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.

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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

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