Relief as flows into Zimbabwe’s Kariba Dam increase

UNSPECIFIED - MARCH 18: Kariba Dam, 1959, hydroelectric dam in the Kariba Gorge of the Zambezi river basin between Zambia and Zimbabwe. (Photo by DeAgostini/Getty Images)

The Kariba Dam’s water level ended last year at its lowest in 23 years, but recent data from three major river flow stations along the Zambezi River, show the encouraging trend that water flow towards the giant lake rose exponentially in December last year.

Kariba Dam is Zimbabwe’s largest power station, with capacity to produce 1 050MW, but is currently generating electricity at curtailed level due to critically low water levels following the drought experienced in the catchment area of the dam’s main feeder river.

While Kariba Dam is designed to operate at about 485 metres (water level) when full, its water level had dropped substantially to about 476 metres as at December 27, 2019 compared to 482 metres at the same time in December the previous year.

The world’s largest man-made inland dam is thus currently generating an average of 200 megawatts in tandem with the reduced lake water levels at Kariba, which ended last year at about 8,36 percent of live or usable water. Live water is water above 475 metres below which the dam cannot be used for power generation, as this poses risk of exposing inlet valves to dangerous weather elements.

It is also critically important to avoid depleting Kariba Dam since the reservoir would require at least three good rainy seasons to fill it up again without using it to generate electricity in the event that it drains out. Even when not used for power generation, the dam must still have water for fishing and recreational activities.

The expansive Barotse plains north of Zambia need to soak up water first during the rainy season and only when they get saturated do they release the bulk of the water, about 80 percent, which then finds its way into tributaries and rivers that eventually empty into Zambezi River.

This state of affairs sees water from this main source of Kariba Dam reaching Zimbabwe around April and May with the dam reaching its peak levels for the season around July of any particular year or slightly later depending on the rainfall pattern for a particular season.

Coupled with generation constraints blighting the country’s only other major power plant, Hwange Power Station, State power utility Zesa Holdings resorts to extensive load-shedding to balance demand and supply of electricity, occasionally augmented with imports from South Africa and Mozambique.

But data gathered by the Zambezi River Authority, which administers the dam and the Zambezi River, shows that the amount of water that passed through major water flow stations at Victoria Falls, Chavuma, Nana’s Farm and Ngonye has increased substantially.

The hydrometric network used for the control and day-to-day operations of the Kariba reservoir comprise of 13 stations where water levels are monitored daily.

“Flows at Ngonye increased due to sporadic rainfall activities being recorded around the catchment, closing the week at 426 cubic metres per second on 27th December 2019, while last year’s flow on the same date was 295 cubic metres per second.

“The Zambezi River flows through Nana’s Farm station have been increasing due to records of rainfall activities on the catchment, closing the week under review at 362 cubic metres per second on 27th December 2019. The flow observed on the same date last year was 293 cubic metres per second,” ZRA said.

At Chavuma water flow steadily increased and closed the week under review at 213 cubic metres per second on the same date in December last year, while the flow observed at the same point last year on the same date was 127 cubic metres per second.  The same pattern has been observed at Victoria  Falls.

However, while rainfall activity around the catchment areas around water flow measuring points has increased inflows going into the lake, only 20 percent of water that comes from catchment areas in Zimbabwe makes up the water that flows into the Kariba Dam.

Zimbabwe requires about 1 800MW at peak period of demand for power, but struggles to even provide half of this from its domestic production facilities, with only two of its plants — Kariba and Hwange — still able to generate tangible amounts of power.

Because of the low lake water level Kariba cannot be used to full capacity despite having its capacity upgraded to current rated capacity after installation of two additional generators from the previous 750MW. ZRA determines the amount of water that can be used for electricity.

Government is, however, working on plans to increase power generation through further expansion of Hwange Power Station, which will have two more generators installed to add 600MW to the grid. The construction, which is nearly 30 percent complete, is expected to last for about 42 months.

Further, Zimbabwe has partnered with Zambia for the construction of the 2400MW Batoka Gorge power station, downstream of Kariba Dam and the process has reached procurement stage with the contractor for the hydropower project expected to be announced soon.