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
