Cyclone Gabrielle: Can we build our way to flood resilience?

About one in seven New Zealanders live in areas prone to flooding. That’s 675,000 people and more than $100 billion worth of homes. Nikki Macdonald asks how we can make them more resilient to the growing threat of torrential rain.

As shovels begin to shift the mountains of silt swallowing houses and cars, the trauma will give way to the stark reality of what’s been lost to Cyclone Gabrielle.

Lives, countless livelihoods, and, for more than 10,000 Kiwis, access to the safe haven they call home.

As each reconnected road reveals more devastation, a question is already rearing its head: How do we brisuild back better? And – in the worst hit areas – should we build back at all?

READ MORE:
* Cyclone Gabrielle: Was the catastrophe at Esk Valley avoidable?
* Hawke’s Bay man dug for 6 hours to free partner buried under slip
* Cyclone Gabrielle: Breeder gets 60 dogs to safety on top of desks at flood-hit school
* Cyclone Gabrielle: How does a town become cut off?

How much water, will it reach me, and what will happen if it does?

Flood risk is a combination of hazard times exposure times vulnerability, says Belinda Storey, managing director of Whakahura, a $10M research programme studying extreme events under climate change.

If you can reduce any of those three things, you can (theoretically at least) improve your resilience to floods.

Hazard is about how much water comes out of the sky and the only way to change that is to reduce emissions to slow down climate change. So don’t expect any imminent relief there.

Exposure is about whether or not the water reaches you. You can reduce that risk by putting things in its way, like stopbanks or floodgates or pumps. (More about that later)

Vulnerability is about what happens when those defences fail and the water begins to rise.

At that point you have two options – try to prevent your home from flooding, or accept that water will get in and try to limit the damage. Or possibly both.

Raise ‘em up

Since January’s downpours turned Auckland’s roads to rivers and its rivers to brown ribbons of destruction, Reuben Turvey’s phone has been running hot.

The Auckland House Lifting director has just been out to a 2-storey in Coatesville that needs a leg up, and he’s off to another in Mt Eden that wants raising about 1m.

Lifting floodprone houses is perhaps the most obvious way to get them clear of potential floodwater.

Just about anything is possible, from lifting a 100sqm pile-foundation house onto taller piles ($50k-60k+GST) to jacking up a two-storey concrete slab house ($400-500k).

If the floodwater is surging rather than still, you’d need an engineered design to ensure the structure could withstand the force of the water.

“We’ve had an unbelievable amount of inquiries with people wanting to do it,” Turvey says. “I would say next year there will be quite an influx.”

Those words fill Bex Hurley with fear.

The West Auckland woman already has a house raised a whole storey on poles, with the garage in the space below. Her home might be floodproof, but her nerves are not.

For the first seven years they lived in the stream-side Henderson home, it was dry and dreamy. In the seven years since, they’ve had 10 floods.

In 2015, the water rose to 1m high in the sloped driveway – the cars were fine. In July 2016, it flooded in the night, writing off one car, but the garage stayed dry.

In April 2017, the property became a 3m deep swimming pool and they were trapped until the water dropped. February 2018, same again. Christmas Day 2019. August 2021 – the fire brigade evacuated them and her cat was stranded up a spindly tree.

Jan 27, 2023, they expected a high tide flood, but at low tide the water was already a rush. She tried to clear the garage. In five minutes the flow was waist-high. She broke palings off the back fence just to get out.

“I was terrified and was screaming as I ran up to the back of the house to get my animals and evacuate.”

Feb 1, the water’s back, nearly to the top of the 6ft fences.

“The emotional trauma is extreme by this stage…As soon as I woke up, I looked outside and saw the rising water, my body started shaking.”

Feb 13 – flooded again. The water lingered for 12 hours. Hurley tried to find time to file their fourth insurance claim, between cleaning up and trying to hold down a job.

Feb 15, she wrote to Deputy Prime Minister Carmel Sepuloni and Climate Change Minister James Shaw, begging for help to get out, for good.

