Control of the Structural Security

The control of structural security is intended to:

• Obtain knowledge about the behaviour of the structures, namely the dam, the slopes and the underground structures (the power station, the transformer room, surge shafts and tunnels/galleries).
• Guarantee the structural security of the undertaking.

To this end the following are taken into account:

• Recommendations of international legislation
• ICOLD (International Commission of Large Dams)

Technical aspects of the Cahora Bassa Dam

Double Curvature Arch Dam with:

• 170 m maximum height
• 303 m length of the crest
• 23 m thick at the base and 4 m at the crest

• 8 radial flood gates half way down and 1 surface spillway. Their total capacity is14,000 3/s

Underground power station, on the south bank, with:

• 5 generator groups each with a capacity of 415 MW
• Maximum turbine flow: 2,250 m3/s

Dam lake with:

• Maximum volume 65x109 - useful volume 52x109 m3
• 270 km long - 30 km wide
• Maximum flooded area – 2,900 km2

Forecast for North Bank Power Station (Project presented in 2002)

Main comments on the Structural Security of the Dam

• The Cahora Bassa dam and appurtenant works are now provided with a thorough monitoring system considered as adequate for diagnosing and follow up the main behaviour scenarios.
• The behaviour of the dam is almost well known and satisfactory, as published else-where, and this is the main achievement of the last years efforts and an incentive to proceed.

Comments by our consultants

• • “...the security of Cahora Bassa is overall highly satisfactory” (Hidrotécnica Portuguesa, Designer of the Cahora Bassa Dam, Lisbon, Portugal – Report of 2001).
  • “The structural and hydraulic behaviour of the Cahora Bassa dam and of its foundation, analysed on the basis of the data of the monitoring system since the lake was first filled, on regular visual inspections, and on the countless specific studies held, has proved satisfactory” (National Civil Engineering Laboratory, Lisbon, Portugal – Report of 2002).
  • “A team of six dam safety and water management experts from the Corps of Engineers were sent to southern Africa in March. They evaluated the structural integrity of dams in the Zambezi River Basin and possible impacts on humanitarian operations...The team found the dams well run in good operating condition. There were no structural problems. The instrumentation at Cahora Bassa Dam in Mozambique was impressive...The dam was in very good working order with no signs of any problems. It was professionally operated and administered. The dam displays no signs of having any problems in the near future” (USACE-United States Army Corps of Engineers, Vicksburg, USA, 2000 Flood Relief Report).

“After 30 years, deformation of the dam wall is within expectation. Plastic deformation (creep) due to swelling of the mass concrete caused by alkali-aggregate reaction (AAR) remains a concern. However, additional monitoring instruments have recently been installed to collect data on vertical wall displacement. The Consultant recommends continued monitoring and research of the movement of the wall due to this phenomenon. The dam wall is well instrumented and the instrumentation system has been maintained and monitored in an exemplary manner by HCB since the dam was completed in 1974. Since 1995 the dam monitoring and instrumentation system is being progressively upgraded and identified deficiencies prioritised and corrected” (NIPPON KOEI, Tokyo, 2006 Due Diligence Report).