Experts analysing storm magnitude

Weather experts say it will be several weeks before the true magnitude of last week's Christchurch floods can be known.

Last week, the Christchurch City Council labelled the southerly storm a "one-in-100 year event".

But National Institute of Water and Atmospheric Research (Niwa) hydrologist Charles Pearson said it would take time to determine its strength.

Three aspects would be used to calculate the "return rate" - meaning how often it recurred - of the storm and subsequent flood. These were: The storm rainfall itself, river flows and land inundation.

The last two would take time to gather and analyse, Pearson said.

Niwa principal scientist meteorology Dr Mike Revell suspected the one-in-100 year figure used last week came from a single raingauge at the Botanic Gardens over an 18-hour period.

"But when we've looked at several of the other raingauges around Christchurch, they span the whole range, really, from return periods of three or four years right up to that one in the gardens," he said.

"Why would you single that particular one out to describe the whole storm?"

Revell said in terms of wind, the Banks Peninsula area received gusts of around 130 kilometres an hour, indicating a one-in-25 year event.

The storm itself created havoc because of the way it moved up the coast, Revell said.

"Often a front will go through and you'll have the rain band last about two or three hours. This one basically because of the position of the storm remained over the Banks Peninsula area for 36 hours."

City council land drainage operations manager Mike Gillooly said saying something was a one-in-100 year event was a slight misnomer. It actually meant in any given year there was a one per cent chance of an event of a certain magnitude occurring.

For example, Gillooly said in 1974 and 1975 Christchurch had two "100-year rainfall events" in consecutive years. Engineers calculated return rates using historical records of rainfall and river flows, Gillooly said.

University of Canterbury coastal scientist Dr Deirdre Hart said, because of the quakes, the rate that water could drain into the estuary via the Avon and Heathcote rivers had decreased. "That means it takes more time to drain water away from the city."

Questions over 'one-in-100 year' event tag

A "one-in-100 year" event. A "once a century" event. Such lovely round numbers, they seem too good to be true. That's because they probably are.

The figures were bandied around last week by the Christchurch City Council and Mayor Lianne Dalziel to help explain the appalling flooding in parts of the city brought on by heavy rain.

However, it's unclear where the numbers came from and what they actually refer to.

Neither MetService nor the National Institute of Water and Atmospheric Research (Niwa) used the "one-in-100" year phrase to describe the rainfall or flooding - in fact, Niwa says it is still gathering data and it will be some time before it can determine return periods for the event.

It's quite possible the council may be quoting that figure in relation to some other specific, highly technical, hydrological parameter. But by using the glib "one-in-100" comment it, ironically, downplays the seriousness for homeowners in flood-prone streets.

It makes it look like it was such a rare event, the chances of it happening again are remote.

Southerly rainstorms are not rare in Christchurch. While actual intensities of rain and wind vary, they often occur every year. In the last 50 years there have been more than half a dozen damaging southerlies that have caused flooding, including December 1963, April 1968, March 1975, July 1977, January 1980, October 2000, June 2013 and now this week.

This latest rain did not break records in Christchurch city. In fact, by the time flooding began in places last Tuesday evening, only 30-40mm of rain had fallen, less than half of the total that fell on the city during the storm.

Dalziel also did MetService forecasters an injustice by unfairly blaming them for "not predict[ing] an event of this gravity". Wrong! As early as the weekend, MetService was warning of the risk of heavy rain in coastal Canterbury, warnings that were firmed up at the start of last week, well before the rain began on Tuesday afternoon.

I believe there were many other factors at play that caused last week's flooding - earthquake subsidence and clogged waterways among them. My concern is that, by blithely attributing it to a very rare event, people have been given false hope.

Unfortunately, it will happen again, possibly as soon as this winter, and is highly likely before flood prevention measures for St Albans-Mairehau residents are in place two or more years down the track.

The Press