Posted on July 13, 2022
Importing sand to nourish St Clair Beach in Dunedin is likely the most effective way to shore it up against erosion challenges, an environmental consultant says.
Effects of a sand-transportation deficit in the area were becoming evident, EC Otago principal and senior environmental planner Ciaran Keogh said.
The beach was not “healing” as well as it used to after storms and it was particularly vulnerable to a storm considered severe enough to happen once in 100 years, he said.
The beach has been battered this winter, putting stress on the St Clair sea wall and sand sausages, which have been used to protect coastal land.
“What we’re seeing now is about to get worse,” Mr Keogh said.
Mr Keogh is a former chief executive of the Southland Regional Council and Clutha District Council and he worked on the Clutha power scheme for five years.
A vast volume of sand had eroded from the Clutha River mouth in the past 60 years and Mr Keogh described a delayed effect for St Clair Beach to its north.
The foreshore had been on “life support” from sand sourced from foreshore erosion to the south of Dunedin, “so what is now occurring is a delayed response at St Clair from the loss of sediment inputs from the Clutha resulting from damming the river in the 1950s”.
Mr Keogh was open to the possibility that groynes could help at St Clair Beach, but that would need to be in combination with sand augmentation, he said.
“Groynes will probably be necessary even if sediment transport is returned to the Clutha, because that will take years for the increased sediment flow to work into the coastal system.”
Dunedin city councillor Jules Radich has advocated for reinstatement of a groyne at the beach, but he lost a vote on that 5-7 at the end of last month.
He has also said he would be keen to pursue a system of sediment transport over the Clutha dams, in the long term.
Mr Keogh referred to images that showed some dramatic changes at Port Molyneux at the mouth of the Clutha since the 1940s.
They included significant regression of the foreshore.
That was the challenge looming for St Clair Beach, at least to some degree, he said.
Sand sausages at St Clair had proven to be a bad idea, as they worsened the situation by deepening the near-shore water, allowing much higher-energy waves to hit the foreshore, he said.
