DAM BREACH ANALYSIS AND FLOOD INUNDATION MAPPING : A CASE STUDY OF KOGA DAM

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 propagation. 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 tested 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 embankment 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 geometric 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 m3/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.