Effects of cyclone measured
Whangamata¯ Beach eroded by 15 metres after Gabrielle
The impact of Cyclone Gabrielle is still being felt — and investigated — one year after it hit New Zealand. New research has found that the cyclone carved away as much as 20 metres from the worst-hit beaches in the upper North Island.
The study, funded by Resilience to Nature’s Challenges Kia manawaroa — Nga¯ A¯ kina o Te Ao Tu¯ roa, revealed just how much impact the Cyclone had, not only on the Coromandel.
“We’ve seen that certain parts of the shoreline are close to their most eroded position we’ve seen over the last 80 years,” Associate Professor Murray Ford, at the University of Auckland, who is leading the study, said.
The research findings also suggest this may be the largest expanse of coastal erosion caused by a single storm in recent history.
“What set Gabrielle apart from other storms in recent decades is that it impacted such a large swathe of the North Island’s coast,” Ford said.
“We’ve seen considerable erosion ranging from Northland all the way down to Hawke’s Bay as a result of the devastating path the storm took and the large swells it generated along this path.”
Losing large chunks of dunes across such large expanses of coastline creates a snowball effect that reduces resilience against future storms.
“Essentially the dunes are our buffer between the ocean and beach, and the assets and infrastructure we value on land,” Ford explained.
“The amount of protection provided by the dunes has been reduced and this makes us more exposed if future storms occur before beaches and dunes have recovered.”
Many beaches in these regions, including well-loved tourist hotspots, have lost more than 10 metres.
Mahanga beach in Hawke’s Bay was among the worst hit, with 20m eroded. Whangamata¯ in the Coromandel lost 15m, Matarangi Beach in the Coromandel lost 10m, as did O¯ maha Beach in Auckland.
The storm also shaved off 10m from beaches along 15km of coastline in the Bream Bay area of Northland, including Ruaka¯ka¯ beach.
Ford and his team have been researching coastal change trends over 80 years. In the wake of Cyclone Gabrielle, thanks to having cuttingedge satellite imagery at their fingertips and additional funding from MBIE’S Extreme Weather Science Response, the researchers pivoted their focus to take an up-close look at how the storm has affected New Zealand coastlines.
Previously, coastal mapping snapshots were few and far between, so the technology has been a game changer.
“Gabrielle is the first large storm of its kind that has taken place since researchers and decision-makers have had near-instant access to very high-resolution and cost-effective satellite imagery,” Ford said.
“It means we’re able to accurately quantify how much erosion there is over large areas.”
The findings will help regional and local decision-makers update their plans in preparing for and responding to worsening erosion caused by more extreme weather events as well as sea level rise.
“Development along the coast must be cognisant of the hazards which might impact the coast, both now and in the future,” Ford added.
“The more we know about how the beaches and dunes respond to storms, the better prepared we are to adaptively plan for future storms.
“Knowing how beaches and dunes are impacted, and potentially recover, from storms is an important first step to make sure we don’t lock ourselves into responses that can often make situations worse. Kneejerk responses in the past, such as the ad hoc engineering of sea walls immediately following storms, can actually make problems worse for decades to come.”
For example, some areas may need to update their strategies on managed retreat (moving away from high-risk areas) or expand existing setback zones further inland.
“Gabrielle is a historically massive storm but we didn’t have widespread coastal housing destruction, which was a big relief. This shows the value of having really robust, scientifically derived setbacks,” Ford said.
The research team are now tracking how the beaches are recovering over time. Against a backdrop of seasonal beach erosion and regrowth, it can take a least a year to see any real signs of recovery.
“We’re aiming to get a good look in the late summer for all of these beaches, so we can see what’s happened over the last year. The key evidence of recovery we’d want to see at this stage is that the land is not still cutting back.
“It’s instantaneous erosion and then a slower process of recovery, where beaches normally accrete during summer and then erode again during winter.
“So it’s certainly not a case of getting a full recovery one year on, but hopefully we might see a start.”