Osborne dam on the Odzi river

Name of Project:

Osborne dam on the Odzi river



Country:
Zimbabwe

Client:
Ministry of Lands, Agriculture and Water Development Department of Water Development

Construction period:
July 1991 – July 1994

The Osborne Dam was designed to store water during the rainy season in order to offset drier periods.

Some of the water is used to supply water to the city of Mutare and other small towns in the area, but most of it is for the irrigation of about 12,000 hectares of land.
The plant consists of the following major works:

  • main embankment of loose material blocking the Odzi river;
  • a secondary levee;
  • a tunneled morning glory spillway;
  • a return system consisting of an intake tower, a pipeline in the body of the dam, and a discharge chamber.


The main dam, which is 75 m high, is a layered embankment with a clay core and earth-fill transition zones, the upper one protected by rock fill and the lower one with grass cover, with the exception of the downstream toe made of rock.
The crest length is 1,010 m and the width is 8 m.
A grout curtain was made with a single line of holes along the foundation cutoff wall line, which went to a maximum depth of 18 m.
The secondary dam in the right-hand portion of the reservoir is a homogeneous earth embankment that is 10 m high and 650 m long, protected by rock on both sides, since it must also serve as an emergency spillway. For this reason, the center portion was left 1.6 m lower than the main dam over a length of 500 m.
The spillway is housed in a crag on the left shoulder. The morning glory hole is 17 m in diameter and 40 m deep; it connects to a well and a tunnel measuring 6 m in diameter, lined with concrete.
The tunnel, which is 260 m long, empties into a short canal which ends with an energy absorber.
The downstream return for irrigation, industrial, and household purposes is controlled by valves and pipes with a diameter of 900 mm, installed in the intake tower, 73 m high, with a hexagonal base, which rises on the lake at the upstream toe of the dam. The water passes beneath the embankment through an artificial tunnel 330 m long, with a horseshoe cross-section measuring 3.4 m² and returns to the river after climbing over a 3 m threshold.
Access to the intake tower is provided by a 150 m bridge which reaches the tower at the crest of the dam.
The start date of the reservoir coincided with the beginning of the rainy season in November 1993, the month in which the embankment of the dam body had reached an elevation of 1090.
The peak workforce occurred in the period of May 1992 to November 1993 with 630 people almost continuously present.
There was a maximum of 28 expatriate employees, whereas during the peak period, subcontracted work required the use of more than 400 people.

MAIN TECHNICAL DATA:
Embankments: 5,660,000 m3
Surface excavation: 606,000 m3
Underground excavation: 15,000 m3
Concrete volume: 18,000  m3
Maximum capacity of the reservoir: 400,000,000 m3
Reservoir surface area: 26,000 km3

Last update: 21/02/2018

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