Extreme event would flood 1000 homes

21:30, Jun 07 2014

About 1000 New Plymouth homeowners have been warned their properties would be left under water if the city's flood detention dams failed.

The scenario is considered "worst-case" in a dam emergency action plan the New Plymouth District Council has put together to comply with new regulations.

The plan details what would happen if one of the three flood detention dams - at Waimea, Huatoki and Mangaotuku - should fail while full, a situation the council's manager water and wastes Mark Hall says is very remote.

"Basically we're required to look at the worst-case scenario. However, the regulations do not consider the likelihood of this happening, which we know is remote because in nearly 30 years we've had these dams they've never been full. We also monitor and inspect these dams regularly and they are structurally sound."

It would take the dam breaking apart because of an earthquake or a landslide, at the same time as the dam was full for the situation to occur.

Two serious rain events within a short time wouldn't cause a problem, because the dams empty in 24 hours.


The three flood detention dams range from 11 metres to 24m in height and can take between 150,000 and one million cubic metres of water.

Hall said flood modelling identified properties which would receive more than 500 millimetres of water and owners and occupiers had been notified. Those properties' Land Information Memoranda (LIMs) were being updated.

"The remote risk to properties is the same today as it was yesterday, and the consideration of inundation during a worst-case scenario has only arisen due to the requirements of government legislation."

Water and waste operations manager Graeme Pool said the New Plymouth CBD had a history of flooding, such as in 1938, 1968 and 1971. "Flooding in the CBD is unacceptable so the council built dams to stop it."

Those dams have been very successful, as shown in 1995 when there was 300mm of rain in 24 hours, peaking at 56mm an hour, Pool said.

"There was flooding around New Plymouth, but none in the CBD and the dams were not full."

The dam emergency action plan will be delivered to the Taranaki Regional Council for approval by June 30.

Taranaki Daily News