| dc.description.abstract | The importance of a safe spillway cannot be sufficiently emphasized; many dam failures have been caused by poorly designed spillways resulting in dam failure and downstream population damage (Lee et. al, 2013). Therefore, the main objective of this study was to evaluate the adequacy of the surface outlet ogee spillways and bottom outlets of Isimba dam to control
overtopping of the dam under normal (design) and extreme flooding conditions (due to climate change and Bujagali dam break). Overtopping of the dam can lead to eventual dam failure and its catastrophic consequences.
The normal flooding constituted analysis of the Probable Maximum Flood (PMF) based on observed River Nile flows and extreme flooding based on analysis of projected River Nile flows under climate change and Bujagali dam break. To achieve this, the necessary data collected was Lake Victoria outflows from 1950 to 2014, flow at Mbulamuti gauging station from 1956 to 2018, and the projected future average daily L.Victoria outflows due to climate change up to 2099 from a water balance model (Vanderkelen et. al., 2018). The sunny day failure dam break hydrograph of Bujagali dam (Eyiiga, 2019) was also adopted for the Bujagali dam break scenario.
Frequency analysis was conducted on the flow data using three frequency distributions; Log Pearson, Normal distribution and Extreme Value Type 1. Log Pearson had the best fitting frequency distribution and highest R2 value for the observed river flows and flows under RCP4.5, but EV1 was the best fitting frequency distribution and highest R2 value under RCP 8.5.These were used to compute the 10,000 year flood under the different scenarios.
To evaluate the spillways adequacy, HEC RAS was used to model the river reach from Bujagali dam to L.Kyoga and Isimba dam added as an inline structure. 10 scenarios were modelled which were based on the gate openings duing the flooding, for the three flooding events; normal, climate change and Bujagali dam break, together with occurrence of the designed-for flood of 4500m3/s. The adequacy of the spillways to prevent dam flooding was judged by monitoring the maximum water surface elevation immediate upstream and downstream of the dam.
Furthemore, the impact of Isimba dam on the flood inundation of the surrounding areas was assessed. This was done by preparing flood maps under all the 10 scenarios which showed the flooding extent and depth, and also calculating the percentage of the area inundated by the flood using Arc GIS pro software. | en_US |
