Central Coast Council recently performed a high-risk inspection of the Mangrove Creek Dam diversion tunnel that was anything but routine.
The diversion tunnel, which is over 30 years old, is 415 m long with a diameter of 2.3 m. The last person-entry occurred during commissioning in 1989. While the Central Coast Council’s current team are well experienced in isolation maintenance, given the timeframe since last entry of this diversion tunnel, they were presented with some challenges to deal with.
Planning for the inspection was carefully done over a number of months to ensure the safety and success of the team, as well as ensuring that the four-week isolation would not adversely affect water treatment and systems operation.
Complications arose when, between the early stages of the project and the beginning of tunnel isolation activities, the dam level rose from 53 per cent to a record 91 per cent due to unprecedented weather. The team was forced to adapt.
According to the Central Coast Council’s Director Water and Sewer, Jamie Loader, the increased dam level meant that the team could not utilise a diver for the process of operating the isolation valve that would allow the tunnel to be drained; the valve was to be accessed via the dam’s intake tower.
“The only other option (other than Navy Seal Divers), [was] the lowering of a remote operating vehicle (ROV) to a depth of 70 m to undertake the work,” Mr Loader said.
Observations during the off-site trial with the ROV allowed the team to make necessary modifications before it could be used for the later trial within the intake tower.
The bulkhead was inspected to ensure it would properly seal and allow the draining of the tunnel. Once drained the tunnel was physically inspected by the team.
“Only two 10 mm defects were identified in the Steelshield coating under the dam wall, as well as a leak through a small crack in the concrete-lined section,” said Mr Loader.
The defects were repaired to minimise the risk of larger repairs being necessary at the next inspection, and a second downstream filler valve was installed, enabling future isolations without having to enter to intake tower.
The success of the endeavour marks an important precedent for the Council.
“Not only have we now introduced a precedent to use robot technology, the team involved say they have learnt so much about this important infrastructure and how to isolate it safely and effectively,” Mr Loader said.
The work was completed, and the dam returned to service at the beginning of July. Equipped with the experience and information afforded by the inspection process, Council will be able to maintain the dam infrastructure well into the future.
For more information, visit Central Coast Council’s website.
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