FLOOD INUNDATION MAPPING AND HAZARD ASSESSMENT USING GIS AND HEC-RAS MODEL FOR BILATE RIVER FLOOD PLAIN, RIFT VALLEY, LAKE BASIN ETHIOPIA

Abstract

Flood is a natural disaster that occurs suddenly which runs from hilly land to low land area of river banks when runoff exceeds the capacity of the natural or artificial water conveyance structures. Currently, the hazard of flood is common in some parts of Ethiopia including Bilate watershed and cause heart breaking losses in terms of property and even human life in flat river flood plains due to elevation difference and climatic factors. The main cause of flood disaster vulnerability is due to unavailability of proper River and watershed management. Hence, the intended aim of this research is to carryout inundation mapping and hazard assessment of Bilate River flood plain. The required data for this analysis were, Digital Elevation Model, stream flow data, soil type and land use land cover map. These data were collected from Ministry of Water, Irrigation and Electricity in legal request. Hydraulic Engineering Center- Geographic River Analysis System was used to prepare RAS import file for steady flow analysis on Hydraulic Engineering Center-River Analysis System. Stream flow data was checked for outlier and consistency at different significance level using Grubbs and Beck (G-B) test and double mass curve method respectively. For flood frequency analysis parameter estimation of worldwide used distribution functions was done by three important parameter estimation methods such as: method of moment, probability weight method and maximum likelihood method. Best fitted Pearson type distribution function was selected by linear moment (L-moment), D-index and correlation coefficient methods. The estimated peak floods in Bilate at Tena Bilate gauging station for 5, 10, 25,50, 100 and 500 years return periods were 153.2, 193.2, 273.6, 395.3 and 335.9 m3 /s respectively, at Guder station 140.89, 195.79, 217.64, 244.98, 270.33, and 323.86 m 3 /s respectively and at Wera were also 70.31,88.46,108.54, 121.9,134.17 and 159.78 m 3 /s respectively . Mapping of flood inundation was processed with inter connection of GIS extension tool HEC-GeoRAS and HEC-RAS software. The flood inundations for 5, 10, 25, 50,100 and 500 years return period were 16658, 16930,17207,17381,17531 and 17828ha respectively. Flood hazard map were prepared by reclassifying depth grid and its area bounding polygon. Also an assessment of hazard level was done with integrating flood causative agent land use land cover map with hazard map prepared after post-RAS processing. Area inundated at very sever to moderate hazard was ranges between 4% to 10.3% and around more than 72% was low hazard area. Hence, the result of the level of hazard indicates that there is significant potential of flood and the consequence risk on livelihood in study area in significant to sever level. It is recommended that, the watershed management party or any stakeholder, should use this research finding to take appropriate measure and reduce flood disaster in the future