Abstract:
Dam breach analysis and flood inundation mapping is generally used to forecast flood on
the downstream of the dam. Dam breach analysis is essential to predict dam breach
parameters, outflow hydrograph and its downstream nature of propaga tion. Results from
dam breach analysis can be used to protect downstream population and it can also be used
while designing and implementing future infrastructure. Koga Dam found in the Tana sub
basin of Abay Basin in Ethiopia has been selected as a case study in this paper. Within this
study the dam has been checked for both overtopping and piping using one dimensional
river analysis model HEC -RAS and empirical equations are used to predict dam breach
parameters for the use in the model. The flow data is t ested by calibration and validation
using HBV hydrological model. PMF inflow is calculated using HEC -HMS model with a
peak 337.9 m3/s is used for overtopping failure. The spill way has adequate capacity for
the flood due to the PMF and breaching of the emb ankment was not possible in HEC-RAS
but Piping failure was simulated in HEC -RAS using breach parameters obtained from the
empirical equations. The modeling process was towards performing unsteady flow
calculations in the intent of routing the breach outflow downstream of Koga Dam from the
dam up to the downstream boundary which is 12 km from the dam. The models HEC -RAS
and HEC-GeoRAS are used alternatively. HEC-GeoRAS extracts topographic data from
counture of collected and surveying data and prepares geome tric file in ArcGIS. The
geometric file is imported into HEC-RAS. Using the geometric file and unsteady flow data
(PMF inflow, initial flow and normal depth) unsteady flow calculation is performed in
HEC-RAS and the breach peak flow estimated is 3071.65 m
3
/se . Geometric file with water
surface elevation attached to it is exported to ArcGIS where HEC -GeoRAS uses it to
prepare a flood map. The flood map cover 218.3 hactar with a flood depth of 0-22.5 m
dawnstream of the embankment dam. The produced map is overlaid on an aerial map of
google to see towns and infrastructures that are affected by the flood.
