Abstract:
Flood is a natural hazard that causes damage of property and life. Flood hazard assessment is
particularly important for policy makers, in order to design mitigation strategies and implement
flood risk management planning. During high rainfall depth at summer season of the year the
overflow of Holeta River course is occurred and cause flood risk consequence. This study aims to
map flood inundation area and hazard assessment, for upstream-Downstream Holeta River by
using Geographic Information System, Analytical Hierarch Process, Hydraulic Engineering
Center- River Analysis System and Hydraulic Engineering Center – Geometry River Analysis
System software. The importance’s of this study is to have detail information about future floods
and to serve for flood preparedness. For this study, Log-Pearson Type-III was used to compute
peak discharge. However, there was limited recorded data at the downstream interested point, so
transposing peak discharge from upstream catchment was employed by using Drainage Area
Weighting (DAW) methods. The transposing Coefficient was estimated about 2.1506. Thus, the
final peak Discharges for 10, 25, 50 and 100 years was estimated 350.75, 417.445, 465.551 and
511.68 m3
/s respectively. HEC-Geo RAS Software was used to develop river geometry such as: the
river centerline, river bank, flow path and cut cross-section for upstream-Downstream Holeta
River. HEC-RAS, hydraulic analysis includes the computation of the water surface profiles. The
flood extent area of inundation mapping for 10, 25, 50 and 100 years was estimated about 22, 24,
26, 26.7 km2
and 49, 51, 53 and 53.5 km2
for steady and unsteady flood flow simulation
respectively. Thus, the inundation mapping was developed for different return period with their
extent areas from river course. Based on degree to flooding, the importance of selected
parameters were ranked to four flood hazard category, namely very low flooding, low flooding,
moderate flooding and high flooding. The weight coefficients were determined for each parameter
by Analytical Hierarch Process (AHP) and overlay of ranked spatial information result the final
flood hazard map of study areas. The flood hazard maps indicate that 77.9, 627.01, 567.05 and
47.07km2
corresponds with high, moderate, low and very low flood hazard areas respectively for
Holeta River. Also, the flood hazard map gives the flood prone areas for upstream-Downstream
Holeta River catchment. Hence, most of the catchment areas are under low flooding and moderate
flooding. Some parts are very low flooding and high flooding is less in area
