Dam safe but work needed say owners
More contingency work is required on the Opuha Dam as the dam company works through the ramifications of its internal erosion scare.
In October the company Opuha Water issued a ‘‘dam safety deficiency’’ report following discovery of signs of possible internal erosion in the dam.
Initial engineering reports suggested there was no immediate risk to the public from a sudden dam failure and a report from international dam safety expert Dr Chi Wan Fan has confirmed that assessment.
‘‘Dr Wan’s initial report has been received. He concludes that the original risk assessment by our own engineers [Tonkin + Taylor] was accurate and there is no immediate need for concern,’’ Opuha Water chief executive Andrew Mockford said.
Mockford said Opuha Water is now working to develop a better understanding of the internal erosion risk.
‘‘At this stage, we’re updating our shareholders and stakeholders. The sooner we know the best approach, the sooner we can let everyone know what else we’re going to do.
‘‘This process will take some time, so the reduced lake level and increased monitoring of dam safety will continue.’’
Mockford said remediation options could include installing a new membrane made out of asphalt or PVC, or installing a grout curtain, which is a barrier that protects the foundation of a dam from seepage and can be made during initial construction or during repair.
‘‘But that’s further down the line. However, addressing the situation remains a high priority for us, we’re working as quickly as we can.’’
Mockford would not give an idea of how much the process had cost Opuha Water so far.
‘‘But we’re happy with the level of expenditure, it’s a complex body of work,’’ he said.
‘‘There are a number of options out there . . . the key thing is there is no immediate safety risk. We don’t have to push the panic button just yet.’’
Mockford said a precautionary approach had been taken, reducing the risk even further by lowering the lake level and increasing real-time surveillance.
He said it had been keeping the lake level at 90 per cent of its ordinary capacity, as it works through the options.
‘‘We lowered the lake to a level which would be comfortable for any subsequent work.
‘‘Over the past week in the wake of the floods, the levels have gone up again, but we’re slowly lowering it back to a suitable level.’’