The Kariba Dam Rehabilitation Project (KDRP), which comprises the reshaping of the plunge pool and the refurbishing of the spillway gates commenced in 2017 after routine monitoring of the Kariba Dam identified the need for rehabilitation works to ensure its longevity, long term efficient operation into the future and its continued contribution to energy security and economic prosperity in the region.
The works, carried out by the Zambezi River Authority (ZRA), a bilateral organisation mandated to operate, monitor and maintain the Kariba Complex comprised of the dam, reservoir and telemetry stations commenced with the plunge pool reshaping.
ZRA is also tasked with developing of additional water storage infrastructure along the stretch of the Zambezi River that forms the common border between the Republics of Zambia and Zimbabwe, i.e., from Kazungula to Luangwa on the Zambian side and from Kazungula to Kanyemba on the Zimbabwean side.
Maricho Magazine (MM) publishes here an in-depth discussion with ZRA Chief Executive, Eng. Munyaradzi Munodawafa (EngMM) on the KDRP.
MM: Kindly share the technical specifications that the rehabilitation exercise aimed to address?
EngMM: The Kariba Dam Rehabilitation Project (KDRP) comprises of two projects of reshaping the plunge pool and refurbishment of the spillway that seek to address identified dam safety concerns. The projects were informed by the analyses of results of dam safety data and dedicated studies that were carried out before a decision was made to reshape the plunge pool and refurbish the spillway.
Reshaping of the Plunge Pool
The reshaping of the plunge pool aimed to address identified concerns related with the scouring towards the dam which if not addressed, could result in the loss of foundation stability and compromising the safety of Kariba Dam. The plunge pool at Kariba developed over the first 20 years of repeated and excessive spilling since in February 1962 after construction of the dam. By June 1981, the plunge pool had scoured to approximately 80m below the original riverbed downstream of the dam.
The scouring of the plunge pool was closely monitored through combinations of plumbing and bathymetric surveys since construction of the dam. Various studies that were carried out between 1981 and 2011 showed that further scouring of the plunge pool could happen should there be need to spill with more than three gates as was the case between 2000 and 2002, and again between 2010 and 2011.
To further inform the decision, quasi-3D numerical and physical model studies were carried out between 2011 and 2012 to establish the optimum plunge pool profile with respect to the spillway discharge combinations. The study did not only confirm the earlier findings, but also showed that reshaping the plunge pool by increasing its volume and flattening the downstream section would increase the jet diffusion angle of the spilling water and in doing so, reduce the erosive power of the spilling water jets and limit the potential for further scouring of the plunge pool.
The reshaping of the plunge pool involved controlled blasting and
the removal of approximately 300,000m3 of rock.
The project was successfully executed between May 2017 and completed in August 2024.
Refurbishment of the Spillway Gates
The Kariba Dam suffers from the alkali silica reaction (ASR) phenomenon, a form of alkali aggregate reaction (AAR) that occurs when alkaline pore fluids react with siliceous minerals in the rock aggregates to form a hard calcium-rich gel that cause the concrete to increase in volume.
While the swelling of concrete due to ASR is mild at Kariba and of no dam safety concerns elsewhere, it had caused cracking and noticeable leaks around built-in parts (BIP) steel structures that hold the groove guides. If not addressed, this posed operational risks associated with jamming and failure to operate the stop logs and floodgates which would further, compromise the safe operation of the spillway and dam safety.
To address the associated risk, a decision was made to refurbish the spillway for long-term dam safety reasons. The scope of works involves the replacement of the secondary concrete around the stoplog guides and installation of new BIPs and groove guides, the fabrication and installation of a bigger gantry crane and the manufacturing and installation of an emergency gate, capable of being operated under flow conditions. Other works under the project include the sealing of cracks within the sluices and replacement of the hydro-electromechanical equipment of the floodgates hoisting mechanisms.
MM: What influenced the selected engineering systems/designs?
EngMM: Selection of the engineering solutions were primarily influenced by the need to secure the long-term safety and reliability of Kariba Dam.
The following factors were central to the selected solution:
- Application – that the engineering solution could be technically implemented without compromising dam safety. In this instance, both the reshaping of the plunge pool and refurbishing of the spillway were technically sound and have now been either fully implemented or nearing completion.
- Economics – the economic consideration extended beyond the direct project costs. The engineering solutions must have guaranteed an uninterrupted power generation and supply from the Kariba Dam, within reasonable costs.
- Socio-environmental considerations – both in respect of avoiding and limiting harm by ensuring that the engineering solution will not introduce hazards to the environment and society. To achieve this, the projects were designed and implemented in accordance with the best practices and monitored using best available safeguards standards.
MM: Kindly share research/assessments carried out before the project began (e.g., rock types, terrain, falling water speed?
EngMM: Both the plunge pool reshaping, and spillway refurbishment works were informed by various studies and the analyses of dam safety data that was collected since the construction of Kariba Dam. For example, the development of the plunge has been monitored through combinations of plumbing and bathymetric surveys since in 1962. Similarly, the ASR phenomenon was first evaluated following the analysis of precision levelling and data collected from strainmeters measurements that have been recorded since the construction of the dam.
