Dam Break Report

We operate three flood detention dams – the Huatoki, Waimea and Mangaotuku dams. These dams were built in the 1980s and have been successful in reducing the frequency of flooding in urban areas along these streams and in New Plymouth’s CBD.

As part of the Government’s dam safety regulations, the owners of dams are required to classify their dams as either high, medium or low potential impact in the event of failure. Our flood protection dams fall into the high-potential impact category due to the presence of urban areas downstream of them. 

The Government’s regulations also require the owners of all high-potential impact dams to prepare an emergency action plan, which includes examining what the effects would be if a dam were to break.

Flooding after the breaking of one of our dams is absolutely a worse-case scenario, and the chance of it happening is remote. It would require:

  • The dam breaking apart, such as from the effects of a large earthquake; and,
  • The dam being full at the time.

The chances of both of these occurring at the same time are very remote. It is worth noting that the Huatoki, Waimea and Mangaotuku dams have never been full in the nearly 30 years since they were built.

Council staff monitor the dams every month, and third-party dam experts are engaged to undertake annual inspections and five-yearly comprehensive dam safety reviews. These inspections confirm that the dams are structurally sound.

We have documented trigger levels to initiate increased monitoring if the dams ever get near to full.

Our modelling of flooding that would occur if a full dam failed, has identified those properties that would receive 500mm or more of water on any part of their property as well as those that would be isolated for a time on a rise of land.

The Dam Emergency Action Plan will become part of an Incident Response Plan that addresses how we will respond in other emergencies, which is part of our overall strategy to be prepared for any emergency in the district.