Dam break analysis to determine the outflow hydrograph and flood inundation impact of Isimba Dam, Kayunga District

Abstract

Dams are essential for water supply, hydropower generation, flood control, irrigation, recreation and navigation. There are several dams in Uganda for example, Bujagali dam, Owen falls dam, Isimba dam among others. The case study considered was Isimba dam located on Victoria Nile in Kayunga district. Though on rare occasions, dam breaks result into floods hence damage to people and property. The main objective of this study was to predict the resulting flood in the event that Isimba dam breaks and the area that would be covered by that flood. To achieve this, a model having the river, dam and two 2D flow areas was created in HEC-RAS this constituted the geometry. The model was calibrated using flow data. Four scenarios were made with respect to the dam types, sunny day failure and hydrologic failures. The results from analyzing the scenarios in HEC-RAS were viewed from the RAS mapper and extracted to Arc-GIS and excel to map the inundated villages and draw hydrographs downstream. The hydrographs downstream showed how the flood attenuated downstream. To obtain the impact on the bridge, profile lines were used to extract water surface elevations that were compared with the design elevations of Isimba Bridge to conclude on the bridge’s failure. For all scenarios, the bridge did not fail. Inundated areas of 78.9km2, 79.2 km2, 79.2 km2 and 75.3 km2 were obtained for scenario 1, 2, 3 and 4.