03-07-2025
Survey set to shed light on wedge
Coastal residents may spot a regular visitor to Clutha waters this month.
The Otago Regional Council is set to conduct a bathymetric coastal survey in Molyneux Bay using the University of Otago's RV Tūhura.
The vessel will observe seafloor elevations of a 6500-year-old sand wedge, made up of sediment from the Clutha Mata-Au River.
Council natural hazards analyst Julion Wright said the survey would determine the latest dimensions of the coastal Clutha sand wedge deposit, which spilled river sediment into the Southern Ocean.
Mr Wright said the sand wedge deposits were an important part of the South Otago sediment system, containing a large volume of sediment that might possibly contribute to the stability of the coast.
"The modern sediment of the wedge is formed by deposits of material from the Clutha River over the last 6500 years.
"This survey will be compared to a similar ORC survey, from 2013, to map any changes in the sand wedge deposit.
"The results of this will have future implications for coastal and river management in South Otago."
One such implication could be a better understanding of sediment accumulation and movement, as well as river mouth and coastal erosion effects, he said.
Molyneux Bay lies to the south of Balclutha and encompasses the mouth of the Clutha Mata-Au River, down to the Nugget Point lighthouse, south of Kākā Point township.
The offshore survey would be conducted by Geomatics NZ and use a single-beam echo sounder mounted on the research vessel.
The Tuhura, a 6.5 m aluminium powerboat for up to six people, can support restricted oceanographic work, limited bottom sampling, trawling and diving, as well as operate around coastal Otago and inland South Island waterways.
The vessel would not be streaming any devices astern, Mr Wright said.
Over a week, it was expected to travel about 180km within the bay.
The survey was scheduled to run this month, but the timing was otherwise weather-dependent.
He said the shallower part of the sand wedge was influenced by discharges from the Clutha as well as coastal processes.
The offshore surveying would also be extended onshore, using land-surveying techniques to cover the coastal shape and structure.
The gap between the onshore and offshore surveys had already been covered by an earlier, 2025 airborne bathymetric LiDAR survey (Light Detection and Ranging), he said.
Once the survey was complete, it would take up to three weeks to process data to form 2D imagery, which would then be analysed to interpret what changes may, or may not, have occurred since the previous survey in 2013.
The results will then be made publicly available.
