Orange–Fish River Tunnel
For many years, large areas in the Eastern Cape experienced severe water shortages because of little rainfall in the arid Karoo. The situation was aggravated by the reduction in capacity of many of the existing dams due to heavy silt deposits.
The Orange-Fish Tunnel, together with its network of canals, weirs and balancing dams, has enabled these areas to be restored and has made the irrigation of thousands of hectares of additional land possible. The main purpose of the tunnel is to divert water from the Gariep Dam to the Eastern Cape for irrigation, household and industrial use.
The inlet tower at Gariep Dam at Oviston: the name Oviston is an acronym based on the Afrikaans Oranje-VISrivier TONnel. After traversing under the Suurberg mountain plateau, it releases the water to the Teebus Spruit (tunnel outlet at ), to the Groot Brak River and onwards to the valleys of the Great Fish River and the Sundays River.takes water from the
Construction started in 1966 and the tunnel opened in 1975. When completed, the tunnel's length of 83 km was the longest continuous enclosed aqueduct in the southern hemisphere and the second-longest water supply tunnel in the world. Over 200 000 m3 of concrete was used to line the tunnel which has a maximum throughput of 54 m3/s (about 2,000 cusecs).
It was engineered in association with Messrs Keeve Steyn and Partners of Johannesburg for the South African Department of Water Affairs. Former Halcrow senior partner, Sir Alan Muir Wood, the "father of modern tunnelling" worked on many of the world's leading tunnel projects, including the Orange-Fish Tunnel.
The intake tower is situated on the south bank of the Gariep Reservoir at Oviston, approximately 19 km upstream of the dam wall. Seen from above, the intake tower is shaped like a four-leaf clover with each leaf containing an inlet gate - all at different levels. In this manner, water can be drawn from different levels to help control the water quality. Each of the four inlets can be sealed off to allow complete de-watering of the tunnel for routine maintenance.
- Doyle, Barry R. (2001). Hazardous gases underground: applications to tunnel engineering. CRC Press. p. 201. ISBN 978-0-8247-0483-4.