~ South African Construction World ~
To facilitate planning, a partial dewatering exercise was carried out to determine the actual extent of the damage and to remove the tailrace screens that had fallen out for closer inspection and repair.
The partial dewatering exercise showed that it was possible to draw down the tailrace Kilburn Reservoir by 29.6 million ml in approximately 65 hours by using the three turbines in pump mode. However, to draw down the reservoir an additional 0.8 million ml to the control level in the tailrace canal would take a further 162 hours using one throttled turbine (provided, of course, that the turbine experienced no problems). This was not acceptable and therefore a coffer dam was needed at the control point in the tailrace tunnel (to retain a 500 mm head) to limit the draw down time to 65 hours.
The proposed sandbag solution would have required 2,900 sandbags and the estimated time of installation was 12 hours. One drawback was that the plastic covers used on the sandbags provide a fairly low resistance to sliding, and would thus have posed a possible threat to the stability of the coffer dam.
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