Eroding New Zealand: New maps reveal where our coasts are crumbling fastest

Parts of New Zealand’s coastline are crumbling at rates equivalent to a football field each generation, new maps reveal - with climate impacts now beginning to compound the problem.

A just-released dataset, compiled by a team of more than 40 researchers over five years, marks New Zealand’s first detailed stock-take of coastal erosion since the late 1970s.

“For the first time in decades, we have a comprehensive, nationwide baseline that will aid in better decision-making and planning for coastal hazards and climate change adaptation,” said one of the project’s leaders, Professor Mark Dickson of the University of Auckland.

The observations, drawing on records dating back to the 1940s, showed some stretches of coast have been growing over the past few decades, while in other places, land was being steadily lost.

At Port Waikato an annual 1.5m of land had been lost to the sea over 60 years – while the settlement's nearby sand spit had been building by around 3m each year. Photo / Mike Scott

“The richness and granularity of this data allow us to see coastal changes with unprecedented detail,” said Dr Murray Ford, another lead researcher.

It comes as some of New Zealand’s best-loved holiday spots have been grappling with chunks of waterfront being swallowed by the sea.

At Port Waikato, officials were last month forced to close the Sunset Beach car park after around two metres of it collapsed in one night.

And last week, Hawke’s Bay councils were told that urgent work was needed to protect erosion-threatened properties in Haumoana, Te Awanga, Westshore and Bay View - at a potential cost of nearly $35 million.

University of Auckland coastal scientist Dr Emma Ryan said in known “hot spots”, it wasn’t uncommon to see erosion rates of many metres each year.

This map shows erosion-driven changes over time at Dunedin's St Kilda Beach.

Parts of the coast at Kaipara Harbour’s North Head were vanishing by about 6m annually, while around 3.4m and 2m of land was being eroded yearly at Washdyke at Timaru and at Otago’s Molyneux Bay respectively.

Near Port Waikato township, the mapping revealed an annual 1.5m had been lost to the sea over 60 years – multiple homes there have been made uninhabitable after recent storms - while the nearby sand spit had been building by around 3m each year.

“Some communities have been grappling with erosion for years, while other communities sit in areas that have been stable, or even growing, for decades,” Ryan said.

“Dealing with this variability makes for very challenging responses, which need to be grounded at the local scale.”

Ryan said some places with “chronic” erosion would only see that trend worsen with sea-level increases, which scientists recently showed could amount to 1.2m by century’s end in large parts of New Zealand.

“In other areas we see changes in coastal erosion and accretion patterns through time,” she said.

This image captures historic erosion rates at Whatipu Beach, on Auckland's West Coast.

“Some beaches that have historically been building out have switched to erosion.”

Nationally, there was some evidence to show an across-the-board increase in erosion rates since the turn of the century – although that couldn’t yet be attributed to climate change.

“That doesn’t mean that sea-level rise isn’t impacting coastal erosion – it just means that we can’t obviously see those impacts nationwide so far, but we expect to at some point in the future,” she said.

Historic erosion rates at Ruby Bay, in the Tasman District.

“Having this baseline dataset means we can monitor for future changes, such as accelerating erosion or slowing accretion.”

The mapping, carried out through the Resilience to Nature’s Challenges National Science Challenge, also captured the impact of recent major weather events such as Cyclone Gabrielle.

Ryan said last year’s extreme summer storms had taken out chunks of coast reaching 15m to 20m in parts of the North Island’s east coast.

Many beaches in these regions, including well-loved tourist hotspots, had lost more than 10m during Gabrielle alone.

Ryan said that observations from these single storms had allowed the researchers to place “event-driven” change in the context of longer-term patterns.

“We are continuing to monitor coastal change in some of the worst hit beaches using satellite imagery, allowing for post-storm recovery to be assessed.”

The carpark at Port Waikato's Sunset Beach - pictured last month - has been gradually falling into the sea. Photo / Waikato District Council

Some councils, such as Tasman District Council, have already begun using the new data for their planning.

Ryan said the data would also be crucial to banks and insurers, who could use it to support hazard assessments or risk-pricing.

“The data set can show that some properties may be more vulnerable than previously thought, while others may be less vulnerable,” she said.

“But vulnerability is complex, and a thorough national coastal vulnerability assessment has not been conducted so far.”

How to prepare our vulnerable coastal communities for a warmer and wilder world has proven a tricky issue for policymakers – with New Zealand largely still taking an ad hoc, council-by-council approach to managed retreat and property buy-outs.

But today, the Climate Change Commission warned the Government wasn’t moving fast enough on adaptation, and set out a raft of recommendations for action.

While the Government has been working toward adaptation legislation, it’s not clear whether that will result in a national policy statement, or a new dedicated law, as the former government attempted to introduce.

People can view the new maps here.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.