Abstract:
When dam breach occurs, the resulting flood can cause enormous damages to infrastructures and
loss of life. The study was conducted on the Dire dam located at 30 km to the east of Addis
Ababa city within the boundary of the Oromia region. The failure modes assumed for dam
breach analysis in this study were overtopping and piping through the dam body. The dam
breach analysis was carried out through HEC-RAS model incorporates with GIS and HEC GeoRAS. The HEC-RAS model performs unsteady flow simulation using Dire dam breach
parameters, Dire dam geometric data, Dire reservoir elevation-volume data, unsteady flow data,
and geometric data of Legadadi River as an input data. The GIS develops flood inundation map
after water surface elevation, maximum flood depth, and velocity were exported from HEC-RAS
to GIS. Dire dam breach parameters of input data for HEC-RAS model were computed through
the empirical method of Froehlich 2008. The computation was done utilizing the Dire dam and
reservoir geometrical data. The breach parameters value of breach bottom width (B), breach
formation time (tf), and breach side slopes (bss) for overtopping failure mode were 51.3m,
0.65hr, and 1. The breach parameters value of breach bottom width (B), breach formation time
(tf), and breach side slopes (bss) for piping failure mode were 45.98m, 0.75hr, and 0.7. The peak
breach outflow discharges at the dam location due to overtopping and piping failure simulated in
HEC-RAS model were 9285.30m3
/s and 7712.10m3
/s respectively. The maximum flood depth
and maximum flood velocity occurred due to dam overtopping failure were 16.41m and
22.74m/s respectively. The maximum flood depth and maximum flood velocity occurred due to
dam piping failure were 14.43m and 19.56m/s respectively. The simulation outcomes presented
that in the event of Dire dam breach due to the assumed dam break modes, some Dire dam
downstream areas were observed as having high flood hazard due to the significant flood water
depth and velocity values
