If the “flood-prone” Ardgowan Dam were to fail it would result in an inland tsunami flooding the Oamaru CBD, with expected deaths, Waitaki district councillors have been told. Infrastructure manager Joshua Rendell issued the stark reminder of the nightmare scenario on Tuesday in the wake of flooding in the district last month. He said if the 140-year-old dam was built today, it would not be built where it is “because of the risks to human life”. Options were being considered to potentially move storage elsewhere, so the operating level of the dam could be reduced, but this was a longer-term project. Mr Rendell was giving a floods recovery update at an assets and infrastructure committee meeting on Tuesday. The dam was classed as “flood-prone” with a “high” potential impact classification under dam building safety regulations, which came into effect in 2024, he said. Following the introduction of these regulations, the council was required to take steps as a “dam owner”, including investigations by engineers to understand the potential causes and impacts of failure. The council has prepared for three potential scenarios — overtopping, an earthquake and issues with the outlet pipe, the likelihood of which were described as “very low” in a report on the council’s website. Mr Rendell said if the dam were to fail, “it would flood to Oamaru central CBD in about one and a-half metres of water, moving at quite a rapid pace”. “We’d be looking at an inland tsunami coming down Oamaru Creek.” A number of houses were in the “flow path”. There would be “expected deaths” if it was to occur. Therefore steps needed to be taken to make sure the dam was preserved, Mr Rendell said. Natural and built environment director Roger Cook said officers had to lower the dam level as part of the dam safety plan ahead of weather events as it was not a "quick" process. "It's a very fine balancing act but nobody wants to be responsible for the dam overtopping and the dam failure." A conserve water notice was put in place last month after heavy rain meant the council was unable to pump water into the Oamaru supply due to its turbidity, leaving the stored water at Ardgowan Dam as the only available source. This source was “vulnerable” during severe weather and a new delivery system, piping water from a secure pond on the Waitaki Plains, which had been described as a priority by the council, expected to be in place within two years. Public meetings were held in November last year organised by the council and the Emergency Management Otago to advise local residents and businesses to be prepared for “catastrophic” scenarios associated with the dam failing. The Otago Regional Council identified the structure in Reservoir Rd as a “dangerous dam”. “Risk reductions measures are being planned and undertaken by the council. “However, there could be potential impacts in a flood event,” its website states.
