Dam Breach Analysis and Flood Inundation Mapping For Lower Awash Multipurpose Dam
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Date
2020-11
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Addis Ababa University
Abstract
Dam breach analysis is essential for investigating future effects posed to human life and
property by a sudden release of water to the inundation area downstream of a dam. Every
constructed as well as proposed dams need to be analyzed for the possibility of dam breach to
clearly identify the extent of catastrophic effects associated with their failure. The aim of this
study is to model the dam breach at the proposed Lower Awash Dam for the development of
flood inundation maps used to emergency action plans.
The proposed dam is located in Afar National Regional State on Logia River at about one
hundred kilometers upstream from the bridge on Logia River of main road to Djibouti. The
analysis was performed for piping failure scenario using HEC-RAS two dimensional unsteady
flow modeling. The inflow hydrograph was taken as an upstream boundary condition for the
unsteady flow simulation. The model was calibrated for the flow years (1998-1999) and its
performance was evaluated for the year 2001. The flood inundation and flood hazard maps
were prepared by exporting the tiff raster files from RAS Mapper to ArcGIS and overlying to
ArcGoogle to visualize coverage of the flood prone area due to the breach.
The resulting breach parameters were 146 meters breach bottom width, 1.17 hours breach
formation time, and 0.5 breach side slope (H:V). The simulated results on HEC-RAS reached
peak breach outflow discharge of 60,072 cumecs. Through model calibration close agreement
has been arrived between model simulated and observed rating curves at Logia gauging
station. As noticed from the flood inundation and flood hazard maps an extended area in the
flat plains downstream of the dam would be affected due to the breach with a maximum flood
depth of 21.6 m. Therefore, the flood inundation and flood hazard maps could be adopted for
proper emergency action plans that have to be taken by stake holders.
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Keywords
Lower Awash, HEC-RAS, ArcGIS, RAS Mapper, Dam Breach, Inundation Map