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The risk of a structure failure at the dammed Young River remains very high and a flood protection scheme is ‘‘urgently'' needed to protect 200ha of prime farming pasture 12km downstream at the confluence of the Young and Makarora Rivers, a University of Canterbury geologist claimed yesterday.
However, Makarora residents, business operators, and opposing submitters claim the three proposed gravel and rock armoured bunds will cause detrimental downstream effects, whenever heavy rainfall floods the Makarora River.
Geologist David Bell, of Christchurch, said the risk of failure of the Young River landslide dam had not diminished and remained very high, because the structure is now saturated and has not been subjected to any significant failure triggers.
Even a partial failure of the upper 10m-15m of the structure could result in a 3m high wall of water surging down the Young Valley, carrying sludge, silt and debris and any vegetation caught in its path. His opinion contradicts studies by the Otago Regional Council which claim the 100cu m landslide dam is structurally sound.
Mr Bell appeared as an expert witness for Lonestar Farms Ltd, the owners of Mt Albert Station, who want to ‘‘urgently'' build the three bunds as part of a flood protection scheme for 200ha of land at Sawmill and Millionaire Flats, under an emergency provision of the Resource Management Act.
Lonestar seeks a non-notified resource consent hearing, which should consent be granted, will allow them to proceed without haste and build three, 14m wide tapering to 2m wide bunds, in two locations.
The proposed bunds will be 250m and 500m long in the riverbed of the Makarora River, where the Young River exits, and 200m long on the true right of the river at a natural convergence, opposite the slip-prone Pipson Creek.
The bunds will require a total of 24,675cu m of gravel and rocks. Commissioners John Matthews and Lyal Cocks will determine whether Lonestar can proceed without a lengthy public notification process and full consent hearing for their application.
Makarora Residents Association chairman Rick McLachlan said the isolated mountain valley community want a fully notified hearing so they can present their views on the proposal.
The bunds would potentially force the river, whenever it was high from frequent heavy rainfall, into ‘‘unnatural'' flood plains and divert it with detrimental effects for downstream river users, businesses, and land owners.
None of the expert evidence presented could say with any certainty what the structure was going to do, and in any event, the bunds would probably be destroyed, if rainfall more than 200mm fell in the catchment area, causing the Makarora River to flood, he said.
Wilkin River Jet owner Patsy Nolan said the differing expert opinions gave no confidence to anyone in the wider community that the bunds required immediate construction and the proposal should be notified.
Natural events and disasters ‘‘just happened'' and resulted in adverse affects. If mitigation structures were allowed for every situation, the landscape would be littered with man-made protection devices, she said.
The hearing continues today when Lonestar's hydrologist gives evidence on the downstream effects of a dam failure.