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There was a 90% likelihood the massive landslip damming the Young River would fail in the next 12 months, an expert hydrologist told commissioners at a determination hearing in Wanaka yesterday.
Hydrologist engineer Robert Hall, of Timaru, said the downstream consequences of even a partial failure would be significant for 200ha of prime farmland at Sawmill and Millionaire Flats at the confluence of the Young and Makarora Rivers.
However, any downstream effects once a potential 3m to 5m-high surge of floodwater and debris reached the Makarora River would be dissipated upstream of Pipson Creek and have little effect on any landowners, or business and tourist operators in Makarora township, he said.
Mr Hall was one of a group of engineering and planning consultants for Lonestar Farms Ltd - owners of Mount Albert Station - who flew to inspect the dam last week.
Lonestar wants to make an immediate start on three large gravel and rock bunds in the Makarora River bed to mitigate adverse effects of a dam breach and protect farmland at Mount Albert Station.
It is seeking consents from the Queenstown Lakes District Council and the Otago Regional Council under a seldomly invoked emergency provision of the Resource Management Act (RMA), which will allow them to begin work immediately and waive the normal lengthy resource consent process.
Mr Hall told commissioners John Matthews and Lyal Cocks people needed to be ‘‘on the ground to make an assessment of these things'', and during an inspection he felt uneasy about being on the dam.
The risk of failure was higher now than when the landslip and first overtopping of the dam took place more than three months ago, he said.
A trigger mechanism was necessary for failure to occur and this was most likely to be a significant rainfall and flooding event, or a magnitude 7 earthquake, within a radius of 50km and at a depth of 25km.
Makarora residents want the proposed bunds to be notified, so they can present their views and concerns at a public hearing. They are concerned about the potential downstream effects of bunds.
Mr Hall said regardless of whether the bunds were in place, no downstream effects, such as floodwater, debris, boulders and/or trees, would reach Makarora township.
A failure was likely to coincide with a flood in the Makarora River, which would take most of the brunt of any Young River dam failure surge.
The bunds were designed to protect Mount Albert Station from the worst of the heaviest debris, but silt and some floodwater could overflow on to Sawmill and Millionaire Flats.
Mr Hall conceded a lot of the potential adverse affects were ‘‘crystal ball gazing to some extent'', but his opinion was based on more than 30 years' experience investigating dam structures and, on two occasions natural dam failures.
The ORC has issued statements which claim the dam is ‘‘structurally sound'', refusing to process Lonestar's application under the emergency provision of the RMA, instead implementing a limited notification hearing.
Lakes Environmental principal planner Christian Martin recommended to the commissioners a notified hearing was appropriate due to landscape and visual amenity affects from the proposed bunds.
Any hydrology issues were the domain of the ORC and despite their absence and no evidence from them, a joint hearing would be preferable, he said.
Makarora residents approached the Otago Daily Times during the hearing to remark on the absence of the ORC, given the significance of their contradictory statements to Lonestar's expert witnesses.
Lonestar planner Simon Hedley said several official requests had been made to the ORC to provide evidence or studies to substantiate claims a dam failure was unlikely but there had been no response.
Mr Matthews said he and his fellow commissioner could not ‘‘second guess'' the ORC and had to make a decision based on the evidence presented at the hearing.
The hearing was adjourned to allow the commissioners to deliberate on a decision, with a target date of before February 8 Mr Matthews said.