Storm surge's wall of water one of the biggest recorded in Firth of Thames

Friday, 12 January 2018

Many of New Zealand's coastal areas were hit by high winds, heavy rains and a storm surge causing widespread damage.

Some called it a wall of water, others compared it to a tsunami.

However you describe it, the storm surge that flooded Firth of Thames locales last Friday was among the worst on record for the area.

The sea level at the peak of the storm was near that of the Hauraki Plains floods of 1938, according to Waikato Regional Council data.

Water levels in the storm were more than a metre higher than a normal high tide, Waikato Regional Council's Rick Liefting says.

This time, homes, farms and roads around the coast were soaked and battered, prompting a Civil Defence response which involved the NZ Defence Force and the Red Cross.

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Several hundred homes on the Thames Coast and in Kaiaua were affected by the storm.

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'As a guide, this is one of the biggest events that have occurred in the Firth of Thames in recorded history,' Waikato Regional Council's Rick Liefting says (file photo).

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The storm ripped seal off the Thames Coast Road and washed out some of the layers under the seal.

'We are still assessing … but, as a guide, this is one of the biggest events that have occurred in the Firth of Thames in recorded history,' Waikato Regional Council's Rick Liefting said.

'The water levels were more than a metre higher than your 'normal' high tide.'

An offshore tide gauge in Tararu recorded a 2.8m water level, the regional hazards team leader said, though waves would have exceeded that.

The biggest 'storm tide event' on the Firth of Thames was in 1938, he said, when much of the Hauraki Plains was submerged and the water level was estimated at 3m.

Stopbanks on the plains stopped them going under this time, Liefting said, but mid-storm, the water was nearing the top of those built by the Piako River.

The Thames Coast has been hit before and will be again, Thames-Coromandel District Mayor Sandra Goudie says (file photo).

Three key factors combined to beef up the storm, he said: low atmospheric pressure, winds, and a king tide.

'It occurred smack bang on the highest tide of the year.'

Farmland and homes near Kaiaua, on the western side of the Firth of Thames, were flooded by the storm surge (file photo).

The only thing missing to create a perfect storm was heavy rain.

'Three potential whammies'

The water was at a similar level to a 1938 flood, which submerged much of the Hauraki Plains, according to Waikato Regional Council data (file photo).

'We were hit with three potential whammies all at one time,' Hauraki District Civil Defence Controller Steve Fabish told viewers in a live-streamed update on Wednesday.

Footage of East Coast Road on Friday morning - the main road out of Kaiaua - on the western Firth of the edge of Thames.

'So [the water level] was at least 0.4m higher than that '95 flood that most of us can certainly remember.'

The storm started in the moisture-filled subtropics, where a strong low pressure system developed, Niwa meteorologist Ben Noll said.

That essentially charged up as it travelled over the warmer-than-usual Tasman Sea - 'a boon in terms of strengthening'.

In the mix were the highest tides of 2018 and northerly winds which blew right down the firth and pushed water on to the coast.

'They all sort of worked together to generate what was a pretty impactful situation.'

Looking ahead, the forecast was for a week of fairly settled weather, with perhaps a bit of gusty rain.

From early February, there could be 'a rather busy time in terms of rainfall, wind and possibly waves' for people in the upper North Island.

'It's not to say there won't be plenty of nice days in there, too.'

Up the Thames Coast

The Thames Coast has been hit before and will be again, Thames-Coromandel District Mayor Sandra Goudie said - though this is the worst she's seen.

The NZ Transport Agency had been going hard and fast to repair the battered Thames Coast Road and keep it open to at least one lane.

People affected by the floods were barely over the clean-up, she said, but in future they would have to think about whether they wanted to lift up their homes or cut their losses.

Some affected areas were quite small, she said, so if council added extra storm or flood protection, there would be few people to share the cost.

The climate change question

The after-effects of the storm have prompted plenty of comments about climate change.

It affects all weather that we experience, Niwa's principal scientist for climate Dr Brett Mullan said.

'The difficult thing is to say this is the climate change part of that storm surge compared to what you might have got anyway.'

Continued sea level rises could heighten the base that storm surges start from, increasing the effect, he said.

'Even for the same intensity of weather system, if the sea level keeps going up, as it has been, we would expect there to be a big impact.'

It's an oversimplification to simply say storms will just get more intense, he said, as so much depends on geography and where the storm is coming from.

But climate change is expected to produce more extremes on the rainfall front - heavier downpours with more dry spells in between - and temperatures that hit extreme highs more often.