Features

Kariba Dam rehab: Getting the big job done

AS REHABILITATION works on one of the world’s largest dams – Kariba Dam – progress, the scope of the engineering work involved is now becoming more apparent.
The project involves reshaping the plunge pool and rehabilitating the spillway.
Reshaping the plunge pool and rehabilitating the spillway will ensure the Kariba Dam’s continued contribution to Zambia and Zimbabwe’s energy security and economic prosperity.
Reshaping the plunge pool
Releasing water from Lake Kariba through the spillway gates over 60 years has deepened the natural river bed where the spilling water lands. Reshaping it will limit the erosion. Reshaping the plunge pool of an existing dam – a world first – calls for the best engineering, construction, blasting and even diving skills, and comprises many carefully planned, coordinated steps.
The first is creating a dry work area where the plunge pool currently is by building a temporary cofferdam. Divers work underwater to place the foundations for the cofferdam columns or piers.
The hollow piers are built on the bank before being floated across the plunge pool to where divers place them on their underwater foundations. When secure, they are slowly filled with concrete.
Prefabricated metal gates called stoplogs are then positioned between the piers, forming a solid, water-tight wall called a cofferdam.
With the cofferdam in place, the water from the plunge pool is slowly pumped out into the river, creating a dry working area for the next step.
That is, carefully controlled blasting to widen the bottom of the plunge pool and shape its sides. Giant steps are created which also serve as roadways for trucks accessing the plunge pool work area.
The next step is strengthening the soft rock in the fault-zone by constructing a reinforced concrete slab.
When the work in the plunge pool is complete, some of the stoplogs in the coffer dam are removed to allow water from the river to slowly fill the plunge pool again. Finally, all the stoplogs are removed, leaving the piers in place below the water. In all, this will take three years.
Rehabilitating the spillway
The six sluice gates in the Kariba dam wall form the spillway. Through them, water is released from Lake Kariba if the water level is too high.
Over time, the concrete has expanded slightly affecting the smooth opening and closing of the sluice gates, possibly causing the gates to jam.
Jamming when open would mean that large amounts of water would pour through the sluice gates leading to higher water levels in the river downstream of the dam, putting people, animals and the environment at risk and loss of power generation.
Jamming when closed, could mean that the dam would overflow, again putting people, animals and the environment at risk.
Refurbishing the spillway will ensure that it will operate as it was designed to for many more years.
Because refurbishing the spillway is complex and the gates will be refurbished at a rate of two per year, it will take several years. The first step is manufacturing some of the components like a new emergency gate and motorised gantry crane off site.
These are then transported to site, first by ship and then by road from Durban, South Africa or Beira, Mozambique. When the components reach Kariba Dam, they will be reassembled near the dam and then placed into position.
Again, the first step is to create a dry workspace at each gate so that the gate can be refurbished.
This entails building a temporary small coffer dam which clings to the dam wall on the lake side.
Experienced divers, working underwater in full diving gear, fix steel supporting members to the upstream face of the dam to allow the cofferdam to be put in place.
A crane on the dam wall then lowers the coffer dam pieces into place one by one. The water trapped between the coffer dam and
the dam wall is pumped out into the lake, creating the water free space in which the workers work.
The final step is the installation of the new gantry crane which is self-motorised so that it can move to place the emergency gate into position.
When all six spillway gates have been refurbished, the spillway will be commissioned.
Large scale engineering projects like the KDRP take many years. Work on the plunge pool reshaping started in 2017, and the spillway refurbishment is expected to start in 2019. The project will be completed by 2023.
Working together the World Bank, the African Development Bank, the Government of Sweden and the European Union; the governments of Zambia and Zimbabwe and the Zambezi River Authority have made the Kariba Dam rehabilitation a reality.

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