The proposed steel coffer dam was a U-channel structure bolted to a trapezoidal concrete base with steel plate gates slotted in. It was envisioned that a PVC liner would also be used to improve the seal. Manufacture of the structure in the Eskom workshop would have taken approximately 3.5 days while erection on site was estimated to be an eight-hour task for a team of six people. Securing the superstructure installation would have involved drilling being carried out in 0.5 in of water with zero visibility and sealing would have been problematic given the uneven profile of the canal.

     Eskom then approached local company Dura-Dams, who specialize in innovative hydro solutions, to propose an alternative system.

     Dura-Dams suggested using an AquaDam®, patented by an American company called Water Structures Unlimited. Widely used in the United States for many water controlling applications, AquaDam® consist of an outer polypropylene tube with two polyethylene inner tubes that are inflated with water. The outer tube confines the inflated inner tubes and the internal friction forces, with the inherent weight, prevents the AquaDam® from rolling or sliding when an upstream water pressure is exerted. The flexibility of the polyethylene means that the AquaDam® can be installed on uneven sedimentary and concrete surfaces, yet still seal effectively.

     AquaDams® come in different sizes to impound different upstream water depths ranging from 200 mm to 1,270 mm but for this application a 900 mm high unit was recommended which could impound a maximum head of 700 mm. The anticipated water depth was only 500 mm but this provided some factor of safety for fluctuations in the tailrace reservoir which could result from summer storm run-off.

     Due to the rapid installation of AquaDams®, it was decided by Eskom to use this innovative product for the first time in South Africa. The 900 mm unit was bought and Dura-Dams, with Water Structures Unlimited, agreed to supply one larger 1,225 mm AquaDam® free of charge as part of their marketing launch and also to allay some concerns about safety.

     For the actual outage, a draw down of Kilbum reservoir started on November 4, 1994, using only three turbines as the fourth turbine was under repair. The turbines were switched off in a staggered sequence to prevent erosion of the unlined section and the minimum static water levels were followed until only one turbine was operational. The draw down took approximately 65 hours (as planned) and installation of the AquaDams® began shortly thereafter.

     Installation of the AquaDams® was supervised by Dura-Dams. The 900 mm AquaDam® was installed first in the 16.6 in wide tailrace canal, approximately 2 in upstream of the control point.