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Lake Victoria is the largest tropical lake in the world and the source of river Nile, the longest river in the world. Due to prolonged precipitation above the average over the past two years, the lake water levels have increased surpassing the highest ever recorded of 13.41m in 1964. To mitigate the negative impacts on the people, navigation and developments in the flood zones, lake outflows were gradually increased from 1000cumecs to 2400cumecs. Isimba Hydropower Plant (HPP) with a reservoir capacity of 170 million cubic meters is the fourth hydropower plant along the upper Nile cascade with a design flood level of 1055m above sea level. Reservoir operation is constrained by the bridge construction and ferry operation works downstream which are affected by the increased discharges. There is also uncertainty of the incoming discharge magnitude and travel time due to lack of an upstream gauging station. This paper seeks to assess the reservoir response to the increasing flood conditions using Hydrologic Engineering software for flood routing and to model an operation water level (OWL) that optimizes power production and flood control whilst ensuring public safety of the downstream activities through real-time forecasts in the short term.

Lake Victoria storage and water levels are affected by changes in precipitation, evapotranspiration, tributary inflows and outflow in the single Nile River outlet at Owen Falls Dam. In May 1964, the lake reached its first highest recorded level of 13.46 m at the Jinja Pier that resulted into the Nile River outflow of 1910 m3/s. Consequently, the decision to construct Owen falls extension later in year 2000 was motivated by the risk assessment to mitigate the potential of regional hydrology reverting to the 1960s conditions. However, the catchment has consistently had above-normal precipitation coupled with accelerating land use changes that have resulted into higher inflows from the lake tributaries. In May 2020, the lake level surpassed the 1964 record, peaking at 13.47 m towards the end of May. These two occurrences (1964 and 2020) greatly affected hydropower generation, disrupted socio-economic activities around the lake and along the river and posed a threat on the safety of Nile Cascade dams. This paper therefore, presents the integrated multi-sectoral framework that was adopted to assess and mitigate the risks involved. The paper also identifies challenges, opportunities, interventions and lessons learnt from such risks and uncertainties.