What caused the Canterbury flood? Three questions answered

After earthquakes, flooding happens to be New Zealand’s second costliest natural hazard – and the weekend’s deluge in Christchurch reflects why. Science reporter Jamie Morton looks at three questions surrounding the disaster.

Was it a one-in-100 year event?

There’s no question the weekend storm was a significant one.

Places like Akaroa, Methven and Winchmore received around twice their normal monthly rainfall in fewer than three days.

At Lismore, near Ashburton, 238mm dropped – that’s the same amount that it had received in the previous 187 days.

Such was the storm’s intensity that some labelled it a “once in a century” downpour.

But it’s really that not simple – and indeed such a calculation could be impossible to make.

Dr Rob Bell, a scientist specialising in coastal hazards, said categorising events in such a way – as had become ingrained because of past practice – was a “problematic issue”.

“We now face the ongoing influence of climate change on weather-related events that no longer fits with the assumption of no underlying change in our weather systems, climate or sea level.”

The term has long been used as a measure of the likely timeframe, on average, an event exceeds a certain level – whether that be a flood height, rainfall total, or storm-tide sea level – based on a record of historic measurements.

That meant calculating a “one-in-100 year” event from 50 years – or even 100 years of records – came with obvious uncertainties.

Bell said a slightly better measure was the “annual exceedance probability”, or AEP.

By this, a “one-in-100-year” event had a chance of one per cent each year of being reached or exceeded.

“So the clock doesn’t get reset once such an event occurs, such that it won’t occur again for 100 years,” he said.

“It’s like a computer picking randomly from numbers one to 100 and getting a one, then five rounds later it could be a one again.”

The usual way the AEP of a flood is calculated was with river gauges, which measure the height of the water in a river at a given time.

This was converted into a “flow” – or how many cumecs, or cubic metres of water, per second, were flowing down the river using a rating curve.

“So for rivers where we have gauges, we have measurements of the flow in that river over time,” said Dr Emily Lane, a hydrodynamics scientist at Niwa.

“To calculate the AEP of a flood in that river, we take the largest flow in the river each year and we look at the values we get for this over many years.

“If we had hundreds of years of data this would be easy and the one in 100 year event would be the one that happened on average once in 100 years.

“It is a bit trickier though, because we don’t have that length of data, so we have to use mathematical techniques to work out what these would be if we have shorter records.”

That meant that, the shorter the record was, the more uncertainty there’d be with the result.

“If there happens to have been a big flood in that time, it might overestimate – or if there haven’t been any floods, it might underestimate.”

She also pointed out that floods didn’t happen regularly.

“Just because there is a big flood now it doesn’t mean there won’t be another one in the near future. They still might on average only happen once in a 100 years.”

And the other obvious problem: the planet was warming and changing our weather.

“One of the big assumptions in this method is that the climate is not changing – of course we know that this is not the case at the moment,” Lane said.

“So we are using the past record to calculate the probability of a flood occurring but because of climate change the probability of that flood occurring might be increasing.”

Especially because of climate change, Bell said using the AEP measure was causing confusion.

“The past measurements are no longer a reliable guide to future events – both the size and how often they will occur,” he said.

“An event that in the past might have occurred, on average, once in 100 years is likely to occur, on average, once every 50 or 80 years or so over the next few decades.”

So how should these big events be categorised?

“That is a tricky one,” Lane said.

“People always ask what the probability of a flood is and a lot of our planning and design and hazard management are based on probability of events happening.

“I think we need to ensure that whenever we talk about the probability of flooding we acknowledge that we are in a changing climate and that probability of flooding might increase.

“We are also working hard to understand how climate change might affect these probabilities but there is a lot of uncertainty in that – not least what we as the human race are doing to reduce our carbon emissions.”

Did climate change play a part?

Such was the complex nature of the atmospheric and ocean processes driving the storm, that it remained difficult to ascribe this or any other weather event to climate change.

The deluge came as a result of a deep and slow-moving low-pressure system that helped siphon moisture through an “atmospheric river” stretching thousands of kilometres back to the subtropics.

