Waves crashing on Tamaki Drive in Auckland during a storm. Photo: Nikki Gibson

A preliminary count of the potential damage to Auckland’s roads from sea level rise has revealed that $500 million worth of roads, footpaths and other assets could be flooded after a big storm.

Auckland Transport has estimated that is the value of the city’s transport assets that could go underwater with a major storm surge after 1m of sea level rise; the figure rises to $700m if seas rise by 2m.

Many people associate Auckland’s vulnerability to flooded roads with images of waves overtopping Tamaki Drive in Mission Bay, something that already occurs intermittently.

But the rough tally-up by Auckland Transport found that, after sea level rise, several hundred hectares of roads will be at risk of going under during a big storm.

The figures come from a “high level screening” the agency did in mid-2014, recently released after an official information request to Newsroom’s Parliamentary press gallery.

Auckland Transport made the estimates by toting up how much road, footpath, bridge, retaining wall and how many traffic lights, street lights, bus shelters and other assets would theoretically be within reach of a one-in-100-year storm after adding 1m or 2m of sea level rise.

The tally excludes damage to storm-water networks and carparks managed by Auckland Council and does not count any increased risk to roads from erosion or landslips caused by more frequent storms.

Much of the at-risk area was roads: after 1m of sea level rise, 2.7 million square metres of roads, worth $270m, would be at risk of flooding by a major storm, according to the analysis. After 2m of sea level rise that area increased to 3.8 million square metres, worth $380m.

As the next step, Auckland Transport plans to expand the estimate to include ferry terminals, wharves, parking buildings and other assets, none of which were counted the first time. A note in the initial screening says that non-road assets might add another $500m replacement costs, even at the lower sea level rise scenario of 1m.

The latest projections from top sea level scientists say seas around New Zealand may increase between 0.5m and 2m by early next century, depending how quickly people curb emissions, and sea levels likely continue to creep up for hundreds of years.

Auckland Council is working with Auckland Transport on coastal issues.

Its Coastal Management Framework for the Auckland Region, dated July, says preparing the city for at least 1m higher seas by the year 2115 is what’s needed to be reasonably cautious, along with mulling the effects of 2m higher seas for big, greenfields developments. That’s in line with the approach espoused by the Ministry for the Environment’s new sea level guidance for councils, which wasn’t yet final when the council wrote its coastal framework.

Auckland Transport’s rough cost tally doesn’t identify what work is need to prepare the most at-risk areas of the city but the agency says it is working with Auckland Council to identify the places likely to be worst-affected and decide how to manage them.

“Given that the potential impacts are unlikely to be limited to one party, Auckland Transport is currently evaluating [Auckland Council’s coastal] framework and discussing its application with Auckland Council experts to determine how we can combine our efforts and … maintain an efficient and consistent approach,” a spokesman for Auckland Transport said.

The council’s framework notes there’s a need to identify coastal hot-spots in the city needing planners’ urgent attention. Some initial hot-spots listed in the document include Glendowie, Buckland beach, parts of Whangaparoa, Orewa, Whenuapia, Maraetai, Kawakaka Bay, Onehunga, Waiuku, Big Bay, Huia bay and Piha.

Although places such as Dunedin and Hawke’s Bay are often cited as the first in line to be affected by rising seas, Auckland’s framework notes that Auckland has the highest population density-to-coastline ratio in New Zealand with 3200 km of coastline and three harbours (Kaipara, Manukau and Waitemata). It identifies about $200m of seawalls, wharves, jettys boats ramps and other coastal structures area manged by community facilities.

“Over the past decade, a significant proportion of Auckland’s development and supporting infrastructure has been concentrated towards the coast; with much of the urban part of the region sited on a narrow isthmus of land between the Manukau and Waitemata Harbours,” it says. “Intensive coastal development exists along the east coast almost continuously from Maraetai Beach (in the south) to Hatfield’s Beach (in the north). Increased development has also occurred on the south-eastern coastline of the Manukau Harbour….Coastal residential property is highly sought-after. As a result, developments have been undertaken in a range of coastal environments including immediately landward of eroding soft sedimentary cliffs and on accreted lowlands behind beach systems,” says the framework.

Auckland Council’s preliminary flood mapping shows inundation by the sea is a significant hazard for the region, with four percent of Auckland’s total land area exposed to coastal flooding after a one-in-100-year storm, even today, with no added effects from rising seas. “This exposure and associated risk significantly increases with the impacts of sea-level rise,” says the framework.

The document notes that very little of Auckland’s existing urban area is protected by structures such as stop-banks or seawalls. “In the event of such structures being built, consideration must always be given in long-term planning to the consequences of failure and the remaining residual risk (e.g. what happens if an event larger than the structure is designed for occurs),” it says.

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