Q&A: What is a one in 500-year flood, and does that really apply to Edgecumbe? video

CHRIS McKEEN/Stuff.co.nz

Work on rebuilding Edgecumbe's flood wall began last week.

Edgecumbe's flood has been identified as the greatest flood on record  - but is it truly "a 500-year flood", as has been suggested?

Bay of Plenty Regional Council made the claim after the Rangitaiki River burst through stopbanks designed for a one-in-100 year flood event, gushing into hundreds of homes and forcing 1600 people to evacuate.

Though the weather has cleared, houses remain sodden and filled with mud, and some will be condemned due to structural damage. Residents will be allowed to return briefly to fetch necessities, but it could be up to 10 days before many occupants can go home to inspect the damage and find out what they've lost. 

The Edgecumbe flood burst through stopbanks designed to stand up to 100-year floods.
CHRIS MCKEEN/FAIRFAX NZ

The Edgecumbe flood burst through stopbanks designed to stand up to 100-year floods.

Meanwhile, work is ongoing to assess how the flood really stacks up against others in New Zealand's history - and how soon we might expect a repeat.

READ MORE:
Edgecumbe and Bay of Plenty have shaky, flood-prone histories
Photo essay: Edgecumbe in ruins
Kayaking into Edgecumbe

 
Whakatane Mayor Tony Bonne, right, says it was a "one-in 500-year" flood, but Bay of Plenty Regional Council won't ...
TOM LEE/FAIRFAX NZ

Whakatane Mayor Tony Bonne, right, says it was a "one-in 500-year" flood, but Bay of Plenty Regional Council won't release the data that assessment was based on.

WHAT IS A '500-YEAR FLOOD'?

Prepare for a maths lesson.

The label "a 500-year flood" refers to what experts call a "return period" or "recurrence interval" - an estimate of how long until the next disaster of the same size or intensity.

Water gushed into hundreds of homes, forcing 1600 people to evacuate.
Chris McKeen

Water gushed into hundreds of homes, forcing 1600 people to evacuate.

Alongside climatic events, like rainfall, floods and tornadoes, return periods can also be applied to earthquakes, tsunami and volcanic eruptions.

Ad Feedback

When we talk about flooding return periods, a one-in-100-year flood means a one per cent chance, on average, of a repeat event in the same spot occurring in any given year, regardless of when the last one was.

A one-in-10-year flood would have a 10 per cent chance; for a one-in-500-year flood, it's 0.2 per cent in any given year.

The council and locals will need to assess whether future stopbanks need to have a higher level of protection.
TOM LEE/FAIRFAX NZ

The council and locals will need to assess whether future stopbanks need to have a higher level of protection.

Of course, New Zealand doesn't have 500 years of flood records - in fact, many cities and towns' records only go back a matter of decades - meaning a hydrologist's job of predicting flood flows for a 500-year event is challenging.

HOW DO WE KNOW HOW BIG A 500-YEAR FLOOD IS?

Without a longer historical record, experts can only make an estimate.

"When you've got a short record, then you're quite uncertain about those very long return periods, or very low-probability events," Niwa hydrologist Roddy Henderson says.

"You're both uncertain about the magnitude of them, or if you get one, you're uncertain about what its exact return period is."

He gives the example of looking at 50 years worth of flood records, finding the most extreme flood, and assuming it was a one-in-50-year event, or perhaps a one-in-100-year event.

The best guess, he says, "often is somewhere in between those two ... You end up with an estimate where the biggest record is maybe 1-½ to 1-¾ times the length of record - that's your sort of expectation".

Estimating what a 500-year flood would look like is, therefore, a challenge, with climate change, climate cycles and weather patterns, and different ways of analysing data internationally all adding to the difficulty.

DID EDGECUMBE REALLY HAVE A ONE-IN-500-YEAR FLOOD?

Bay of Plenty Regional Council has repeatedly said Edgecumbe is experiencing a 500-year flood - a claim "based on technical information from the regional council", according to Whakatane Mayor Tony Bonne.

The council is refusing to make that information public.

The Rangitaiki River's flood records go back to 1952, and it's fairly hard to estimate exactly how big a 500-year flood would be from 65 years of data. 

The town's stop banks were built for a 100-year flood, and the council says the flood was "30 per cent bigger" than that.

However, on Saturday, council flood manager Roger Waugh walked the original estimate back, saying: "Once the analysis has been done, it's likely to be in the order of a 200 to 500-year flood."

Henderson says it'll be some time before the flood's actual return period is known.

"To get something exceptional after 50 years of recording does put you up quite likely beyond 100 [years] usually, but to be precise about it, no, I certainly wouldn't be going that far at this stage before someone's had a chance to have a good look at the data."

The regional council and Niwa staff were still working to assess the flood's flows, "and until that's done, it's not really feasible to put a return period on the event", he says.

