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July 29, 2021

by NIWA

The flood currents on the Buller River this month were the largest of all rivers in Aotearoa New Zealand for almost 100 years, NIWA measurements show.

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NIWA hydrodynamics scientist Richard Measures says records show even greater flooding of the Buller River in 1926.

“It is not the largest river in New Zealand, but it experiences the greatest flood currents. The flooding through Buller Gorge is incredible, “he said.

NIWA environmental monitoring technician Mike O’Driscoll used a radar cannon to measure the speed of the Buller River on its surface on July 17th from the Westport and Orowaiti bridges.

These measurements, combined with investigations of the river bed level, which were carried out by a jet boat after the flood, provided the data for the flood discharge calculations at the bridges. When the bridges were closed, the measurements showed a flow rate of 7640 cubic meters per second – the largest direct measurement of the river ever made in New Zealand.

The previous record was a flow measurement of 5870 cubic meters per second in the Gray River in the Year 1988. For comparison: The mean flow rate of the Buller River is 454 cubic meters per second.

Meanwhile, an NIWA measuring station on the Buller River near Te Kuha, about 10 km upstream, continuously recorded the water level during the flood. The monitoring station is part of the national NIWA network of reference monitoring stations for the hydrometric monitoring stations and an important location for flood forecasting by the West Coast Regional Council.

Due to its importance, the location has three independent monitoring systems – a sensor system for rapid updates in the event of flooding as well as primary and backup sensors to record data for future analysis.

However, the magnitude of the flood meant that debris put two sensors out of service, so only the backup provided real-time information to make emergency management decisions .

The water level in Te Kuha peaked at 13.35 on Saturday, July 17, at 12.8 meters, several hours before the high tide in Westport. During the flood of 1970, the flood peak was 11.9 meters. This was the largest flood recorded since the Te Kuha monitoring station was set up in 1963.

“We can say with confidence that the recent Buller River flood currents are the highest of all rivers in New Zealand since 1926, as historical Recordings and photos of floods show an even greater flood on the Buller, ”said Mr Measures.

Work is in progress to gather more data to refine NIWA’s flood analysis and forecasting tool. Repairs to the monitoring systems have already been completed.

Measuring the river during major floods is essential. The data is used to model and map flood risks, design flood protection measures, create flood forecasts and understand the effects of climate change. The data collected during this event will be used as part of the project to develop improved flood forecasting for Westport, which began in April.

River flows during this flood, as well as observations of the extent, depth and impact on residential buildings, which are measured afterwards, will also be used Providing invaluable information to NIWA Mā te Haumaru ō te Wai’s Endeavor program.

The program aims to improve Aotearoa New Zealand’s flood resilience by providing a consistent national understanding of our flood threats and risks across the country Developed and worked with communities, iwi and other stakeholders to develop strategies to mitigate this risk – especially in places that may be more vulnerable to flooding due to climate change.

Observations of this flood will help reduce the flooding used in the program – and Establish impact models to ensure we have a better understanding of the risks we face.

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