FLOOD PLAIN ANALYSIS AND RISK ASSESSMENT ON ABELA-ABAYA FLOOD PLAIN AREA IN BILATE RIVER

Abstract

Flood is the most devastating extreme events across the globe. The floods phenomena are common in Abela-Abaya flood plain in Chokare and Guricho Kebele in the course of Bilate River. The main objective of this study is to conduct the flood plain and flood risk assessment in the flood plain Abela-Abaya area. Different input data were gathered and analyzed. Rainfall and air temperature data for a period of 1980 to 2015 were analyzed for characterizing the climate in the area. The streamflow data for a period of 1980 to 2015 was checked by stationarity and independence for input of unsteady process of HEC-RAS. Spatial inputs digital elevation model with a resolution of 12.5*12.5 m and land use map with a resolution of 30m * 30m were used for the HEC-RAS 2D model. Various methodological approaches were employed to achieve the objective. The easy fit software tool was used to identify the parent probability distribution which best fit the stream flow time series. HEC-SSP was used to compute the flood magnitude for various return periods. The HEC-RAS 2D 5.0.7 model was used to model the flood inundation depth, extent and velocity for various return periods in the flood plain area. Flood hazards are estimated based on flood depth, duration and velocity. Vulnerability index analysis was done from Questioners. Flood risks are then computed as product of flood hazard and vulnerability of different location in flood areas. The result of the frequency analysis show that the Extreme-Value Type I (Gumbel) best fitted the time series as evidenced through the goodness of fit statistics. Consequently the estimated quintiles for 2, 5, 10, 25, 50, and 100 years return periods were found to be 104, 156, 195, 252, 292 and 338m3 /s respectively. The HEC RAS 2D model from RAS Mapper showed that the 2, 10, 25, 50 and 100 years return period flood magnitude inundate about 674, 726, 752, 783, 804 and 998 ha of land. The relation between flood velocity and flood depth indicates that 79.2% is in ranges of medium to high hazard level and the other relation between flood duration and flood depth shows that 65% is in long-duration with a higher depth value. The map illustrates that most of the agriculture land and rural settlements area are in moderate risk ranges. The total crop loss for 100years flooding is 233,871.1 quintal and damage cost is 901,584$. To reduce the impact of risk a structural measures such as levee and drop structure are recommended along the river reach to safely pass the flood into the river.