Flood Hazard Modeling of Omo River: Case Study of Dasenechand Gnyangatom Woredas

Abstract

Flood occurs frequently in Ethiopia and causes tremendous losses in terms of property and life. The majority of flood disasters‟ victims are people living in floodplain. Therefore, this study was carried out to perform floodplain analysis of the un-gauged part of lower Omo-Gibe basin of Dasenech and Gnyangatom woredas of SNNP using CFSR data. The data used in this study includes; measured daily flow at Abelti sub basin, Gibe I II maximum outflow, soil data, and daily climate data. The time frame covered by this study is 30 years from 1980 to 2009. The study describes the flood inundation map of the flood prone area using probability occurrence of different return period. The study area lies along sides of Omo River with annual average rainfall of 657mm and average discharge of 1245m 3 /s. Flood frequency analysis for 5, 10, 50, 100 and 200-years return period have been done by Gumbel, Log Pearson Type III, and Log Normal method based on Maximum flow recorded at the flood prone area. One dimensional hydraulic model HEC-RAS with HEC-GeoRAS interface in coordination with ArcView was applied for the analysis. Triangulated Irregular Network (TIN) was prepared from 5m interval contour in ArcView GIS for the study area including Dacenech and Gnyangatom woredas. Required data sets as stream centerline, banks, flow paths and cross-sections were prepared in HEC-GeoRAS. In HEC-RAS, boundary conditions for upstream and downstream were defined. Flood discharges for different return periods were calculated using Hyfran plus software. Steady flow analysis was done for the results. The boundary condition set to critical depth. The maximum floods for 5, 10, 50, 100, and 200 years return period of different flood frequency analysis methods found to be 2100, 2340, 2940, 3220 and 3530 m 3 /s respectively at the upstream. Areas inundated by 5, 10, 50, 100 and 200-years return period floods were 1642.1, 1737.4, 1938.3, 2027.7 and 2124.4 ha respectively across the flood prone area. The classification of flood depth area showed that 41% of the flooding area had water depth greater than 2.5m. Flood area increased with flood intensity. Higher flood depth increased and lower flood depth decreased with increase in flood intensity. Thus, finding of the study may help to establish pre warning system to the flood prone area of Dasenech and Gnyangatom woredas