While we could assume this week's flood is the biggest Edgecumbe - or New Zealand - will ever get, Henderson says that's probably not the case.

"The fact that we've seen it that large means there probably could be something a bit bigger. And so the debate is all around, right at that end, exactly how rare were those biggest three that we've seen and how much bigger could it get."

DOES IT REALLY MATTER?

To a degree, yes - but it's not the biggest issue here.

Auckland University associate professor Dr Asaad Shamseldin, an expert in hydrology and water engineering, says the main point is that regardless of the flood's return period, the town's stopbanks were inadequate to stop it.

"The debate should focus on whether they should increase the level of protection, rather than trying to debate whether it's 200 or 500 or 300 or 350 ... You are talking about events which are very, very rare, and their estimation is very, very uncertain," he says.

"The most crucial thing is that [the floods] exceeded the design capacity, and the issue here, the dilemma which will face the politicians and the council and the public is: Should we go for more protection?"

While Edgecumbe could build higher stopbanks, other options might be raising floor levels, or relocating parts of the town.

A higher level of protection, he adds, will have extra costs - and the council and locals will need to decide how much they're willing to pay.

HOW DO OTHER EVENTS MEASURE UP?

* In 1978, a "100-year flood" swept through Southland. Just six years later, another flood saw waters at Invercargill's airport rise 1.48m deeper

* A magnitude 7 quake in Wellington would be a one-in-500-year event

* Flooding in St Louis from Hurricane Katrina was described as a 500-year flood

* Auckland's Volcanic Field has an estimated return rate of 900 years

* Japan's 2011 magnitude 9 quake, which sparked a deadly tsunami, was a one in 1000 year event

* The Okataina Volcanic Centre, including Mt Tarawera - which erupted in 1886 - has a return rate of about 4000 years

* The Greendale fault, responsible for kicking off the Canterbury earthquakes, is estimated to be a one in 10,000-20,000 year event.

 - Stuff

Ad Feedback
special offers
Auckland

Fire causes 'substantial damage'

Fire fighters are battling a blaze at a rural house near Warkworth, north of Auckland.

Occupants of the burning house - near Warkworth, north of Auckland - fled and are safe.

Fatal Northland crash

The crash occurred on Kapiro Road, north of Kerikeri.

A crash involving a car and a motorbike in Kerikeri has left one person dead.

'Phantom' to remain jailed

Loizos Michaels, pictured before being sent to prison. He has been decline parole twice.

A conman who calls himself 'The Phantom' and once duped Jonah Lomu has been denied parole.

Thousands gather in Auckland

The Anzac service at the Auckland War Memorial on Anzac Day 2017.

A crisp, clear morning greeted thousands of Aucklanders at the War Memorial Museum Anzac Day service.

Wellington

Services draws thousands video

Willie Apiata greets a soldier with a hongi during the Anzac Day National Commemoration Service at Pukeahu National War ...

Thousands stood in silence in remembrance of those who have fought under the Anzac banner.

Missing hunter found

Search and rescue operation on Mt Taranaki.

A search team airlifted the 62-year-old man unharmed from the Kaweka Ranges.

Crook denied parole 

Kelly Leigh Crook was jailed for her role in the murder of Matt Stevens, which she helped attempt to cover up by faking ...

Kelly Leigh Crook lured a man to a deadly ambush then attempted to cover up murder by faking a car crash scene.

Protest mars service video

Newshub was interviewing peace activists Ellie Clayton and Laura Drew when they were interrupted by David Broome, and ...

"Do it tomorrow, do it the day before - but today is wrong, wrong, wrong." Father and son clash with peace activists.

Canterbury

Wreath behind locked gates video

Jim Lilley, left, Allan Luchford and Cathy Luchford lay their poppies at the Christchurch Citizens' War Memorial.

The Christchurch Citizens' War Memorial has been off limits for six years. But this Anzac Day is different.

Climbers' bodies found

The bodies were found in the Darren Mountains.

The bodies two missing people have been recovered in Fiordland.

River surges to life

Sarah McCall and her family stand on the banks of the Eyre River, normally a dry riverbed, which surged to life on Good ...

"I don't know how it's doing that; I think someone's filling it up," 6-year-old Ruby McCall suggests.

Rentals smelly, musty and mouldy

Rental homes are more than twice as likely to have poorly maintained features than owner-occupied buildings, according ...

Rental homes are twice as likely to smell musty, and three times more likely to feel damp.

Waikato

'Today means everything to me'

The Anzac dawn service attracted thousands to Hamilton's Memorial Park.

It's the one day the world remembers yet every day reminds this family of the price of war.

Body found in bush

Thames

A hunter alerted police to the grim find just after 1pm on Anzac Day.

Five hurt in highway crash

Emergency services are at the scene (file photo).

Two people suffered moderate injuries - three others got away with scrapes and bruises.

Stolen war medals come home

Sam Healy and daughter Jorji, 9, are reunited with their family's war medals.

