Flood flows on the Buller River this month were the largest of any river in New Zealand in nearly 100 years, measurements by Niwa showed.
The river was flowing more than 10 times faster than normal during flooding on the west coast earlier this month, the scientists found.
Niwa hydrodynamic scientist Richard Meathers said records show that the Buller River flooded even more in 1926.
"It is not the largest river in New Zealand, but it has the largest flood flows," he said. "The flood level in Buller Gorge is incredible."
Niwa environmental monitoring technician Mike O'Driscoll used a radar rig from the Westport and Orowight bridges to measure the speed of the Buller River on its surface on July 17, at the height of the recent flooding.
These measurements, combined with riverbed level surveys carried out from a watercraft after the flood, provided data for the calculation of flood runoff.
When the bridges were closed, measurements showed a flow rate of 7,640 cubic meters per second - the largest direct measurement of river flow ever taken in New Zealand.
The previous record was a flow measurement of 5870 cubic meters per second on the River Gray in 1988. For comparison, the average discharge of the Buller River is 454 cubic meters per second.
The Niwa monitoring station on the Buller River in Te Kuha, about 10 km upstream, continuously recorded the water level during the flood.
The monitoring station is part of the Niwa national network of water flow monitoring stations and is also a key flood forecasting site for the West Coast Regional Council.
The station has three independent monitoring systems - a system of sensors for promptly obtaining information during a flood, as well as primary and backup sensors for recording data for subsequent analysis.
But due to the magnitude of the flooding, two sensors were disabled by debris, and only the backup sensor provided real-time information for emergency management decisions.
The water level at Te Kuha peaked at 12.8 meters at 13:35 on July 17, several hours earlier than the flood peak at Westport.
During the 1970 flood, the flood peak was 11.9 meters. It was the largest flood on record since the establishment of the Te Kuha monitoring site in 1963.
“We can confidently say that the recent floods on the Buller River are the highest for any river in New Zealand since 1926, when historical records and photographs of floods showed an even larger flood on Buller,” Mizers said.