Hydrological analysis of the Middle Zambezi and impacts of the operation of hydropower dams on flow regime in the Mana Pools National Park

Abstract

The Mana Pools flood plain along the Middle Zambezi River and part of a popular Mana Pools National Park of Zimbabwe is experiencing both ecological and morphological change since the construction of upstream hydropower reservoirs. It is against this background that hydrological analysis was done to investigate the hydro-dynamics from natural and human influenced flows, particularly at the Mana Pools floodplains upstream and downstream stations on each reservoir were indentified to represent what could be natural and modified flows respectively. Comparison of hydrographs and flow duration curves constructed from flow data recorded at each station was done to investigate the effects of each reservoir on the downstream flow regime. Descriptive statistics was used to assess the differences in flow regime upstream and downstream of hydropower reservoirs. A standard t-test (α=0.05) was done to compare the natural and modified mean annual flows. Monitoring of the effects of reservoirs operation to the Mana Pools flood plain was done by tracing contours from water levels recorded at Chirundu. A rainfall-runoff model, Soil Conservation Services utilizing a curve number system was used to estimate flows in six un-gauged catchments flowing towards the Mana Pools. Results indicated that, the presence of upstream reservoirs had reduced the high peak average monthly flows by 17 % and increased the average monthly low flows by 5 % at Mana Pools. However this effects observed at monthly level have not significantly affect the mean annual runoff. This was confirmed by a t-test at 95% confidence level. Although the presence of reservoirs upstream of Mana Pools reduced the peak flows, operation of sluice gates to release water can result in excess flooding at Mana Pools. The Mana Pools catchment contributes 7.2 % of the total flows at Mana Pools. From the findings of this study, In order to avoid unexpected flooding, operation of the sluice gates at Kariba Dam should be directly communicated to the communities residing in the low lying areas, downstream of the dam. With the establishment of a gauging station on the Rukomechi River, further studies that will make use of observed flow data from the same catchment are encouraged in order to support the findings of this study.