The town of Waipawa would suffer catastrophic damage and 1000 people would be put at risk if a proposed irrigation dam failed, according to a report considering worst-case scenarios.
The Hawke's Bay Regional Council report looks at the impact of a failure of a proposed irrigation dam on the Makaroro River, west of Waipawa.
The dam is expected to be 85 metres high, about 500m long and 6m wide. It would consist of rockfill with a concrete face on the upstream side, and would hold about 91 million cubic metres of water.
The report does not consider what would cause a failure, but said that if the dam failed its water could discharge at a rate of up to 45,000 cubic metres a second.
By the time the flood wave had entered the Waipawa River and reached Waipawa 2 to 3 hours later it would be flowing at 10,400 cubic metres a second - eight times the level expected in a 100-year flood.
Much of Waipawa would be under three to five metres of water. The flood wave would take about 13 hours to reach the coast at Haumoana, 116km from the dam.
The further it travelled the less damage it would cause, but it was likely to cause "significant to severe" damage to about 16 bridges, two sewage treatment plants and many kilometres of roads.
"The actual consequences of this type of failure are likely to be costly repairs and rebuilding," the report said. "However, the actual disruption to the population was likely to be limited to the towns of Waipawa and Waipukurau."
It estimated at least 50 houses would be flooded to depths of 3m and about 373 houses would be flooded to a depth of more than 50cm. About 1000 people would be at risk, nearly all of whom would be in Waipawa.
The report stresses that the dam would be designed and constructed in a way that meant the likelihood of failure was extremely small. The regional council believed the $230 million dam would improve the water quality of the Tukituki River as well as provide irrigation to 25,000 hectares.
It could boost the region's GDP by $320m and create up to 2600 jobs. A discussion document is out for submissions until October 5.
- © Fairfax NZ News
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