Water levels in Kariba Dam have dropped markedly as a result of the drought that plagued the 2018/2019 season. The Zambezi River Authority (ZRA) — the body responsible for managing the reservoir on behalf of Zimbabwe and Zambia — has been progressively reducing water allocated for power generation to the country’s power utilities.Power generation has been affected as a result.

Our Senior Reporter Lincoln Towindo spoke to (ZRA) chief executive officer Engineer Munyaradzi Munodawafa on developments along the Zambezi basin, including imminent works on the Batoka Gorge Hydroelectric Power Station.

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Q: Can you give us a brief background of the current challenges affecting power generation at Kariba Power Station?

A: Power generation at Kariba is subject to water levels, which are subject to the rainfall received and associated inflows into the lake.

In this case, the rainfall received during the 2018/2019 season was below normal, leading to below average inflows into the lake and low lake levels recorded at Kariba for the most part of 2019.

As a result of the poor rains, the below average lake inflows and associated low lake levels recorded at Kariba, there was reduced availability of water for power generation at Kariba Hydropower Station. This implies power generation levels would be below optimal levels at Kariba for the 2019 period.

Q: What is the current status in terms of water levels and power generation at Kariba?

A: Due to the poor rains experienced during the 2018/2019 rainfall season and the associated low lake inflows, the lake is currently three metres above the minimum operating level and receding. Normally, it should be about 11 metres above the minimum operating level and rising. Stored usable water is currently at 22 percent (14 billion cubic metres of stored usable water) when last year during the same period it was at 84 percent (55 billion cubic metres usable storage).

Combined power generation at Kariba is on average around no more than 580MW.

Q: We understand there are plans to temporarily decommission Kariba Hydropower Station owing to depressed water reserves. What does the proposed decommissioning entail and what would be the implications for Zimbabwe and Zambia?

A: What is meant is that should the volume of stored usable water be exhausted (seeing that the water levels at Kariba are still receding due to below average inflows into the lake, which have resulted from the poor rains received in 2019), this would lead to shut down of power generation at Kariba.

However, ZRA, working with the two power utilities at Kariba, continue to implement measures to conserve the available stored usable water to mitigate this risk.

This includes a reduction in the water allocation for power generation and associated generation levels at Kariba, which was implemented at the end of April 2019 to avoid exhaustion of stored usable water that would have otherwise occurred by the end of October 2019.

The water allocation was reduced from 36 billion cubic metres to 34 billion cubic metres (shared equally between the two power stations) to ensure continued availability of water for power generation into the 2020 period.

The power stations adjusted their generation plans to accordingly utilise water within the respective revised water allocations (17 billion cubic metres for each power station for the whole of 2019).

Q: When should we expect the water situation to start improving at Kariba?

A: The improvement in water levels at Kariba is mainly subject to improved rainfall performance. Should the rains for the 2019/2020 season turn out to be normal to above normal, the lake levels will steadily improve. In the meantime, the Authority is working with the two utilities at Kariba in implementing measures to conserve the stored usable water at Kariba going into the 2020 period.

A further mitigation of the situation with regards to this is fast-tracking the development of the Batoka Gorge Hydroelectric Scheme

Q: The Governments of Zimbabwe and Zambia recently awarded the tender to develop the Batoka Hydropower Stationt to a consortium of General Electric and Power Construction Corporation of China. Can you give us the timeline and processes leading to the commencement of construction work?

A: The Batoka Gorge Hydroelectric Scheme (BGHES) Project Steering Committee — comprising government officials from the Ministries of Energy and Finance in Zambia and Zimbabwe, the utility companies (Zesco and Zesa) and the ZRA — is working out a development programme which will be considered soon by the two governments before the timelines can be published.

Q: When should we expect the power plant to be commissioned?

A: The implementation of the project will be over a period of five to six years. The developer will give a more detailed programme that will confirm the proposed date of completion.

Q: We understand that ZRA plans to introduce high-tech equipment, including closed circuit television (CCTV) and drones, to improve security at Kariba Dam following a series of vandalism of infrastructure there. What is the status regarding the rollout?

A: Use of the CCTV was discussed during meetings with the security organs based in Kariba and Siavonga. Studies on how this can be done effectively are still in progress. The use of drones is, however, not part of these security measures.

In the meantime, ZRA has constructed two observation buildings on both banks to enable the security personnel to effectively monitor and inspect the dam environs from these advantageous positions.

Q: The ZRA also recently announced plans to rehabilitate Kariba dam through reshaping the plunge pool downstream of the dam wall. What does the scope of the rehabilitation work entail and what is the current project status?

A: The rehabilitation of Kariba Dam comprises two project components, which are: reshaping of the plunge pool, which involves construction of a cofferdam just upstream of the tailrace’s outflows; (and) dewatering of the plunge pool and reshaping the pool by controlled blasting of the rock to create the required optimum shape that will be more efficient in dissipating the energy of the water jets during spilling.

The current project status is on construction of the cofferdam, which will be followed by dewatering of the plunge pool before start of the excavation works.

Then there is refurbishment of the spillway upstream control facility and this involves removal of the steel guides and the supporting secondary concrete in the upstream stop-logs guide-slots and replacing these with new ones; adding an emergency gate and replacing the old gantry with a new one.

This contract was awarded in May 2019, and the contractor is mobilising resources to start works by manufacturing the hydro-mechanical equipment in Europe.

The equipment can only be completed and transported to Kariba at the end of 12 months.

Q: Some areas around Kariba, particularly Zimbabwe, have witnessed a series of moderate earthquakes over the last few months. Has the ZRA established why there has been a sudden increase in these occurrences and has this not affected the integrity of the dam and its infrastructure?

A: The tremors are a result of “reservoir-induced seismicity” — a phenomenon that is caused by significant variations in lake levels, which may either be very low levels or very high levels.

The recent events have been caused by the very low lake levels that are currently obtaining in the lake. The magnitudes of the tremors are, however, below the levels that would cause any structural damage to the dam and associated features, and, hence, no impact has been registered on the infrastructure.