What would happen if 'storm of the century' hit Auckland?

The February 1936 storm destroyed buildings, derailed a train and damaged ships - including this coastal steamer that was driven ashore at Wellington. Photo / Supplied

It flooded every major North Island river, destroyed buildings, derailed a train, forced an interisland ferry aground and left several people dead.

Such was the massive scale of the deluge that hammered half of the country in the first two days of February, 1936, that it went down as the "storm of the century".

Now researchers are looking what would happen if such a system hit Auckland and the upper North Island today, in an era when climate change is predicted to increasingly intensify extreme weather.

The 1936 storm, an ex-tropical cyclone, caused more than $1m in damage, yet didn't affect our biggest city as badly as other areas.

That also proved the case with three other recent cyclone-driven storms which barrelled into our neighbourhood – Ita in 2014, Pam in 2015, and Cook in 2017.

"If a repeat of the 1936 storm occurred, or other more recent storms had taken different tracks, it is likely that damage would be in the billions, as a much more developed city with significantly more exposed assets would have been impacted," Niwa meteorologist Dr Richard Turner said.

As this century progressed, that risk would be compounded by seas growing higher and warmer.

"This means these storms may maintain their most intense phases for longer periods as they pass New Zealand."

In a new study, funded through the national science collaboration Resilience to Nature's Challenges, researchers will simulate the impacts of an upper North Island event using ultra-high resolution, multi-hazard weather models.

Scientists are modelling the impact of a large ex-tropical cyclone on Auckland and the upper North Island. Photo / NZ Herald

"To help emergency management planners, we want to understand what those impacts might be, and what we can do to reduce them for future events."

By calculating everything from strong wind and intense rain to storm surge and landslide, the study could improve flood forecasts and stop bank networks.

It was just one part of a wider programme, led by Turner and GNS Science hazard and risk management researcher Dr Sally Potter, exploring the effect that one-off, major weather events might have on the country.

Another looked at a severe winter storm hitting the south, and what an extreme blast of ice, snow, wind and heavy rain could do to infrastructure in Canterbury and Otago.

"It may seem counter-intuitive, but we're still anticipating big snowfall events under a warming climate," Turner said.

"We also want to examine how high wind loads affect electricity transmission lines and pylons.

"Up until now, there has not been much research in these areas in New Zealand."

A particular focus was on current snow-loading standards at low elevations, which have long been questioned – especially after Stadium Southland's roof collapsed in 2010.

A 2016 snowstorm which knocked out power to 50,000 homes in Hawke's Bay - and another event in 2006 that caused widespread disruption to power networks throughout South Canterbury – similarly raised concerns.

"There are several reasons to suspect the adequacy of our snow load standards, but the main one is that there is a lack of good low-level snow observations for New Zealand and a reliance on continental – or Australian or North American - data when we have a maritime climate."

A 2006 snowstorm caused widespread disruption across South Canterbury. Photo / File

But Niwa's latest generation models – measuring mean wind speeds and direction, gust speeds, temperatures, humidity, and snowfall rates – were now advanced enough to get an accurate picture of how pylons might fare in heavy winter storms.

A third scenario focused a on a hypothetical wildfire breaking out in a subdivision on Mt Iron, near Wanaka.

Turner said a range of factors put Otago at high risk of wildfire: among them, highly combustible fuels like manuka, limited road access and water resources, and summers that would become even drier under climate change.

"The Mt Iron community in the Wanaka area provides an opportunity to study a suburban development in progress within a high-risk zone," Turner said.

"This includes some homes which have already been built and others yet to be built."

Potter said a general goal of the research programme was to develop new "fragility functions" – or better understanding how vulnerable something is to a certain hazard.

"A brick building may be strong against flooding, whereas a wooden building may be weak, and this is the opposite for earthquake shaking," she explained.

"We will provide these fragility functions as inputs to modelling, to understand the impacts of the scenarios."

All of the new insights would be fed into New Zealand's main model for natural hazard impacts – a tool developed by Niwa and GNS Science and called RiskScape.

"The impacts might be building damage, injuries and fatalities, downtime from disrupted infrastructure, and economic losses if combined with other models," Potter said.

"At the moment, it can be hard to get accurate hazard models into RiskScape in real time as an event occurs, in order to rapidly calculate the potential impacts.

Wanaka's Mount Iron community, which was hit by a bushfire in January 2016, is being used as a case study to study effects of wildfire. Photo / File

"But we have done this for earthquakes, including the Kaikoura earthquake in 2017, to help inform impacts on roads in the area, and to contribute towards building legislation.

"Our desired end outcome is that, through more accurate and detailed understandings of the impacts of severe weather and wildfire events, agencies can better plan for these events – ultimately leading to more resilient communities."