DAM BREACH ANALYSIS AND FLOOD INUNDATION MAPPING 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