Decades of neglect of ageing infrastructure is now causing damage to dunes at Otago Harbour’s entrance "at an alarming rate", a local community group says.
The Aramoana League is concerned significant habitat for a wide variety of birds, insects and salt-tolerant plants — the Aramoana Ecological Area — behind the dunes, could soon be inundated.
However, Port Otago, the owner of the "long Mac groyne" at the end of the Aramoana sandspit, says building up the groyne would not make a difference towards the community’s concerns.
The port company instead is seeking permission from the Department of Conservation (Doc) to replenish the beach where erosion is under way.
Aramoana League vice-president Paul Munro said for about three decades the league had lobbied for maintenance of the groynes at the end of the sandspit, installed not long after the 1884 Aramoana mole.
The Aramoana mole, originally called Cargill Pier, extends from Aramoana Beach to protect the harbour entrance from silting up and to keep the channel clear for vessels.
The groyne was installed south of the mole shortly after to further that end.
The configuration helped with the channel, but a secondary process was that the sandspit and the dunes protecting the saltmarsh had built up thanks to the protection afforded by the groyne.
Then, as the groyne fell into disrepair, and as the wash from larger vessels passing by increased, the protection it afforded waned.
By Mr Munro’s measure there was about 10m of dunes left before the water, or the channel, broke through into the conservation area.
"We’ve lost at least, in my estimate, 10m or 11m in the last five years.
"It’s eroding at an alarming rate."
Frustratingly, the groyne structure the league wanted repaired was similar to the groynes recently installed by Port Otago across the harbour at Te Rauone Beach, an effort that was already being heralded as a success by some.
Port Otago chief executive Kevin Winders said the port company was not planning any repairs to the groyne.
The long Mac groyne had been in place for a long time, "and there’s always been a debate around its effectiveness, or not".
"We got the experts in and the long Mac groyne doesn’t make a difference there and clearly it’s just a sand replenishment issue," Mr Winders said.
"We’re quite comfortable with the technical advice we’ve had, we’ve shared that with Aramoana League as we always do," he said.
"I think they might have a differing view, but we’re pretty comfortable with the science we’ve got."
He said Port Otago had offered to replenish the beach with sand, but the company was not consented to do that.
"We’d love to do it and we’ve got dredging equipment going right past most weeks so it’s not a high cost for us to be able to divert some of that and put some sand up on to that beach."
Port Otago was instead seeking approval to do that from Doc, which owned the land, Mr Winders said.
Doc Coastal Otago operations manager Annie Wallace said while some of the groyne structure was within the Aramoana Ecological Area, Doc had no view on its effectiveness.
However, the ecological area was "very worthy of protection".
It was the most extensive area of saltmarsh in Otago and one of the most extensive areas of "dune slack", or wetlands lying between former dunes that were now above the high-tide mark, in the country.
"It is also a habitat for intertidal marine species and birds such as banded dotterels and bar-tailed godwits.
"It provides a buffer between the sea and the Aramoana township which will become increasingly important in the face of sea-level rise, and it is highly valued as a place for people to enjoy and appreciate nature" Ms Wallace said.
