Coasters on clean-up duty have till Sunday before the rain returns

Wednesday, 27 March 2019

The sun broke through the clouds scudding over the sodden West Coast on Wednesday as Coasters began cleaning up in earnest and started counting the cost of a one-in-20 year deluge.

Rivers which ran wild across the narrow coastal plain on Tuesday, washing away bridges and road approaches and adding to flooding from the downpours, are now subsiding as the torrents of water running off the western flanks of the Southern Alps start slowing down.

Coasters will have a reprieve over the next few days before the next frontal system approaches from the northwest on Sunday.

Flooding on Hoffman St in Hokitika on Wednesday morning after 200mm of rain fell on Tuesday.

This will bring another spell of possibly heavy rain, though forecasters are at pains to point out it will be nothing like the exceptional event this week.

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Flood waters took out a section of the Waiho River Bailey bridge near Franz Josef township on Tuesday afternoon.

* Woman named after being killed in West Coast floodwaters as scale of destruction unfolds

* Hokitika family's home flooded for a third time in four years

The raging Waiho River destroyed the Bailey bridge on Tuesday afternoon.

* Marine heatwave a factor in the West Coast's torrential rain**

However, MetService head of weather communications Lisa Murray said it still had the potential to be significant.

New Zealand's wettest raingauge, at the Cropp River at Waterfall site, inland from Hokitika, set a new record of 1086mm of rain in 48 hours on March 25-26 2019.

'It's not going to be the same type of system but it could have an impact, given what's already lying and the soil being sodden.'

A ridge of high pressure over the next few days would bring settled and less windy conditions to much of the country, she said.

On a rare dry day... Niwa is amazed at the new 48-hour rainfall record set at the country's wettest raingauge at the Cropp River inland from Hokitika, but says other places may well have recorded even more rain.

Roads around the West Coast and through the Alps affected by either slips or flooding slowly opened during the day on Wednesday.

However, NZ Transport Agency system manager Pete Connors, said it could be a week or 10 days before a bridge is built to replace the Bailey bridge across the Waiho River near Franz Josef which was washed away on Tuesday afternoon.

The Cropp River at Waterfall raingauge is 975m up and inland from Hokitika. It set a new record of 1086mm of rain in 48 hours on March 25-26 2019.

An NZTA spokeswoman said State Highway 73 across Arthurs Pass reopened about 3.15pm, but was down to one lane in several locations.

Delays of up to 15 minutes were likely while road work was carried out. A slip at Candys Creek at the top of the Otira gorge had closed the road on Tuesday afternoon.

All other affected roads were open, although there were sections with just one lane and caution was advised.

The slow-moving rainband that crept up the West Coast brought astonishing amounts of rain to locations tucked in close to the western slopes of the main divide.

New Zealand's wettest monitored location, Cropp River at Waterfall, set a national record during the deluge.

The particularly pluvial location, about 975 metres up and about 40km inland south from Hokitika, recorded 1086mm in 48 hours. The National Institute of Water and Atmospheric Research (Niwa) said this was New Zealand's highest 48-hour total since records began there in 1982.

Cropp River's previous highest two-day fall was 1062mm in 2013.

Niwa climate scientist Nava Fedaeff said it was likely even more rain had fallen elsewhere in the Southern Alps.

'We just can't measure everywhere,' she said.

Niwa also reported a 24-hour rainfall at the Cropp River of 698.5mm – the third highest 24-hour total for the site and well behind the wettest day in January 2013, which had 868.5mm of rain.

Fedaeff said other records were challenged but not quite broken.

Hokitika recorded 173mm of rain in 24 hours, making it the second wettest March day since records began in 1866. Milford Sound had 401mm of rain in a day, its second wettest day in March since records began in 1929, and Mt Cook recorded a daily total of 291mm of rain, again the second wettest March day since records started there in 1928.

There were several other recent storms of similar magnitude, Fedaeff said.

'The December 1995 event was quite a significant one and caused a washout in the Waiho River. Another noteworthy event was in 1979.

'The wettest 24 hours in Mt Cook was in December 1979 when 537mm of rain was recorded. In more recent history a significant rain event occurred in April 2009 during which an active front moved northwards up the West Coast of the South Island, bringing thunderstorms and heavy rainfalls as well as widespread flooding.'

West Coast mayor Bruce Smith said the magnitude of this event was 'incredible'.

'I have never seen one so widespread. To get all the rivers from Haast to Hokitika all in flood at the same time - I've never known that.'

It was also unusual to have such a major northwesterly storm in March, he said.

'All the laws have been broken.'