The way that moisture was slung around the low and on to the eastern South Island – where moisture-packed air masses dropped their loads before climbing the Southern Alps – was something seen more commonly with big West Coast rain events.

Further in the background, the moisture coming from the tropics had been charged by the presence of a pulse of rain and thunderstorms that circulates the planet every 30 to 40 days called the Madden-Julian Oscillation, or MJO.

But that wasn’t to say a warming climate wasn’t a factor.

“As the climate warms, there is more moisture in the air on average, so when it rains it is likely to rain harder than it used to,” Victoria University climate scientist Professor James Renwick explained.

“That’s why we expect the occurrence of heavy rainfalls and floods to increase over time as it continues to warm up.

“At the same time, the very heaviest rains are getting heavier, so records will be broken and the once-rare events will become more commonplace.

“Unfortunately, the terrible damage we’ve seen done in Canterbury over the past couple of days is something we are likely to see more often in future.”

Lane said the sheer amount of water that fell was the main factor in Canterbury’s storm, but there were also other exacerbating factors.

“The sea level is higher than usual at the moment because of the spring tides from the super moon last week and storm surge driven by strong winds and low air pressure,” she said.

“This intensifies the flooding in coastal areas as the water can’t drain away as fast. With the drought conditions Canterbury has been experiencing recently, the ground is less able to absorb water so more of it ends up in the rivers.”

Renwick added that rivers actually flooded naturally, and could change course unpredictably over their floodplains.

“For instance, the Waimakariri River has in the past flowed through the area where Christchurch now is,” he said.

“Building stop banks reduces the chances of such events and are a good protection against most floods.”

The downside, he said, was what was called “maladaptation” – or where communities developed a false sense of security through the presence of the stop banks.

“That can lead to increased urban development close to the river, and even greater damage and misery when the really big flood comes.”

As for climate change, Lane said the deluge highlighted an important point.

“Canterbury was in the grips of a drought recently and lack of water was a far bigger problem,” she said.

“Then suddenly when the water came, it came all at once. These sorts of extremes are expected to occur more frequently under climate change.

“The expected increase in these types of drought-flood cycles needs to be incorporated into future planning.”

Are "atmospheric rivers" new features?

Atmospheric rivers aren’t anything new – but scientists are learning more about them all the time.

Otago University senior lecturer Dr Daniel Kingston described them as “long, thin filaments” of atmospheric moisture transport that could carry more water than the Amazon River.

“Atmospheric rivers more commonly occur on the West Coast of the South Island, but this one made landfall on the east coast instead.”

Associate Professor Asaad Shamseldin, of Auckland University’s Faculty of Engineering, said these rivers were known to a major contributor to the water cycle, severe floods, droughts breaking, and strong winds worldwide.

On average there are only three to five atmospheric rivers present in each hemisphere, covering just 10 per cent of the globe’s mid-latitude circumference but accounting for 90 per cent of moisture transport in the same region.

But so far, there was a very limited understanding about the impacts they’d had in New Zealand specifically.

Around 40 atmospheric rivers make landfall in New Zealand every year, with around 20 being weak, 10 to 15 being classed as “rank one”, or at the lower end of scale, and four or five being strong, predominantly occurring during summer.

Atmospheric rivers had the greatest impact on the West Coast of the South Island, accounting for much of the heavy rain commonly seen there.

One dramatic example was the March 25, 2019, storm which washed away the Waiho Bridge near Franz Josef on the West Coast.

That event packed the highest ever total of rainfall over a 48-hour period in New Zealand recorded history – reaching 1086mm.

While the most severe happened around once every five years, evidence suggested that these bigger events may become more frequent and more intense in a warming climate.

Earlier this year, an Otago University study provided the first detailed analysis of their effects on local weather events.

“In very basic terms, one of the results of a warmer climate is a wetter atmosphere,” said the study’s lead author, Hamish Prince.

“With more moisture in the atmosphere the frequency and magnitude of atmospheric rivers making landfall in New Zealand is expected to increase.

“The increase in extreme precipitation is not expected to occur equally throughout the nation however, with indications that the landfalling location of atmospheric rivers is shifting southward in New Zealand.”

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