WHEN a 60-year-old dam that stands 128 metres tall, 716 metres long and holds 181 billion cubic meters of water needs repairs, it is never a small job and it comes with a huge budget: US$294 million. But, consider the cost and consequences of disregarding dam safety and non-repair. That is the story of the Kariba Dam in southern Africa, on the stretch of the Zambezi River which forms a common border between Zambia and Zimbabwe.
What is dam safety?
Dam safety refers to the state and performance of a dam compared to design expectations. Dam safety management includes measuring how the dam responds to forces or loads arising from the stored water , temperature induced loads and loads from other factors like earthquakes. To ensure a dam’s safety, dam operators implement dam safety monitoring programmes which essentially involve design and installation of instruments to carefully monitor the dam’s responses to the forces or loads. The information or data collected from the instruments enables the dam operators to understand how the dam responds to the loads and whether these responses are in line with the design expectations. If the responses are not as expected, cause and effect will be investigated and appropriate remedial measures are prescribed and implemented.
Dam safety monitoring includes monitoring and recording of water inflows; managing the reservoir levels to ensure the availability of sufficient storage space should there be a flood; monitoring earthquake activity in the vicinity of the dam; checking the behaviour of the banks of the dam and the ‘health’ of the concrete in the dam wall; checking and the functionality of the mechanical and electrical parts of the equipment used to open or close the spillway for flood passage, amongst others.
The Kariba Dam, has many instruments in the wall which measure stress and any changes or minute movements in the wall, water pressure and drainage. These instruments are continually inspected to ensure that they function optimally.
The dam safety monitoring programme is complemented by dam maintenance plans which comprise well-structured routines as well as targeted remedial measures that seek to correct any safety deficiencies observed.
Why dam safety matters
While large dams bring benefits they also pose risks to the public. Effective dam safety monitoring and maintenance programmes and effective communication help to mitigate the risks that large volumes of water stored behind large dams pose to communities and the environment in the downstream valleys, to infrastructure and to the economies of the regions in which they are located.
In the case of the Kariba Dam, the Zambezi River Authority’s mandate includes implementing a precisely structured dam safety monitoring and maintenance programme to ensure the continued safe operation of the dam. This ensures uninterrupted power generation for Zambia and Zimbabwe. The current Kariba Dam Rehabilitation Project (KDRP) entailing the reshaping of the plunge pool and the refurbishing of the spillway also ensures the long term safety and reliability of Kariba dam.
Plunge pool
Where there is an excess of water above the maximum storage capacity of the Kariba Dam, the water is released from the reservoir through the spillway, which is centrally located along the length of the double curvature concrete arch dam. This is done to protect the dam in line with the international best practice, dam designs and the standing dam operational procedures for flood control.
In the 60 years of its operation, the dam has spilled 597 billion cubic meters of water through the six flood gates. The spillage has led to the gradual development of a scour hole in the natural rock floor of the river bed, which is now about 81 metres deep.The plunge pool is located about 100 metres downstream of the dam wall. Although the deepening of the pool itself is an expected feature of the spilling process, the irregular erosion of weak rock material is a safety concern and has been carefully monitored over the years with mitigation measures undertaken. The reshaping of the plunge pool is the definitive next step.
According to the Zambezi River Authority’s KDRP Project Manager, Eng. Sitembinkhosi Mhlanga, the existing pool will be widened from bank to bank and will also be extended in the downstream direction. The result will be an enlarged pool with an increased volume. The enlarged shape will create a water cushion which will improve the capacity of the pool to absorb the potential energy from spillage, thus reducing the development of the pool, and ensuring the stability of the dam foundation. In its quest for a solution, the Zambezi River Authority carried out detailed investigations and modelling processes which showed that the force of the spilling jets of water are significantly reduced once the plunge pool shape is widened.
Spillway operations
The Kariba Dam spillway comprises six sluices or openings through the dam concrete allow flood water to pass from the reservoir into the river downstream of the dam. These are controlled from the downstream face by floodgates and have a total discharge capacity of 9,000m3 per second.
Over time, alkali aggregate reaction can occur in concrete exposed to water causing the concrete to swell. This causes distortions to the steel-lined guides of spillway flood-gates, affecting the spillway functionality.
“In the case of the Kariba Dam, careful dam safety monitoring has ensured that the alkali aggregate reaction has not gone unnoticed. In accordance with its internationally recognized dam safety protocols, the Zambezi River Authority has taken measures to lessen its impact and to ensure the dam remains operational thus guaranteeing the storage of water for the generation of electricity for Zambia and Zimbabwe. We like to ensure that our work reflects internationally accepted standards of managing a dam,” says Chief Executive of the Zambezi River Authority Engineer Munyaradzi Munodawafa. The spillway refurbishment is part of this dam safety assurance process.
The current state of the Kariba Dam
The dam safety monitoring and dam maintenance programmes followed by the Zambezi River Authority in the management and operation of the Kariba Dam confirm that there is no abnormal behaviour in the general operations of the dam, apart from those relating to the plunge pool and the spillway that are already undergoing remedial action.
Precautions
Considering that the excavation of the plunge pool will be done by controlled blasting, a rigorous extended dam safety monitoring programme will be followed to ensure that the safety of the dam is not compromised during the works.
This includes the installation of additional instrumentation to intensify the already careful monitoring of the dam during the works: instrumentation to monitor pore-water pressures in the dam foundations, vibrations sensors and equipment to monitor seepage
Risk mitigation
As a responsible organisation, the Zambezi River Authority is cognisant of the potentially devastating impacts of a dam breach on the lives, livelihoods and power supply for the citizens of Zambia, Zimbabwe and the countries downstream, should such a risk materialise.
“We are being very cautious. The KDRP is our largest attempt to ensure the further long-term full operability of the dam,” says Engineer Mhlanga.
He adds: “As part of our risk mitigation process, the Zambezi River Authority is also undertaking a Dam Break Analysis (DBA) study to fully understand the implications of a dam breach in the Zambezi Basin, given the locations of other existing dams in the basin. Furthermore, we have established an Emergency Preparedness Plan (EPP) to ensure a rapid response. The EPP links up with national disaster management agencies in Zambia and Zimbabwe, and it is networked with dam operators downstream of the Kariba Dam, in Mozambique.”
Kariba Dam Rehabilitation Project status
The plunge pool works are now underway, having commenced in 2017, while the spillway refurbishment is expected to start in 2019. The Kariba Dam Rehabilitation Project will safeguard the longevity of Africa’s largest dam and guarantee a continued supply of electricity, contributing to the economic and social development of Zambia and Zimbabwe, and the southern African region.
The KDRP is jointly financed by the European Union, the African Development Bank, the government of Sweden, the World Bank, and the Zambezi River Authority on behalf of the governments of Zambia and Zimbabwe.
While the works at the Kariba Dam are going on, engineers at the Zambezi River Authority have their eyes on two other sites on the Zambezi River that have huge potential for power generation where the same international best practice standards of dam safety will be implemented. The most immediate one is the Batoka Gorge Hydro Electric Scheme dam site and the other at a site with an unforgettable name - Devil’s Gorge.
