Artificial Wellington reef will be a home to crayfish and pāua

Saturday, 22 June 2024

Civil engineer Chris Barber says an artificial reef offshore from Petone will one day be a home for crayfish.

A five hectare reef being built offshore from the Petone beach should one day be home both crayfish and paua.

On Friday, the last of 56 concrete pyramids was placed on the seabed. Each pyramid is 4 metres long by 4m wide, and 5m tall, and weighs approximately 22 tonnes.

The pyramids have been placed in clusters of three, up to 10m below sea level. The units are designed to attract and accommodate fish while providing a surface for marine plants and shellfish to grow on.

The reef is an important but largely unseen part of the $312m shared path and seawall, known as Ngā Ūranga ki Pito-One.

New Zealand Land Transport Agency Waka Kotahi civil engineer Chris Barber said the reef is part of the resource consent and will mitigate the negative environmental impacts of the project. It will also improve water quality in the harbour.

One of the last of 56 pyramids was placed on the seabed near Petone on Friday to create a five hectare artificial reef.

It’s hoped the underwater reef will boost fish numbers and provide a home to octopus, crabs, crayfish and starfish. The reef has been designed in collaboration with local Māori, using mātauranga (traditional Māori knowledge).

The first step to getting species like starfish to move in is to encourage kelp to grow on the concrete structures.

The reef is an important but largely unseen part of the seawall and shared path being built between Petone and Ngauranga.

The reef is a first for New Zealand. Work began in May and a large rig has provided the heavy lifting.

Barber said the pyramids will embed themselves in the seabed over time. At low tide the reef will be more than 2m metres below the surface, and he said the local rowing and water skiing clubs are aware of the new structure.

General manager Jetesh Bhula said the reef was a cost effective way to improve water quality, which was one of the requirements of the consent.

Waka Kotahi regional manager Jetesh Bhula (left) and civil engineer Chris Barber say an artificial reef will be a major boost for the ecology of Wellington Harbour.

Although the reef faces one of the most exposed sections of the shared path and seawall, Barber said it has nothing to do with calming the sea to protect the seawall.

Those monitoring progress on the project will also have noticed large interlocking concrete blocks shaped like an X on the coastal edge.

It is one first projects where a seawall’s seismic performance had been considered and the blocks will add to its resilience. Not only will they mimic the existing coastal layout, they are also designed to cope with a one in 100 year storm.

The project is often referred to as a cycleway, but the seawall will help protect the road and rail corridor from the damaging effects of storms and climate change.