations. The best fit distribution model at each station was then utilized to predict return period floods for each station for return periods of 2, 5, 10, 25, 50, & 100 years. The best fit probability distribution models obtained for the different return period is lognormal distribution for this paper. Results of HEC-RAS model were exported in ArcGIS to perform inundation mapping. Flood inundation maps of the whole study area were prepared for 2, 5, 10, 25, 50, and 100 year of return period flood. Floodplain maps show flooded depth from 5.717 m, 5.8977 m, 6.2034 m, 6.4397 m, 6.6108 m, 6.7813 m for (2, 5, 10, 25, 50, and 100) year return period floods, respectively and flooded area from 286.92 ha to 418.08 ha for the 2-year to 100-year of return period, respectively and flooded velocity from 4.795 m/s to 6.959 m/s for the 2-year to 100-year of return period, respectively. Flood inundation maps will be an important tool for engineers, planners for emergency actions plans and for flood management.