Ardgowan Dam
History and Function
- Ardgowan Dam was constructed nearly 150 years ago, is situated at Reservoir Road
- It stores around 160,000m3 of untreated water for supply to the Oamaru Water Treatment Plant, which supplies more than 16,000 consumers in Oamaru and south to Moeraki
Legislation
The new Building (Dam Safety) Regulations 2022 commenced 13 May 2024 and initiated a series of steps for 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.
The new regulations require us to have a Dam Safety Assurance Programme and demonstrate compliance with it. We have completed the programme and are working hard to implement the required plans and processes. We are in a good position to manage events that pose a risk to the dam and prevent failure and are now working with the community to support residents and businesses in emergency preparedness.
This investigative work found that in the unlikely event that the Ardgowan Dam failed, it would have a high potential impact on homes, businesses and infrastructure.
What are you planning for?
As part of the hydrology study, potential failure scenarios were considered. There are three potential scenarios, the likelihood of these happening are very low, but we have prepared for:
- Overtopping during an extreme flood event
- Issues with the outlet pipe
- A strong earthquake
The first two of these can be monitored and handled by taking action – like lowering the dam levels before heavy rain events, draining the dam if there were outlet pipe issues, and turning off the feed pumps.
As part of the hydrology study, flood modelling was undertaken to understand the extent, depth and velocity of flood waters from a dam failure.
The following plans show flooding extents during a significant 1:500 rain event. The first plan reflects flooding that would be expected from the rain event, without a dam break and the second plan shows flooding from the rain event plus floodwaters from a dam break.
When compared to tsunami modelling for Oamaru, we can see the flooding extents are very similar, highlighting the importance of general emergency preparedness.
Flood Modelling Plans
What is Council doing?
Council has significantly strengthened its dam safety management over the past year. We know more and monitor more closely to develop comprehensive plans that include:
- Regular inspections and monitoring
- Hydrology studies and dam safety reviews
- Methods for dewatering the dam
- Emergency Action Plan developed with Civil Defence and emergency services
- Clear evacuation procedures and communication systems
- Ongoing maintenance and infrastructure upgrades where needed
The dam is safer today than it was in the past because of these enhanced processes and our increased understanding of dam management.
WDC has also set aside money for dam safety improvements. This includes increasing spillway capacity to reduce the risk of overtopping and potentially replacing pipework to prevent leakage.
In the unlikely event there was a failure?
Our plans would mean evacuating by alerting households and businesses by text, door knocking and with the support of Civil Defence and the emergency services.
Water would flow down into Oamaru through Glen Warren and have a severe impact on homes, businesses and key infrastructure.
That is why we have made evacuation plans and mapped safe routes for all potentially affected areas.
More things in place – Safer today than yesterday
- Council has worked with, and will continue to work with, key stakeholders to develop and implement the Programme and Plan
- It is important for the community to be prepared for any type of emergency event or disaster – including tsunami, earthquake and fire – and to be ready to respond as per the information available to all New Zealanders (https://getready.govt.nz)
- An evacuation will be ordered through an Emergency Mobile Alert, local emergency services and all other available channels – including Council’s social media and website
- All residents or visitors should evacuate on foot via the routes and in the direction indicated on maps for their location within the flood zone.