The 128-meter-high Kariba Dam is one of Africa’s biggest. Operated by the Zambezi River Authority on behalf of Zimbabwe and Zambia, it has been a cause for concern on a number of safety issues, including from its earliest days when the filling of its reservoir caused earthquakes, to more recent times when rumors began to surface that the huge dam had structural problems and suffered from poor maintenance. Kariba has also worsened the region’s floods in recent years.
The filling of what was then the world’s largest reservoir was followed by considerable earthquake activity. The weight of Kariba’s reservoir translates into a mass of 180 billion metric tons. The reservoir is located in a tectonically active area, at the southern end of the African Rift Valley. Since its construction and filling in the early 1960s, Kariba has caused numerous earthquakes in the area, 20 of them larger than magnitude 5 on the Richter scale. Project documents did not discuss the possibility of reservoir-induced seismicity and the need to take this into account in the design of the dam, so the seismic activity’s affect on the dam’s safety is unknown.
After the devastating March 2000 floods in Mozambique, the US Army Corps of Engineers was requested to do site visits at three Zambezi dams, including Kariba. The team was given full access to Cahora Bassa and Kafue dams, but not Kariba. “Though our mission was to inspect the dam, this was not possible due to the reluctance of the Authority to share operational data with the Team,” notes the Army Corps’ final report. “The inability to obtain operational data seriously hampers informed consideration of the dam and raises serious questions concerning the operation of Kariba.” Most recently, emergency releases from Kariba to prevent damage to the dam from rapidly rising waters led to flood alerts and evacuations for thousands of downstream dwellers
Kariba, like many dams, has been affected by a condition that mars its concrete, known as “alkali-aggregate reaction.” While regional concern about the dam’s condition is widespread, little is known about the extent of the problem or its implications for dam safety. The US Army Corps noted in its 2000 report, “the severity of this reaction was not disclosed.” The Corps report notes, “With the limited information provided, no conclusive assessment can be made of the hydrologic and hydraulic conditions, structural stability, or the overall dam safety aspects.” The World Bank reports that inspections of Kariba are carried out every five years by the engineering firm Coyne and Bellier, but these reports are not made public. A Zambezi river expert who lives downstream of Kariba notes that concern about the dam’s safety arises especially during large floods, because “the dam walls vibrate when several or more sluice gates are open and pass large discharges.” He adds, “The dam operators adamantly deny that there are any problems, and it is certainly in their interest to make sure the walls are stable – so I hope they are doing so!”
Africa Dams at Risk from Earthquakes, BBC, May 9, 2002
Zambezi Dams at Risk from Earthquakes, JA!, March 14, 2006
Download the US Army Corps of Engineers’ report (March 2000) on Zambezi Dams’ safety
Download a detailed case study on Kariba submitted to the World Commission on Dams