17 July 2021
Flood risk in the Buller and Inangahua Rivers has triggered warnings for people to evacuate to higher land. Video / Supplied
The West Coast deluge that swamped Westport, damaging hundreds of homes, also brought "incredible" flood flows that were the largest recorded in any river here in nearly a century.
The devastating flooding on July 17 came as the result of downpours brought by a "atmospheric river" stretching from the tropics – and just the type of event scientists expect to become more common under climate change.
Newly released Niwa measurements showed the flows recorded in the storm-swollen Buller River weren't the largest ever recorded in New Zealand – data from 1926 in the same river still topped them.
But they were the highest seen since.
"It's not the largest river in New Zealand but it does experience the biggest flood flows," Niwa hydrodynamics scientist Richard Measures said.
"The flood levels through the Buller gorge are incredible."
NIWA environmental monitoring technician Mike O'Driscoll used a radar gun from the Westport and Orowaiti bridges to measure the speed of the Buller River at its surface on July 17.
These measurements, coupled with surveys of the riverbed level carried out from a jetboat after the flood, provided the data for flood flow calculations at the bridges.
As the bridges were being closed, the measurements showed a flow of 7640 cubic metres per second - the largest direct measurement of river flow ever completed in New Zealand.
The previous record was a flow measurement of 5870 cubic metres per second taken in the Grey River in 1988.
As a comparison, the mean flow of the Buller River is 454 cubic metres per second.
Meanwhile, a Niwa monitoring station on the Buller River at Te Kuha, about 10km upstream, was continuously recording water levels throughout the flood.
The monitoring station is part of Niwa's national network of benchmark hydrometric monitoring stations as well as being a key flood forecasting site for the West Coast Regional Council.
Because of its importance the site has three independent monitoring systems in place - one sensor system to provide rapid updates during floods, plus primary and back-up sensors to record data for future analysis.
However, the magnitude of the flood meant debris knocked two sensors out of action leaving only the back-up providing real time information to inform emergency management decisions.
The water level at Te Kuha peaked at 12.8m at 1.35pm on Saturday, July 17, several hours ahead of the peak flooding in Westport.
In the 1970 flood the flood peak was 11.9m - the biggest flood recorded since the Te Kuha monitoring site was established in 1963.
"We can confidently say that the recent flood flows in the Buller River are the highest of any river in New Zealand since 1926, when historic records and photographs of flooding show an even bigger flood on the Buller," Measures said.
Niwa meteorologist Ben Noll said the big storm had links back to the tropical eastern Indian Ocean, thousands of kilometres away, via an "atmospheric river" that funnelled down moisture and warmth and packed it into the low-pressure system that hit the country.
That moisture had traversed northern Australia, he said, "before being dumped into the Tasman Sea, and then right into the west of South Island".
Scientists have been astonished at the flood flows they recorded in the Buller River in July 17. Photo / George Heard
The rain was likely made worse due to sea surface temperatures also being warmer than average over the past few months.
Specifically, a climate phenomenon called the Indian Ocean Dipole had been in a negative phase – priming the part of the globe where the atmospheric river came from to produce more moisture.
"Underneath that, as well, there's now more moisture in the atmosphere than there used to be."
In most parts of the world, atmospheric rivers are expected to become more intense and frequent with climate change.
Work was now ongoing to gather more data to refine the flood analysis and Niwa's flood flow forecasting tool, while repairs to the monitoring systems had already been completed.
A flooded Westport, pictured on July 18. Hundreds of homes have been yellow-stickered from flood damage. Photo / George Heard
Measuring river flow during large floods remained essential in better understanding the risk.
The data was used to model and map flood risk, design flood defences, provide flood forecasts and understand the effects of climate change.
Data collected during this event were being used as part of project that began in April to develop improved flood forecasts for Westport.
River flows during this flood, as well as observations of the flood extent, depth and impacts to housing measured afterwards would also provide invaluable information for Niwa's Endeavour programme Mā te Haumaru ō te Wai.
That programme aims to improve New Zealand's resilience to flooding by developing a consistent national understanding of our flood hazard and risk throughout the country.