A year ago thieves stole Sam Healy's grandfather's war medals. On Anzac Day eve, she got them back.

Taranaki

Teen to take on snooker's best

New Plymouth's Mario Hildred, 16, in preparing to take on the world's best in China later this year.

"Obviously I feel a lot of pressure when I get a high break to keep going to get more."

Anzac Day in Waitara

The Anzac Day Parade in Waitara before the civic service.

The weather was perfect for the Anzac Day services held around the mountain on Tuesday.

Missing Taranaki man

Jared Shaun McLean has not been heard from since Saturday.

"We all are actually worried, it's just unlike him to do this."

Bramhill for Greens

Dr Stuart Jeanne Bramhall has been picked as the Green Party's candidate for the New Plymouth electorate in the 2017 ...

Long time environmental activist Stuart Bramhill is the Green's candidate for New Plymouth. 

Manawatu

Gorge could be closed for weeks

About 3,500 cubic metres of earth is blocking the Woodville end of State Highway 3 through the Manawatu Gorge.

Manawatu Gorge set for another long closure after two slips block lanes.

Woman badly hurt

Police are at the scene of a crash in Dannevirke, where a motorcycle hit an elderly woman.

An elderly woman is in a critical condition after being hit by a motorcycle in Dannevirke.

Child basher in denial

Peter Ross Moran wants to live near a child whose brain he damaged.

He caused his victim's brain to become "watery mush", but still denies doing anything wrong.

Smoke and coffee to start the day video

Vietnam War vet Alby Armstrong gets ready for the dawn service in Palmerston North.

Alby Armstrong thinks about other soldiers when he's at a dawn service.

Nelson

Richmond remembers lost heroes

Poppy laying during the Anzac Day service at War Memorial Gardens, Richmond.

Anzac Day crowds overflowed into the streets around Richmond's memorial gardens.

A city pays tribute video

Anzac Day dawn parade and service at Anzac Memorial Park, Nelson.

Thousands woke before dawn to attend Anzac Day dawn service in Nelson.

Nelson: The new Silicon Valley?

Markus Erdmann and Vivian Escorsin are part of the entrepreneur group Local.Foundation who are organising a business ...

Business group says Nelson has potential to become "start-up hub of New Zealand".

Flowers revive wartime memories

World War II veteran Allan Carson holds poppies from Flanders Fields in Belgium, brought back from a visit there in ...

A Nelson man's souvenir from a WW1 battlefield stirs up two generations of memories.

Marlborough

Pak 'n Save's unusual policy

Two German men have stolen meat, cheese, beer and herbs from Pak 'n Save Blenheim.

What to do with food found on a shoplifter's person? Put it back on the shelves, of course.

Anzac day treats

Kaikoura council earthquake recovery economic lead Mel Skinner with a plate of Anzac cookies from Wellington's Good ...

The Good Bitches' homebaking brings a sweet reassuring note to earthquake recovery effort.

'It's a reality check'

Dawn services at the Seddon Memorial Hall.

Friends, family remember those who made the ultimate sacrifice in Anzac commemoration.

'The supreme sacrifice'

Australian Army veteran Betty Binnie gets her two remounted medals pinned to her chest by RAAF Squadron Leader Marcus Watson.

Rocked to the core by earthquakes, the people of Ward stand strong to remember the fallen this Anzac day.

South Canterbury

Honouring sacrifice

War veterans march on Memorial Ave towards the cenotaph at the Anzac Dawn Service in Timaru.

A crowd of a few hundred stood at the cenotaph in Timaru to commemorate Anzac Day.

Raising awareness 

Volunteer firefighter and former NZ Army infantryman  Maaka Mckinney is escorted into Timaru from Waimate by Timaru ...

Maaka McKinney is raising awareness for depression among first responders.

New book on war hero

Tom O'Connor has published a book on Waimate war hero Eric Batchelor.

The life of one of New Zealand's most decorated soldiers has been detailed in new book.

Man killed in crash

A man is dead after a crash on Freehold Creek Bridge at Lake Ohau.

One person has died after a single-vehicle crash on a bridge in the Waitaki District.

Southland

50-year-old receipt returned

Sue Samson with the 50-year-old receipt left in the Winton Night 'n Day store on April 6.

They'd kept their honeymoon receipt for five decades. So when a shopkeeper found it, she knew she had to return it.

'Two shot' in city

Police have cordoned off Otepuni Ave in Invercargill after a suspected double shooting.

Police indicate two people have been shot and one person is in custody following an armed incident in Invercargill.

Kiwi climbers perish in Fiordland

The bodies were found in the Darren Mountains.

The pair were roped together traversing a "climber's Mecca" when a fall cost them their lives.

Nasa balloon seen from Chch

Ashley Marles managed to photography the balloon with his own observatory.

That "bright stationary object" is headed for outer space - with an Anzac poppy in tow.

Ad Feedback