Field work backs river model

"There is no substitute for good field work," river management consultant Henry Hudson said on his second day of defending the computer model he used to predict how TrustPower's proposed hydroelectric power scheme would change the Wairau River.

Dr Hudson is an expert witness for TrustPower in an Environment Court case called to reconsider a Marlborough District Council's decision to grant the company consent to build the scheme.

On Wednesday, Dr Hudson was questioned by Fish & Game lawyer Maree Baker on the accuracy of his two-dimensional model which she said was unreliable and inaccurate. Yesterday, he faced Save the Wairau lawyer Mike Hardy-Jones and the Environment Court panel.

Dr Hudson used hydraulic-habitat modelling to predict water depth and substrate with conditions suitable for particular species. Deciding how much habitat was required to sustain a population of fish, birds or aquatic insects was left to stream ecologists.

He described modelling as a tool to assist in making decisions which quantified the changes which could be observed in the field. "One should never take modelling results in isolation," he stressed.

Dr Hudson conceded that floods and high flows compromised the topographical measurement phase of his work and follow-up measurement of water velocity and depth. This included a 1000 cubic metre per second flood which shifted a channel 60 metres and buried $7000 worth of instruments. The survey was repeated in less extreme conditions but flows were still higher than ideal.

However, Dr Hudson argued that his model was sound because of supporting field work. This included the establishment of 30 benchmarked photo-points at critical points for recording changes in river topography which could influence flow. Series of aerial photographs had been commissioned and historic photos dating back to the 1930s were being collected.

Dr Hudson also noted relevant bed features such as banked-up stones which, if shifted, would influence whether flow went down a certain channel.

Commissioner Alex Sutherland, himself a specialist in hydraulic engineering, asked whether it made sense to strive for great accuracy in modelling. "No," Dr Hudson replied. "Modelling is to show direction and magnitude of change.

"We aim for excellence, not perfection," Dr Hudson said.

"I would say that this is a very good piece of modelling."

Asked to consider the impact of increasing residual flows through the affected reach of river on habitats, Dr Hudson said, "The only way we can have nothing changing is by doing nothing."

Major points of his evidence were that flows in the Wairau naturally fell below minimum flows proposed for the scheme and that disturbance of habitat by frequent floods was more likely to dominate aquatic life than habitat availability.

His modelling suggested variation of channels in the affected reach of the river was expected to remain within the natural range.

The number of islands and access to them would also not change, so there would be only minor effects on black-fronted tern nesting habitats, Dr Hudson said.

- The Marlborough Express

Newest First

Oldest First