“We cannot go on like this, the stress and anxiety is so extreme that I can barely speak of it without crying,” she wrote. “The fact that there is discussion that more houses should be put up on poles to keep people safe is ridiculous and very upsetting. The way we live is not how anyone should live.”

The need to see the big picture is why Tonkin+Taylor flood risk and emergency management adviser Alex Cartwright advises any homeowner to get a specialist to survey their property before doing any flood resilience work.

The first thing everyone needs to do is understand their property’s flood risk, he says. And then consider options to keep the water out, or to speed up recovery if it gets in.

While jacking up buildings can work, they can become inaccessible during flooding, which creates risks both for residents and those who might have to rescue them, Cartwright says.

“Raising houses is an option, but it needs to come with a flood risk assessment and that strategic thinking around how we’re planning our communities, to make sure we’re not actually creating an island of trapped individuals or trapped properties.”

Protect and accommodate

There’s an acronym for flood resilience – PARA. It stands for protect, accommodate, retreat and avoid.

At a household level, protect means trying to keep water out. According to UK guidelines, you can do that pretty well for filthy floodwater licking up to about 60cm above your floor level. (More than that could strain the house structure).

Obviously, that requires a flood barrier. You can buy entire floodproof doors, or flood gates that slot into rails with inflatable seals. For brick homes, you can get bricks with self-closing air holes to keep water out.

Those measures aren’t completely watertight, but you might get a damp doormat instead of 20cm of water throughout your house, Cartwright says.

If you’ve already been flooded, think about building back better, he says. As well as putting in flood protection, you can better waterproof your home, so it’s easier to clean up after the next washout.

That could mean replacing carpet with tiles, fitting the kitchen with marine ply and raising up power points. That’s the accommodate bit.

The British government set up a subsidised insurance scheme to encourage flood-hit homeowners to upgrade rather than just replace.

BRANZ senior materials scientist Kathryn Stokes also advocates building back better.

“Maybe we can’t avoid floods completely, but we can choose materials that will either withstand exposure to moisture, or not get as badly damaged.”

At a community level, temporary a-frame barriers can help divert water away from houses, Cartwright says. Earth bunds around properties – like mini stop banks – might be feasible, but like lifting houses, they can have unintended consequences.

“If we put a brick in the bath, the bathwater rises up. So if we’re talking about bunding properties, you have to think about what that means for the community as a whole.”

At the most extreme end of the protection spectrum sit stop banks and hard engineering to hold back the waters. If they’re breached, you can build them higher or stronger, for a cost.

But that cost isn’t just financial. Climate change means unprecedented floods will become more common, and protections – however high or strong – will periodically fail.

Defences don’t reduce risk, they just suppress it, says Storey. And they can create a dangerous false sense of security.

“They make the 1-in-100-year event less likely [to impact you], but they encourage development behind those stopbanks, those pumps and those floodgates. So you actually end up with more assets in the way when that 1-in-200-year event occurs. So they suppress the risk that is damaging, but they can increase the risk that is devastating.”

How big a flood do you design for anyway?

Flood defences are only as good as their design brief. So what do (and should) we design for?

The Building Code requires houses to withstand flooding from surface water caused by a 1-in-50-year flood.

Deciding what frequency of flooding we should design for is tricky, says Cartwright.

“I think it’s a really important question, and whatever design flood level we’re picking, we should always be thinking around plausible future flood events, and what happens if those flood events that we’ve designed for are exceeded. That includes safe failure mechanisms.”

Niwa’s natural hazards and hydrodynamics principal scientist, Graeme Smart, points out that the flood resilience requirement for buildings is arguably much less stringent than earthquake resilience rules.

“As a comparison, Wellington office buildings are designed to withstand earthquake shaking that could be expected to occur every 500 years on average. On the basis of these statistics, an at-risk Wellington building would be inundated ten times before it was damaged in an earthquake.”

