Oamaru could be inundated with a tsunami-like wall of water, leaving the homes and the town centre uninhabitable for a year, should a catastrophic earthquake break the Ardgowan Dam.
That is the scenario local residents and businesses are being advised to be prepared for during a series of public meetings organised by Waitaki District Council and Emergency Management Otago this week.
The dam, which has existed for 140 years, has a high potential impact classification and could potentially fail due to over-topping, earthquakes or other significant events.
‘‘The dam itself has a high PIC which is a high potential impact classification, which essentially means that if it failed for any reason it could cause loss of life or catastrophic loss of infrastructure,’’ Waitaki District Council infrastructure manager Josh Rendell told residents at a public meeting on Tuesday.
While the council is able to control and limit potential for over-topping during rain or tsunami events for which there would be a warning, Mr Rendell said impacts from an earthquake were out of its control.
If there was a dam failure during an earthquake, ‘‘an awful lot of water’’ would rush down Glen Creek and Glen Warren Reserve ‘‘very quickly’’ - up to 4m in places around the creek, and within one hour water would cover the area around State Highway 1 and Humber St and Chelmer St, near the Oamaru Public Gardens, at a depth of about half a metre.
The area covered by water would be very similar to that estimated to impacted by a tsunami in the council’s tsunami modelling.
However, in the event of a dam break, 160,000 cu m of untreated water would take out the Oamaru water treatment plant for at least a year.
As an earth dam, the Ardgowan dam is yet to be assessed for its ability to withstand an earthquake.
Mr Rendell said the council was in the midst of that process.
The meeting highlighted the importance of community resilience, business continuity planning and understanding evacuation procedures.
Emergency Management Otago emergency adviser Danny Fountaine emphasised the need for community preparedness, recommending that residents and businesses in the area have seven days of supplies, create emergency plans and establish communication methods.
Given there were contingencies in place for most scenarios he said there was ‘‘not nearly as much to fear’’ as it might sound.
Mr Fountaine suggested having grab-and-go bags, backup communication methods like battery-operated or wind-up radios, and staying informed through various channels including social media, emergency management websites and local radio stations in the event of an evacuation order.
Any potential evacuation would be ordered through an Emergency Mobile Alert, local emergency services and all other available channels — including the council’s social media and website.
All residents or visitors should evacuate on foot.
The new Building (Dam Safety) Regulations 2022 took effect on May 13, 2024 and initiated a series of steps for the council to take as the dam owner, including commissioning of detailed investigations by specialist dam engineers to understand the potential causes and impacts of dam failure.
Waitaki District Council natural and built environment director Roger Cook said while the risks existed, proper monitoring, planning and communication could help mitigate potential emergencies.
The council also has stringent monitoring, water controls and emergency plans in place.
‘‘The dam is basically a pump storage scheme in that it’s very controllable [in regard to] the amount we can put in. So, in terms of our civil defence preparedness, we are constantly monitoring the weather forecast and rain predictions and things like that, and we will lower the dam level when a high rain event is forecast.
‘‘It’s got the capacity to take the inflows from the sides because we just stop filling it out for a while and it’s quite a controllable environment. It’s not like a river dam where the headwaters just keep piling in.
‘‘We are very confident that we’ve got it managed and the alternative is unthinkable, really.’’
