Cyclone Gabrielle: Native tree planting may have prevented worse damage

Monday, 20 February 2023

Herald Island's Twin Wharf Rd was devastated by the impacts of Cyclone Gabrielle.

Native bush planted as recently as three years ago may have prevented some of the worst damage on Auckland’s Herald Island during Cyclone Gabrielle.

Twin Wharf Rd was partially wrecked thanks to a large gum tree coming loose, taking the footpath, retaining wall and much of the bank down into the sea with it.

Herald Island Environmental Group chairperson Jan Diprose said it had been both discouraging and encouraging – while some of the bank her team intended on restoring was gone, the section they did plant survived the storm well.

“We plant things that will prevent erosion, but of course a huge tree coming down, it’s going to take everything with it,” she said.

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Herald Island's Twin Wharf Rd after Cyclone Gabrielle.

The team had planted more than 1000 native plants of some 30 species since 2020, with funding and support from Auckland Council.

They planted kōwhai and nikau for the birds, as well as cabbage trees and flaxes that have deep root structures to keep the ground in place.

The road, footpath, retaining wall and bank are damaged.

“We’ve had a really good overall plant survival rate at this site, with hardly any losses and the plants are now beginning to flourish,” Diprose said.

Herald Island is a small island connected to Whenuapai by a causeway.

It has two main roads circling the land, with interconnecting streets and one community hall in the centre.

University of Auckland coastal erosion expert Professor Mark Dickson said he could see how erosion may have been prevented thanks to the planting.

“Bank stability in soft sediments is quite complex and can vary depending on rock and soil characteristics as well as hydrological conditions and the nature of tidal currents at the base of the slope,” he said after seeing photographs of the site.

“If all of these factors were similar across the two sites, then the observed differences in erosion during the cyclone could well be due to differences in the vegetation type and stage of growth.”

The team had intended to restore the bank where the gum tree came down, but it was less accessible than other sections.

Among their plans was to throw clay “seed bombs” of native seeds to see what might germinate there.

The road has a reduced speed limit while contractors get to the site amid a slew of immense damage from Cyclone Gabrielle around the city.

Auckland Transport has also made some repairs, with a tarseal patch over the worst of the road cracks.

The speed limit has been dropped to 30kph and cars are only allowed to go through one way at the time.

Diprose said the scale of the damage showed how much work the city had to do to ready itself for the impacts of climate change.

In the wake of the flooding, which came after unprecedented rainfall, experts have called for Auckland to become a “sponge city” to avoid further disaster.

University of Auckland senior lecturer in urban planning Timothy Welch wrote that cities had been built to be vulnerable to major weather events.

Streets were impermeable surfaces and need to be built to better absorb and filter the water, he wrote.