Moving protective rocks from one part of Kai Iwi Beach to another has killed a mussel colony and exposed a fragile headland to erosion, George Matthews, the spokesman for the local Tamareheroto hapū, has said.

Matthews said moving the shellrock was a short-sighted, cheap fix and was "robbing Peter to pay Paul".

And the southern headland that the rocks were placed to protect will be gone in five to 10 years, leaving the beach exposed to south and southeast wave action, a Whanganui District Council report said.

Pieces of limestone rock rip rap were placed at the bottom of the headland 30 to 40 years ago, a council spokeswoman said. Storms in 2017-18 dislodged them, and the beach lost a lot of sand as a result.

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Meanwhile the coast to seaward of the Kai Iwi toilet block was also eroding. About 200 tonnes of the "ineffective" rock from the headland was moved there, with another 100 tonnes from elsewhere added. Concrete and steel from the foot of the headland was also removed, because it was unsafe.

No resource consent was needed for the work, the spokeswoman said.

Mussels on the rocks that were moved have died, which is a serious issue for the hapū. Mussels are a source of kai and the hapū has a caretaker role for the area.

"It's unforgivable and despicable to destroy the mussel colony."

The rocks the council deemed "ineffective" were still protecting the headland, Matthews said, and he and Mōwhānau resident Michael Dick say they should have been left.

"The only reason the peninsula is still there is because those rocks were put there."

Matthews has sent letters to the Conservation Department, Horizons Regional Council and Whanganui District Council setting out the hapū concerns. He said under the Resource Management Act physical and material aspects of the environment should not be destroyed or altered, especially without consultation.

He's setting up a community science project to measure the erosion of the headland.

The southern headland at Kai Iwi Beach is eroding and could be gone in five to 10 years. Tuesday, December 18, 2018 Whanganui Chronicle photograph by Laurel Stowell.
The southern headland at Kai Iwi Beach is eroding and could be gone in five to 10 years. Tuesday, December 18, 2018 Whanganui Chronicle photograph by Laurel Stowell.

A council report said the neck of the headland has eroded 10m since 2005. The sea is now attacking it from both sides and the result will be a "stack" within five to 10 years.

With the headland gone, the beach will be more exposed to south and southeast wave events.

The current council budget to protect the beach from coastal erosion is $7000 a year. A total of $1.2 million would be needed for full protection.

The matter is to come before the council early next year.