For many years, large areas in the Eastern Cape experienced severe water shortages with the result that thousands of hectares of irrigable soil which previously obtained water from one or other of the dams in the region could not be irrigated. 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 Orange-Fish Tunnel was completed in 1975 and is the key structure by which water is delivered from the Gariep Dam, to the Teebus Spruit and the Great Brak River and from there to the valley of the Great Fish River and the Sundays River valley. With a length of 82,8 km, the 5,35 m diameter tunnel is the longest continuous enclosed aqueduct in the southern hemisphere and the second-longest water supply tunnel in the world. The main purpose of the tunnel is to divert water to the Eastern Cape for irrigation, urban and industrial use. Over 200 000 m3 of concrete was used to line the tunnel which has a maximum capacity of 54 m3/s.

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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 to allow complete de-watering of the tunnel for routine maintenance.

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