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Cheap design linked to problems at Tokwe-Mukosi

By Nelson Chenga

A decision to change the initial design for the Tokwe-Mukosi Dam could be behind the present structural challenges now threatening the multi-million dollar reservoir.

Cheap design linked to problems at Tokwe-Mukosi
Cheap design linked to problems at Tokwe-Mukosi

The Financial Gazette can reveal that while a recent risk assessment ruled out the possibility of the dam wall collapsing, the conundrum dramatically playing out at Tokwe-Mukosi has everything to do with a 1992 decision to change the dam’s design in order to cut costs.

The dam is being constructed by Salini Construttori, an Italian construction company, that has built more than 20 large dams in three continents. With the lake fast filling up due to well above normal rains currently pounding the drought-prone Masvingo province, water is now seeping through the dam wall, heightening concerns that if the seepage persists it could further weaken the massive structure.

Fears are that if the surging waters beat the builders on the race to the spillways, the massive water volume would spill over the wall and wash away the dam wall. Reports are that Salini Construttori is building the dam wall at a rate of one metre per day, the same rate at which the water is rising in the lake.

The Financial Gazette can reveal that the designers of the initial dam wall, Coyne and Bellier, had proposed a concrete arch dam wall with crest overflow which would have cost the country Z$641 million as at October 1992.

“This was considered excessive and so the Designs Division of the Department of Water Development undertook a thorough investigation into the design report from Coyne and Bellier and came up with a cheaper alternative, in the form of a concrete-faced rock fill dam,” reads part of a 1998 report on the dam.

When complete, the dam rock fill wall will stand at 90,3 metres high and 8,5m wide, containing 672 000 tonnes (or 1,9 million cubic metres) of rock. A 0,55m thick concrete face veneer with a volume of 15 270 cubic metres is part of the dam wall.

The colossal stone wall was designed to hold back 1 751 780 megalitres of water from a 7 120 square-kilometre catchment area. This makes the lake 30 percent larger than the country’s current biggest internal lake, Mutirikwe, also in Masvingo. The Tokwe-Mukosi reservoir, at a maximum depth of 82,7m and mean depth of 18,7m, will have a surface area of 96,4 square kilometres, compared to Lake Mutirikwe’s maximum depth of 56m and a mean depth of 14,5m.

The dam has twin drop inlets (16,6 metres in diameter) and tunnel (six metres in diameter) spillways that will each discharge 1 090,0 cubic metres of water per second along each of the approximately two-kilometre long tunnels. The outlets each have a diameter of two metres. Constructors are currently racing against the rising waters which they hope to beat to get to the spillways.

Work at Tokwe-Murkosi, which will have a power plant that is expected to generate 12 mega watts (MW) at peak and 6MW at lowest peak, began in June 1998 at a cost of just under Z$400 million. However, 16 years later, 12 years behind schedule, the dam is still incomplete and shrouded in serious controversy.

Completion of the dam, with four additional saddler dams to compensate for topographical constraints, is now literally five decades behind schedule, having been conceived way back in 1965 when the preliminary report was prepared by the then ministry of water development.

Coyne and Bellier were then commissioned to study the dam design that included a hydroelectricity power station and the company issued their report in September 1967.

However, the project was shelved due to the volatile political situation in the country as the indigenous population began to agitate for majority rule. The project was then revived soon after Zimbabwe gained independence in 1980 with Coyne and Bellier undertaking further studies and reviews of the dam design in 1985.

A detailed report with updated costs was submitted in December 1985. The ministry of energy and water development undertook further design studies in 1989, completing its report in 1990. However, the shape of the arch dam was unsatisfactory, which called for more sophisticated stress analysis that saw Coyne and Bellier, through French government funding, undertaking more extensive feasibility studies whose results were availed in 1993.

The report included studies of the arch dam design, irrigation works, the hydroelectric power plant, the water conveyance system, land preparation costs, proposed crop patterns and the economic analyses.

The completion of Tokwe-Mukosi will result in the development of five irrigation schemes, Tokwane North, Tokwane South, Hippo Valley, Runde South and Matibi II with Matibi II to be located south of Runde River billed to be the largest small-scale irrigation development in Zimbabwe. Financial Gazette