A THESIS SUBMITTED TO THE FACULTY OF HYDRAULIC AND WATER RESOURCES ENGINEERING, ARBA MINCH WATER TECHNOLOGY INSTITUTE, SCHOOL OF GRADUATE STUDIES, ARBA MINCH UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN DAM ENGINEERING MARCH, 2025

Abstract

Dams are critical infrastructures designed to provide water storage, flood control, power generation, and recreational benefits. However, despite their numerous advantages, dam failures though rare can have catastrophic consequences for communities, ecosystems, and economies. A dam breach occurs when a dam fails due to structural failure, improper maintenance, or external factors such as natural disasters. The sudden release of large volumes of water can lead to rapid flooding, loss of life, and the destruction of property and natural habitats downstream. Dam breach can result in huge damages to downstream areas of the dam. Embankment dam breach may be caused by overtopping or piping mode of failure. Due to these reasons our world has experienced some catastrophic dam failure. The general objective of the study is to model the Ribb dam breach phenomenon and analyze the resulting consequence at downstream areas. Studying of the dam breach analysis helps to give effective emergency action plan that requires accurate prediction of inundation levels. The analysis was performed for piping failure and overtopping 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 (1951-2021). The flood inundation and flood hazard maps were prepared by exporting the tiff raster files from RAS Mapper to ArcGIS and overlying to Arc Google to visualize coverage of the flood area due to the breach. The resulting breach parameters were 169 meters breach bottom width, 1.45 hours breach formation time, and 1.4 breach side slope (H: V). The simulated results on HEC-RAS reached peak breach outflow discharge of 147,379.12 cumecs. Through model calibration close agreement has been arrived between model simulated and observed rating curves at Ribb 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 16.86 m and flood velocity range from 0.00 to 8.208m/s. Mitigation is the proactive effort to reduce loss of life and property by lessening the effect of disasters. Therefore, the flood inundation and flood hazard maps could be adopted for proper emergency action plans that have to be taken by combine.