The drought is a boon for rescue teams replacing seized floodgates and apertures and also for reshaping the plunge pool on the Zambezi River bed.
In the months ahead, the Kariba Dam hydroelectric scheme must deal with the twin challenges of not holding enough water on the one hand and too much of it on the other.
The capricious moods of the weather in the Zambezi River catchment leave everyone guessing. The flood plain is a huge swathe of mainly bush land carved by tributaries that deposit water into Africa’s fourth-biggest river for its journey to the monstrous Lake Kariba, some 181-billion tonnes of water in extent. So far, the rainy season has been reasonable, but worryingly patchy. There’s an upside, however.
Kariba was built on a gorge on the Zambezi River. The dam wall forced the rising waters to spill out of the gorge and back up over the land upstream creating a 250km long lake.
“Most of the storage is in the lake’s upper levels and, even when most of the storage is depleted, the water levels are still quite a lot higher than the turbines,” notes New Zealander Bryan Leyland, a global expert on hydroelectric schemes.
“I checked to see if they’re running it down to levels that put the turbines at risk of cavitation or vibration,” says Leyland. “The rule of thumb is that the Francis turbines can operate down to 65% of the design head. At Kariba, the design head would probably be about 3m below the maximum level. As it is now, about 18m below full, the low level is not putting the turbines at risk. The problem might well be that, if the level goes any lower, air will be sucked into the turbine intakes.”
Questions were put to the Zambezi Water Authority and Zambia’s Institute of Engineering, but elicited no response.
The situation at both Kariba’s north and south bank power stations is that despite exceptionally low water levels, Kariba Dam continues to generate electricity, thanks to a geological stroke of luck and engineers with the skills to take advantage of it. The 56-year-old, 1,600MW scheme hit the headlines two years ago, when reports circulated, eventually confirmed by the World Bank, that urgent intervention was needed to arrest erosion of the dam wall foundation and to replace faulty floodgates. Kariba’s capacity will increase to 1,926MW when turbine upgrades are completed at the Zimbabwe south bank power station in 2019.
Kariba’s geological configuration on the Zambezi on the borders of Zambia and Zimbabwe, while not unique in the world of hydroelectricity, is propitious in coping with limited water volumes to produce more energy than such low levels could normally allow, even though the maximum power output is reduced.
In generating hydro power, flow and head are the vital components. The flow is the volume of water that can be captured and redirected to turn the turbines and the head is the distance the water will fall on its way to the turbines. The larger the flow and the higher the head, the more energy is available for conversion to electricity.
Double the flow and double the power, double the head and double the power again. Where Kariba scores is that it can produce a decent amount of energy from dam levels as low as 12%. The power generator can get away with not having a large flow of water, because gravity will provide the energy boost.