Monitoring of sediment movement proposed

Environment Court panel (from left) commissioner Alex Sutherland, Judge Jeff Smith and commissioner Ann Leijenn question a panel of specialist witnesses at the second day of a hearing in Dunedin into an appeal of conditions relating to Port Otago depositing sediment off-shore of Taiaroa Head. Photo by Peter McIntosh.

Port Otago has proposed three monitoring sites be established to track sedimentation movement as a result of it dumping dredged spoil off Dunedin's shores, in an effort to protect the coastal environment.

Fisheries interests have appealed to the Environment Court conditions around Port Otago's consent to dump dredged sediment from Otago Harbour, 6.5km off Taiaroa Head.

An Environment Court panel led by Judge Jeff Smith yesterday heard evidence from a panel of experts representing Port Otago, Otago Regional Council and the fishery interests as well as from Port Otago infrastructure general manager Lincoln Coe, fisheries consultant Richard Boyd and Otago Rock Lobster Industry Association executive officer Simon Gilmour.

Judge Smith said the aim of yesterday's session was to work out how conditions might be worded or framed so the process could be caught before it went wrong and adversely affected the inshore environment.

He warned that if the parties could not find solutions to the issues, the court would.

"I know you have made an honest attempt to get it solved in the past six months."

Len Andersen, counsel for Port Otago, said discussions between the parties had resulted in a proposal to install three monitoring points, at 500m south of the dumping site boundary, 500m northwest of the boundary and 1km north east of Cornish Head.

Regular sampling was proposed while the large dredge was operating and for four months during the smaller New Era dredge's operation in a variety of conditions.

From there, he saw an exceedance level being agreed to and below that a suite of management actions.

Niwa programme leader hazard and risks Dr Robert Bell, for Port Otago, said in answer to a question from Judge Smith, modelling showed moving the dump site 2km east would not make a lot of difference to the amount of silt deposited inshore.

University of Waikato senior lecturer in coastal oceanography Dr Willem de Lange, for the Otago Regional Council, said if it was moved further offshore it would put the sediment in stronger currents which meant it would travel further in a northerly direction than at the dump site (AO).

"In terms of the options presented, AO was the most desirable location."

Capping the silt at the dump zone would increase the mound; it would be more effective to mix silt and sand during dredging, he said.

Trialling the monitoring method at the existing maintenance dump site would give parties a chance to fine-tune the process and get a "handle" on what would happen if the plume from the dumping bypassed the sensor.

Judge Smith said there appeared to be some agreement on using turbidity measure NTU to judge if sedimentation was going where it should not.

University of Otago lecturer in aquaculture Dr Chris Hepburn, for the fishing interests, said photosynthetically active radiation sensors should also be used to judge light levels around the reefs and kelp beds near shore.

A suggestion by Judge Smith that Port Otago use a back hoe to remove silt from the harbour as it would not disturb sediments as much as the proposed suction dredge was "not a realistic possibility", Mr Andersen said.

Mr Coe said a back hoe would have to be sourced off-shore, was less mobile, would take longer to do the job and was noisier.

Mr Boyd told the court any risk to rock lobster from the dumping was very small but agreed it could not be ruled out.

However, Mr Gilmour said they were concerned the dump site would affect larvae that populated the coast's rock lobster beds.

The hearing was adjourned until Friday to allow the parties to develop conditions and an environmental management plan with assistant from commissions Alex Sutherland and Anne Leijenn.

rebecca.fox@odt.co.nz