Dam Breach Analysis and Flood Inundation Map the Case of Koga Dam, Abay Basin, Ethiopia

Unsteady-flow simulation of the dam break is performed using HEC-RAS and results are
mapped using the RAS mapping. Inundation mapping of water surface profile, Velocity and
depth results from dam failure models and provides a preliminary assessment of the flood
hazard and provides insight for emergency preparedness. The Unsteady process is for
gathering and preparing data, creating an unsteady-flow model in HEC-RAS, entry of dam
breach parameters, performing a dam failure analysis, and mapping of the flood progression.
Break parameters prediction, the understanding of dam break mechanics, peak outflow
prediction were shown as the essential for the dam break analysis and flood mapping. The
results show that dam break is a complicated and comprehensive process involving lots of
principles. Koga Dam break due to overtopping elongates the time period of high water
surface level, which increases the duration of risk. The design Probable Maximum Flood
(PMF) is 570 m3/sec statically estimated and PMF also estimated in HEC-HMS model after
calibration and validation is 537m3/sec. The calibration result of HEC-HMS model is the
Nash-Sutcliffe Efficiency criteria for the calibration N=0.543 coefficient of determination (r2)
of 0.7334 daily based. Validation result is the Nash-Sutcliffe Efficiency N=0.6 and coefficient
of determination (r2)=0.8.The result of the simulation under overtopping failure showed that
the inundated area surrounding Koga River downstream of the dam is about 10.2 km2 with
maximum flood depth is 16 m, water surface velocity ranges from 0.00 to 11 m/s. Most of
floodplain areas varied from middle to high flood hazard level, and only few areas had very
high level of flood hazard especially on river side. Whereas under piping failure inundated
area is 9.1 km2 with maximum flood depth is 13 m, water surface velocity ranges from 0.00 to
6 m/s. This flood affect the downstream town Wetit Abay town settlement near Koga
river and irrigation canal also affected. Dam break has a greater impact on the downstream
location where is closer to the dam is high flood depth in accordance with the comparison of
the hydrographs at different downstream locations. The structural flood control measurement
to be recommended is to construct flood dike and flood gate. The non-structural measurement
one is to improve watershed management and socialization of flood awareness.