Dam break analysis report angers residents

The Ruataniwha Dam is planned downstream of this location in the Makarara river valley in Central Hawkes Bay. Photo / Paul Taylor

Local residents are infuriated by a dam break analysis report on the proposed Ruataniwha Dam in Central Hawke's Bay.

Half of Waipawa's population of 2000 would be put at risk if the proposed $600 million dam was to fail in an earthquake, the Hawke's Bay Regional Council report states.

Failure of the main dam after construction, assuming the reservoir is full, would likely result in significant damage to infrastructure including bridges, roads, stopbanks, and sewage treatment plants, and environmental damage to the river corridor and surrounding floodplain.

The resulting flood wave from a dam break would overtop the stopbanks at the town of Waipawa, and water depths in the area of Bibby Street near the Waipawa sewage treatment works would likely be in the order of two to four metres.

Due to the potential risks, along with the size of the main dam, the potential impact category of the main dam is classified as high, the report states.

"Accordingly, the dam is designed to reflect this standard."

Waipawa resident Paula Fern told Radio New Zealand she was shocked when she read the dam break risk analysis while researching the dam's potential environmental impacts.

"Our community is not being informed ... we've been let down ... not only [by] the regional council because they've sat on this information for quite some time, but also [by] the district council because I'm sure that they know.

"We are the ratepayers and our wellbeing should be the top of their priority."

Approximately 373 houses were at risk of being inundated in the event of a dam failure, Ms Fern said.

"It is built within 1km of the Mohuka Fault, the reservoir is going to be directly on top of the fault - and that can actually cause an earthquake as well.

"The Mohuka Fault is a very active fault and they're expecting a large earthquake to come from it within the next three to 500 years."

If the 83 metre high dam wall were to collapse, it would sent 90 million cubic metres of water towards Waipawa, potentially causing major damage to the town's infrastructure and putting half of its population of 2000 at risk.

Central Hawke's Bay mayor Peter Butler told Radio New Zealand he could see nothing but positives from the dam proposal, however he conceded the council had not done enough to inform residents of what might happen in the event of a catastrophic dam failure.

"I just presumed it was public knowledge because I've read about it."

The report stated there would likely be a minimum of two to three hours warning time between the time of start of failure, and the time when the population and infrastructure of Waipawa and Waipukurau would be at risk.