Coastal survey of Molyneux Bay about to begin

Otago Regional Council (ORC) is about to begin a bathymetric coastal survey in Molyneux Bay to observe seafloor elevations of transects of the 6500-year-old “sand wedge” made up of sediment from the Clutha – Mata Au River.

ORC’s Natural Hazards Analyst, Julion Wright, said that the survey is to determine the current dimensions of the coastal Clutha sand wedge deposit, which ultimately spills river sediment into the Southern Ocean.

“The sand wedge deposit’s an important part of the South Otago sediment system, containing a large volume of sediment that may, or may not be, contributing to the stability of the coast,” he said.

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,” Mr Wright says.

Those future implications could potentially include a better understanding of sediment accumulation and movement, as well as river mouth and coastal erosion effects, he says.

Molyneux Bay is just south of Balclutha township and encompasses the mouth of the Clutha Mata Au River, down to the Nugget Point lighthouse, which is south of Kaka Point township.

The offshore survey will be conducted by Geomatics NZ and use a single-beam echo sounder mounted on the University of Otago’s Marine Science vessel, RV Tūhura.

Once the survey is complete, it will take up to three weeks to process the raw data, to form 2D imagery, which will then be analyzed to interpret what changes may, or may not, have occurred since the previous survey in 2013. The results will then be made publicly available, ORC said.