Concerns About Kariba Dam’s Stability

By: 
Rudo Sanyanga

The Kariba Dam holds back Africa’s biggest reservoir. Millions of people live downstream of it in the Zambezi River Basin. The safety of the 53-year-old dam has been called into question a few times, for various reasons.

Multi-beam bathymetric photo of the Kariba Dam plunge pool.
Multi-beam bathymetric photo of the Kariba Dam plunge pool.
Source ZRA ppt, 2012

Most recently, at a meeting of dam operators in July 2012, engineers from the Zambezi River Authority (ZRA) revealed that the plunge pool below the Kariba Dam has deepened beyond expectation: it has now eroded to a depth of more than 90m into the rock substrate. The plunge pool is the area where the water is released after going through the dam’s spillways. The main concern is not the depth of the plunge pool, but that it has been eroding towards the dam wall, with the likely possibility of undercutting the foundation of the 128-meter-high wall. This is of great concern, as an unstable foundation can lead to dam failure, a potentially catastrophic event for the hundreds of thousand people living downstream of the Kariba Dam.

Kariba holds back over 64,800 Mm3 of water – about a year’s worth of the Zambezi’s flow. The design has six spillway gates that release water at the rate of 9000m3/s when the reservoir levels go above the maximum flow stipulated by a “rule curve.” At most times the spillway gates are opened in advance of predicted floods for dam safety reasons.

In the July presentation, ZRA noted that dam releases eroded approximately 150,000m3 of rock between 1962 and 1981, when the 19th dam-safety survey was done. Since 1981 there have been no significant changes due to limited spilling during this 30-year period. With the current situation, the stability of the plunge pool will be affected if the spillway is operated with more than three gates open at the same time. The need to open the gates may be necessitated by large floods resulting from heavy rainfall events in the Upper Zambezi catchment – a condition that is likely to increase with a changing climate.

ZRA has engaged consultants who have proposed a number of options to address the problem. The preferred option is one that will excavate and widen the hole in order to dissipate the energy from the spillways, thus removing 295,000m3 of rock. The bigger plunge pool will reduce the excessive turbulence that scours the hole. This work will involve building a cofferdam 18-20m downstream of the plunge pool, dewatering and blasting the pool area in order to widen it. The work will cost millions of dollars and will take over a year to complete. ZRA said they prefer the work to be done during the nine-month dry period and that generation of electricity will continue during that time. A second opinion regarding methods and estimated design is being sought before the work can begin, possibly as early as next year.

Questions about the dam’s safety in regards to the plunge pool issue include:

  • Will the vibrations resulting from plunge-pool blasting damage the dam wall or compromise its stability?
  • How effective will the new plunge pool be in limiting future damage to the 128-metre wall?
  • What do experts believe is the remaining life span of the dam?
  • And how will the current situation affect flood management?

Kariba Dam operators have been advised to open only three spillways in an event of a flood, to alternate the gates that they open, and not to open Gate 6 at all. The downstream Cahora Bassa Dam is far smaller and will not contain the flood from Kariba. How are they going to cope with huge floods in the face of climate change? How safe will the proposed Mphanda Nkuwa Dam in downstream Mozambique be? What kind of evacuation plan would be needed should the dam fail in some way, and is that plan in place?

We’ll be monitoring this situation and reporting on any new developments.

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