Kariba plunge pool rehabilitation

The Kariba Dam is a 128 m high arch dam located on the Zambezi River, between Zambia and Zimbabwe. It forms the Kariba lake, with a surface of 5400 km2 and a capacity of 185 km3. In the 20 years following dam construction, heavy rainfalls required frequent spilling operations at high discharges. Excess water from the upstream reservoir is conveyed to the downstream river through six gated spillways with a section of 9.1 m in height and 8.8 in width, which generate free-falling jets with high impact velocities of around 21 m/s. Intensive operation of the spillways progressively scoured a deep and steep-sided hole with its current bottom at about 70-80 m below the original riverbed and about 80-90 m below the normal tailwater level. Between 2010 and 2012, a comprehensive study including both physical and numerical modeling was carried out to investigate the plunge pool situation, with the aim to assess and control any potential future scour development (LCH/AVE Report 04/12). The results of the study indicated unacceptable scour evolution, and therefore several countermeasures were proposed and tested. The study concluded that plunge pool geometry reshaping in the downstream direction was both an efficient and technically plausible solution to limit future erosion. Different reshaping geometries were tested, and based on pressure and velocity measurements at selected locations, a final configuration of the plunge pool was proposed. In order to perform the necessary excavation works for plunge pool reshaping, a cofferdam is foreseen downstream of the pool. Furthermore, a major geological fault will be treated during excavation by means of a concrete structure. The objectives of the present study are i) to validate the design of the cofferdam; ii) to assess dam safety if only partial excavation of the plunge pool is carried out; iii) to investigate the role of the cofferdam in energy dissipation, and iv) to identify relevant hydrodynamic loads for the design of the fault treatment structure.