The following studies were critical in arriving at the decision to proceed with the implementation of the KDRP:
Plunge pool reshaping project
- The plunge pool scour development at Kariba has been previously studies by various authors including Mason (1984), Mason and Arumugam (1985), Bollaert (2005), Mhlanga and Goguel (2007), and Noret at al. 2013.
- It was anticipated that the plunge pool would scour in the early years until after construction of the second north bank power station. The dam safety risk related to the scouring was considered acceptable, for a limited time.
- In 1967, a consultant who was engaged to review the risk recommended the construction of a US$4 million spillway bypass to ease pressure from spilling using the floodgates. The bypass was not constructed, and the recommendation was superseded by the decision to pursue construction of the second north bank power station.
- Results of monitoring after successive periods of spilling showed that the plunge pool continued to scour, reaching a depth of about 80m by 1981.
- After two decades without spilling from 1981, the spillway was once again used to pass huge floods between 2000 and 2002, and between 2010 and 2011, with four floodgates used in February 2011. This renewed use of the spillway necessitated the need to carry out detailed studies to understand the implications of sustained spilling with several gates on dam safety.
- Between 2011 and 2012 the Authority in collaboration with the Laboratoire de Constructions Hydrauliques (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) and AquaVision Engineering (AVE) of Switzerland developed quasi-3D numerical and physical models to study the optimum plunge pool profile with respect to varying spillway discharges. The studies confirmed earlier findings that the pool was not stable if more than three gates were operated. The studies further showed that reshaping the pool would reduce the erosive power density of the spilling water and overall risk of further scour.
- Without reshaping the pool, a non-erosive required that spilling was limited to no more than three non-adjacent floodgates. If allowed, this would have reduced the spillway capacity by 50% while increasing floods related dam safety risks. For public safety interests, operating the Kariba Dam without attending to the plunge pool scouring concerns required that the flood rule curve be lowered by about 3.5m, which stores 33% of the live storage that is required for hydropower generation. It was based on these findings that a decision was made to proceed with reshaping of the plunge pool.
Spillway refurbishment project
- Refurbishment of the spillway facility was necessitated by the operational challenges related with the distortions and loss of tolerances to safely move the stoplogs and floodgates due to ASR.
- The swelling in concrete around the spillway at Kariba was established in the early years through the analyses of dam safety data that was collected since construction of the dam. It was however, established with certainty in the 1990s that the swelling was ASR-induced.
- The Authority has experienced the jamming of stop logs and used divers to unjam the stop logs while grinding the sealing plan to allow unrestricted movement of the floodgates. This intervention approach was temporary.
- Between 2010 and 2019 the Authority carried out various studies in collaboration with Tractebel Engineering and the Laboratoire Matériaux et Durabilité des Constructions (LMDC) to characterize the ASR using core samples of concrete extracted from the Kariba Dam. Based on the results of these studies, also with jamming experienced when operating the stop logs, it was decided to remove and the replace the concrete with non-ASR reactive concrete. Further, the scope of works included the removal and replacement of groove guides and BIPs, the manufacturing and installation of a large gantry crane to restored serviceability of the spillway. Also, by introducing an emergency gate that is capable to close the sluiceway under flow conditions, the project also seek to ensure that water is not lost should a floodgate jam during operations.
MM: Which company did the designs?
EngMM:
- Plunge pool reshaping design was done by Tractebel Engineering of France in collaboration with the Laboratoire de Constructions Hydrauliques (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) and AquaVision Engineering (AVE) of Switzerland.
- Spillway refurbishment design was initially done by Tractebel Engineering of France and was optimized by Gruner Stucky of Switzerland and KGAL of the United Kingdom.
- Gruner Stucky supervised the implementation of the designs at Kariba Dam.
MM: What are some of the challenges they faced?
EngMM:
- The plunge pool project was primarily affected by unforeseen poor geological conditions that caused delays in completion of the access road and construction of the cofferdam.
- COVID-19, as the manufacturing, completion and factory inspections before delivery of various mechanical components of the project were delayed during the lockdown period between 2019 and 2021.
- The spillway refurbishment works were also affected by issues such as rebar clashes arising from incomplete information on the “as-built” drawings. Some differences were discovered between information on the as-built drawings and the actual installations that were implemented during the construction of the dam. This caused delays as the design and implementation approaches had to be modified to incorporate the new information.
MM: How were these challenges addressed?
EngMM:
- Designs were updated to accommodate newly uncovered information.
- Specialised structural engineers adapted the designs to align with evolving project conditions.
MM: What would ZRA have done differently regarding the KDRP Rehabilitation?
EngMM: The Authority recognises the need to conduct comprehensive investigations during the project appraisal stage to minimise risks related to inadequate information during the implementation phase of the project, thereby reducing potential delays and extensions of time on project execution.