The resilience of flood defences such as stopbanks varies across the country, with design parameters ranging from 1-in-50 years to 1-in-2000 years, Smart says.

The trouble is, the goalposts are shifting. Given climate change means more frequent storms and heavy rainfall events, the past no longer predicts the future. For the Buller River at Westport, what was a 1-in-100 year flood 20 years ago is now a 1-in-60 year flood.

So how often might today’s 1-in-50 year design threshold be breached in future?

“The unscientific answer is ‘more often’,” Smart says. “There is no simple technical answer to this question.”

Deciding how big a flood to design for is always a balancing equation between the cost of protection and the value of the assets being protected, Smart says.

“Human “assets”, or loss of life, have always had intangible value. What is the value of a lost human life?

“Now we have uncertainty on the cost side of the balance because of the unstable climate. We cannot reliably estimate how often extreme events will occur in future and whether what was considered extreme will become commonplace.

“One way out is to clearly delineate hazardous areas and withdraw assets from these areas. But this can have difficult economic and social consequences.

“This significant question also has no simple answer.”

Retreat and avoid

Storey wants an immediate moratorium on any new developments on floodplains. That’s the avoid bit.

Cartwright agrees development in flood-prone areas should be avoided, at least building in the way we have historically. Turning parks into retention ponds and roads into water storage can reduce the chance buildings get swamped in the kind of flood engineers design for.

Storey says we must also think twice about rebuilding on floodplains. That’s the retreat bit.

“I understand that that is a significant number of houses, in the middle of a housing crisis.”

The government needs to speed up its intensification reforms, so we can build six-storey apartment blocks in safer areas. (Nowhere in New Zealand is risk-free).

But can you really move 675,000 people? Storey doesn’t think it would come to that. Only some of those will be in the most vulnerable houses – the first to flood and last to drain. Other houses, on the margins of floodplains, might have their life extended from 30 to 50 years by flood defences or being raised up.

“But for the tens of thousands of houses in the centre or the lowest elevation of the floodplains that are most exposed to flooding, we’re going to need to move. And we can either move now, or we can move in 10 or 20 years’ time when those houses have been flooded multiple times and more lives have been lost.”

Bridge to nowhere

Hikuwai Bridge, on the East Coast north of Tolaga Bay, looks like a piece of toy racetrack that someone snapped underfoot. Brookfields Bridge just stops mid-river. Redclyffe Bridge split in two.

Three of at least seven bridges across Hawke’s Bay and Tairāwhiti destroyed by Cyclone Gabrielle, their rupture cut off traumatised towns without supplies and hampered rescue and repair efforts.

The scene reminds Bruce Melville of his research tour around East Cape after Cyclone Bola in February 1988, shortly after he joined Auckland University. Then, as now, the landscape was dotted with busted bridges.

“Lessons were learned as a result of Bola, but perhaps not quickly enough, or well enough.”

Two processes can cause a bridge to fail, says the professor of engineering.

The first has been captured in images and video – islands of forestry slash trapped against the roadway, slammed against the structure by the raging river. The load on the bridge is proportional to the square of the flow speed. So if the water speed triples, the load increases nine-fold. And eventually, the bridge breaks.

If a destroyed bridge is being replaced, it can be built higher to let debris pass underneath. But there’s only one option to protect existing bridges, Melville says.

“The only solution, really, is for …the various government or regional agencies to get together and enforce a much tougher line on the forest companies.”

He’s not a fan of the idea of “breakaway” bridges, that open on hinges, so they move with the flow.

“That’s not a good idea. They may well break away permanently. Much better to build the bridge higher.”

Bridges can also be undermined by the flood flow scouring out the river bed around the foundations. Imagine you bang a nail into a plank, then chisel out the surrounding wood. The deeper you chisel, the more it wiggles.

Newer bridges are built deeper to withstand scour, but many older ones predate the new design standards, Melville says. He couldn’t put a number on how many. But given there are almost 4200 bridges on our state highways alone, the answer is definitely “A lot”.