The Rehabilitation of the Kariba Dam Wall (07.01.2016)Author: Nick Blevin
Published In: Diwa Zambezi News
At 128m tall and 680m wide the Kariba dam wall stands as a testament to the pioneering spirit and economic development of the British colonial Federation of Northern Rhodesia, Southern Rhodesia and Nyasaland in the early 1950s. This thin strip of concrete, which stands as the border between Zambia and Zimbabwe, holds back a body of water some 250kms long with an estimated capacity exceeding 185bn tonnes of water. It is fair to say then, that in the event that the wall fails, the results would be catastrophic for both Zambia and Zimbabwe. However, more significantly, the downstream effects of this impending catastrophe have caused significant concern amongst the international community. A recent study by the World Bank and the Zambezi River Authority (ZRA) suggests that failure of the wall has the potential to affect 3 million people residing on the floodplains of the Zambezi downstream of the Kariba wall. Furthermore, given the volume of water that would be released, it is undoubted that Cahora Bassa Dam would also collapse and take with it 40% of Southern Africa’s electrical capacity. This coupled with a general consensus that Zambia and Zimbabwe have only three years to avert this crisis is certainly causing some concern.
Thankfully, funding for the project has been obtained with works set to begin in 2016. Reports suggest that funding is be provided through the Zambezi River Authority (‘ZRA’), The World Bank Group, Africa Development Bank, European Union and the Swedish Government with total contributions amounting to 289 million USD. Initial predictions from The World Bank Group suggest that the works are expected to be completed between 2019 and 2023.
From an engineering perspective, the ZRA has identified that there are two areas of the wall which require immediate attention. Namely, the plunge pool and the swelling of the wall caused by alkali silica reaction (‘ASR’).
The initial concern is the so called ‘plunge pool’ which has formed immediately below the spillway. The plunge pool has, over decades, eroded the riverbed gneiss to a point where there are now significant concerns that this erosion has the potential to undermine the structural integrity of the wall. As a result, it is proposed that the plunge pool is reshaped in order to dissipate the water from the floodgates in a more controlled and less turbulent manner. For reshaping of the plunge pool to be conducted, a coffer dam will need to be constructed so that the plunge pool can be pumped dry. This will undoubtedly be the most significant undertaking of the wall’s rehabilitation. Moreover, according to The World Bank report, notwithstanding the new works, erosion controls will still require that no more than three nonadjacent gates are operational at any one time. This control strategy inhibits the ability of the dam to rapidly discharge water in a flood event and it is likely that the safe operational capacity of the dam will have to be decreased. It is however unlikely that any flood event would compromise the safety of the wall.
The second major concern identified in the report points to the effects of ASR which is causing the concrete in the wall to swell. This swelling effect inhibits the normal function of the floodgates and other hydro-mechanical functions. When the dam was first established in the 1950s, ASR and its effects on concrete were not known resulting in many dams sharing this problem worldwide. In order to compensate for the swelling of the wall, an emergency gate is to be installed to be utilised in the event of floodgate failure. Furthermore, the existing gates are to be re-engineered to be made adjustable and to avoid scraping.
According to The World bank report, funding has been apportioned between the Zambezi River Authority, Africa Development Bank, European Union, Swedish Government and The World Bank Group. Currently, the ZRA is undertaking social and environmental impact studies to ensure the sustainability of the project and to ensure appropriate safety controls are in place before works are commenced this year.
Whilst it appears that the restoration of the wall is in-hand, there are certainly significant challenges to overcome in the completion of this project. The biggest hurdle facing engineers will be the time constraints of the project. Failing to meet timeframes in the world of construction is commonplace. Rarely however, do people’s lives and livelihoods depend on timely completion. With this in mind, Diwa Zambezi as a concerned stakeholder will certainly be keeping a finger on the pulse of this project and ensuring that information surrounding the project is readily available.