Securing the inner and outer tubes on the ledge above the canal, the rolled up AquaDam® was lowered into the canal and the inner tubes inflated using two small portable pumps. The AquaDam® was restrained from unrolling so that the head of water inside the inner tubes was always higher than the reservoir water level. This was necessary to ensure that the AquaDam® followed the profile of the canal and created a good seal, particularly important at the corners which are at 70 degrees to the horizontal. The AquaDam® was secured with sandbags on the opposite ledge and then inflated to its full height.
An identical procedure was followed for the installation of the 1,225 mm AquaDam®, which was positioned approximately one meter upstream from the first unit.
Installation of both units was completed in one hour using four people and two portable pumps. One throttled turbine unit was then used to pump out the fore bay.
Despite the uneven concrete surface of the tailrace canal, the AquaDams® sealed very effectively. Seepage underneath the length of the AquaDams® was trapped and drained into the rock trap below. This was emptied by the two Flygt B S220 submersible pumps operating for ten minutes every three hours.
The project team proposed using sky climber platforms to work on the screens, as opposed to conventional scaffolding, in a bid to save time. The proposal was accepted and four sky climber platforms were erected on the fore bay headwall, two per side. All old bolts were removed and replaced with new M16 bolts and new M20 bolts were used to secure the renovated screens. The M16 bolts, which had sheared, needed to be drilled out to M20 and new locking plates were installed on all the bolts before tack welding and paint repairs were carried out. The bolts were grade 8.8 high strength steel and galvanized to protect against corrosion. The repair work to the tailrace screens took six days to complete.
It was fortunate that the two AquaDams® had been installed. While work was being carried out on the tailrace screens, a high wind came up and pulled the inner tubes of the 900mm AquaDam® off the parapet ledge of the canal. The inner tubes began to deflate and it deformed badly, losing its effective seal. Efforts to restore it were unsuccessful and the unit was removed.
Sometime after the 900mm AquaDam® had been removed, it was noticed that seepage water pumped back from the rock trap, as well as the river inflow into the reservoir, had caused the upstream water depth to rise to approximately 850mm, which was considerably higher than the design depth of 500mm and the recommended maximum of 700mm. The increased water depth caused the AquaDam® to roll forward 1.5m and the Kilburn Reservoir sluice valve had to be opened to reduce the dam level.
Once the contractor had removed his plant and personnel, the tailrace fore bay was flooded until the water level on both sides of the AquaDam® was similar. The 1,255 mm AquaDam® was then deflated and pulled out. Neither of the units was damaged during recovery and they were rolled up and stored for future use.
The application of the AquaDam® was very successful and contributed significantly to the Drakensberg Pumped Storage Scheme being handed back to Eskom National Control three days ahead of what was already a tight schedule.
With local manufacture of the Water Structures® secured, Dura-dams says that it is confident of many new and interesting applications of this innovative product.