Dr Bell, a National Institute of Water and Atmospheric Research (Niwa) coasts and hazards principal scientist, was giving evidence for Port Otago on the third day of the company's resource consent hearing in Dunedin yesterday. Concerns about the plume modelling at the offshore disposal site, called AO, was raised by about 26 submitters.
Dr Bell prepared computer modelling for the project which assessed the effects of the works on physical coastal processes and turbidity during dredging and once the channel had been deepened.
In his opinion, it was very unlikely fine sands from the dredged material at AO would reach the Otago coastline in any discernible quantities, he said.
"The model shows that long-term sand transport from the AO disposal area would predominantly be to the north or noth-northeast."
There was also no hydrodynamic mechanism that would move any significant fine sand from AO to the Otago coastline and turbidity levels would be very small as a result, he said.
"The plume modelling ... shows that virtually no sediments from the disposal operation would be deposited in Blueskin Bay or along the coast either side of Karitane Point."
The effects of a widened and deepened channel on tides at Otago Harbour would be minor and the effect of a sediment mound of up to 1.7m at the disposal site would have negligible effect on offshore wave heights, he said.
The other two areas investigated as possible disposal sites were ruled out as it became clear sediment plumes could regularly encroach on the coastline of Otago Heads or lead to increased suspended sediment concentrations along the coastal zone north of Cornish Head, Dr Bell said.
He recommended Port Otago monitor the dredge plume and its suspended sediment as well as turbidity at the chosen disposal site.
The modelling had been peer reviewed by other Niwa staff, externally by Tonkin and Taylor and independently for the Otago Regional Council by the University of Otago's Dr Ross Vennell, he said.
Environmental consultant Dr Martin Single said apart from the physical change to the seabed topography from the dredging and sediment deposition, he considered the effects on the physical coastal environment would be minor.
It was likely the proposed deepening and widening of the channel would result in a reduction in the magnitude of wake generated by commercial vessels at present travelling the channel and no change in the overall effect of wake waves on the shore and beaches of the lower harbour, Dr Single said.
"There should be no increase in erosion of the Aramoana ecological area due to the effects of the Port Otago proposal."
Modelling results indicated the "surfability" of the waves at Aramoana would not be compromised, he said.
"The perceived build-up of sediment volume at Aramoana was part of long-term variability to the seabed topography, and not as a result of oversupply with placed dredged sediment."
Niwa scientist Dr Christopher Hickey said the chance of chemical contaminant-related adverse effects as a result of Port Otago's proposed dredging and disposal of sediment was of "very low concern".
Sediment sampling was undertaken in 2010 at 26 channel sites to assess chemical contaminants and measure toxicity, Dr Hickey said.
"These analyses indicate a negligible risk relating to chemical contaminants associated with the dredging and disposal operations."
No additional exposure would be experienced by marine mammals or humans consuming seafood as a result of chemical contaminants associated with the project, he said.
Port Otago Hearing
Panel: John Lumsden (chairman), Hugh Leersnyder, Dr Mike Johnston.
Proposal
Upgrade channel berth and swinging areas disposing 7.2 million cu m at sea.
Extend multipurpose wharf by 135m. Construct fishing wharf at end of Boiler Pt walkway.
Where: Otago Regional Council, Dunedin.
Day 3
Giving evidence yesterday: Niwa principal scientist Dr Robert Bell, Fonterra general manager strategy Nigel Jones, environmental consultant Dr Martin Single, Niwa principal scientist Dr Christopher Hickey.
Quote of the day
"The plume modelling ... shows that virtually no sediments from the disposal operation would be deposited in Blueskin Bay or along the coast either side of Karitane Point."
- Dr Bell, Niwa coasts and hazards principal scientist.
