A clay cap stretching 150m on top of sand dunes at Dunedin's Middle Beach has been removed by Dunedin City Council contractors, amid concerns further erosion could have caused it to collapse. Photo by Craig Baxter.
As waves continue to erode Dunedin's Middle Beach, major excavations have been carried out in recent weeks to remove a clay cap covering sand dunes, which was at risk of collapse.Dunedin City Council staff yesterday confirmed a clay cap stretching about 150m along the top of dunes at Middle Beach had been removed by contractors, amid fears it was being undermined by waves.
Parts of the cap had already fallen away, leaving plant roots exposed and the remaining cap at risk of collapsing on to the beach below, council parks manager Lisa Wheeler said.
Instead, the cap - up to 1m deep in places - and vegetation in the area was removed by council contractors last month, leaving a flat shelf, she said.
The clay was then replaced with about 7000cu m of sand dredged from Otago Harbour, to restore some of the top of the dunes, Mrs Wheeler said.
"When we took the clay capping off, it was just a flat shelf. We've actually brought the sand back on to try and build the dune back up again.
"It was quite significant [work]," she said.
Otago Regional Council consents required the city council to collect any clay falling on to beach, to prevent it being washed into the marine environment, she said.
The work was undertaken as a "precautionary" measure to avoid that, at a cost of $70,000, but also to ensure public safety for beach-goers, she said.
Public access to the dunes was already restricted, with the area above fenced off since it was discovered the dunes were contaminated by material from a historic landfill located under nearby Kettle Park.
However, plants left leaning over after parts of the cap collapsed were adding to the pressure on the shelf, Mrs Wheeler said.
"Some of it had [collapsed], and there were big cracks where even the pressure of just someone standing on it was moving some other areas of it.
"You just need another little bit of imbalance and it all would have just gone."
Council community and recreation services manager Mick Reece said the clay had been placed on top of the dunes "years ago" - although the exact date was not known - to help promote plant growth.
Adding the clay was "not a good move, as we now know", and would have had to be removed regardless of what decisions were made about the long-term future of Middle Beach.
"As the face of the dune was tested in any way by the weather, you have this great big pie crust on the top that if it was undercut at all, the whole thing collapsed, like slabs of clay down on to the beach.
"Whatever happens in the future ... that clay had to go anyway."
