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Kariba Bureau
The Zambezi River Authority is now working out if allocations of water for the Kariba South and North power stations can be further increased or cut from the present level which allows each station to generate an average of 550MW.
Kariba North has an installed capacity of 960MW and Kariba South 1 050MW, so both Zesa of Zimbabwe and Zesco of Zambia will get larger allocations if the authority believes enough rain is falling in the catchment. With the present ration both utilities can generate a lot more for some hours a day so long as they cut right back at other times to keep the average at 550MW.
Ongoing works on the rehabilitation of the plunge pool downstream of Kariba Dam has seen both Zesa and Zesco reducing output in daylight hours to allow those erecting the diversionary coffer dam to work on the downstream end of the plunge pool. That has led to increased load-shedding in both Zimbabwe and Zambia.
ZRA chief executive, Engineer Munyaradzi Munodawafa, said increased rainfall in the Zambezi River catchment and Lake Kariba had seen increased water inflows, with levels marginally below those recorded in the last rainfall season.
“Owing to the delayed onset of the 2021/2022 rainfall season, the authority has cautioned the two power utilities not to generate at higher levels (currently 550MW for each Kariba power station) than that associated with the water allocation made for 2022,” said Eng Munodawafa.
“The authority is now preparing to undertake the hydrological review for the first quarter of 2022 that will further inform the water allocation for power generation operations at Kariba.”
He said water allocation of 45 billion cubic metres was subject to review if hydrological simulations showed the region experienced an unexpected drought. The allocation, said Eng Munodawafa, was on the basis of predicted normal to above normal rainfall provided by the regional weather and climate experts.
The rainfall season was expected to start by October 2021 ending around March 2022 but started in late December 2021. As a result, lake levels receded up to the first week of this month but have started rising due to increased inflows.
The lake now holds 14,17 billion cubic metres, or 21,87 percent of stored usable water for power generation. The water below the level of the intakes for the two stations cannot be used for generation and is not counted in the totals.
The authority needs to check the rainfall throughout the catchment. Most of the water that ends up in Kariba Dam starts as rainfall in south eastern Angola, with a good extra slice from rainfall in north western Zambia. The inflows from Zimbabwe and southern Zambia are very small in comparison.
ZRA had instructed Zesco and Zisco to reduce power generation levels for six hours on selected days of the week to facilitate construction of a secondary dam (cofferdam)
“The specific works under the pool reshaping sub-project that called for this need have since advanced and are due to be completed by January 25, thereby ending the authority’s request to the two power generation utilities.
“Going forward, the remaining works of the Kariba Dam Rehabilitation Project will not result in the two utilities seeing reduced generation levels at their Kariba power stations,” said Eng Munodawafa.
The rehabilitation of the plunge pool and the floodgates is expected to be completed by 2024 and seeks to improve Kariba Dam’s safety and integrity. While the floodgates have not been used for around 15 years and are unlikely to be used again in the ordinary course of events considering that both power utilities have extended their Kariba stations, they must be kept operational for emergencies.
The plunge pool was carved out downstream in the early days when only Kariba South was operational and the floodgates were frequently opened. While not erosion has taken place in recent years, the authority still wants the to remove any danger that the pool could cut back to the dam wall. Engineers have worked out that if the downstream end of the pool is excavated more and reshaped then this danger is no longer of concern.
Online Reporter