Thursday, April 25, 2024

Gabrielle resets what we know about HB flooding

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Probability of such a flood now expressed as a ‘one-in-550-year’ Annual Recurrence Interval.
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Cyclone Gabrielle has reset the scale for flooding in Hawke’s Bay.

The National Institute of Water and Atmospheric Research (NIWA) undertook analysis of the flood flows that occurred at 20 river gauge sites across Hawke’s Bay during the cyclone. 

The work was funded by the Ministry of Business, Innovation and Employment (MBIE).

Pre-Gabrielle, the probability of a flood this size occurring in a given year, known as an Annual Recurrence Interval (ARI), was as low as a one-in-1000-year event at some river sites, according to NIWA’s modelling. 

Post-Gabrielle, that probability has changed to a one-in-550-year event – meaning the cyclone has changed the standard going forward.

At 13 of the 20 sites, it was deemed the largest flood on record.

NIWA’s principal scientist-natural hazards and hydrodynamics, Dr Emily Lane, said the work was to understand Cyclone Gabrielle’s flooding in the context of what was previously known about flood flow values in the region.

“This event reset the scale for flooding in Hawke’s Bay. It’s vital we factor this into our understanding of future flood hazards.” 

NIWA undertook analysis of the flood flows that occurred at 20 river gauge sites across Hawke’s Bay during the cyclone.

Flood events occur randomly, and so just because a large flood with a high ARI has just occurred, there is no guarantee that there will not be another one in the near future, she said.

Hawke’s Bay Regional Council chair Hinewai Ormsby said she welcomes the report. Its data is one piece of the puzzle to inform flood review and scheme reviews, she said.

“The modelling reinforces that the cyclone flooding was largely unprecedented. The sheer volume of the cyclone’s flooding and its impacts on flood mitigation infrastructure could be compared to having a 100-bed hospital where suddenly 500 patients turn up. 

“This report is incredibly valuable in understanding ecosystem dynamics and managing flood risk, in a world that is changing around us. It is an essential first step, but not the only step in informing the regional council in its development of future flood resilience designs and mitigations.”

The regional council worked with NIWA and provided regional expertise and understanding of the river’s dynamics, including long-term data and knowledge of historic flood levels.

Chris Dolley, group manager, asset management for the regional council, described the NIWA modelling work as “extremely beneficial”. 

“It will allow concept designs for stopbanks and other infrastructure to be fine-tuned.”

Danette Olsen, general manager science system investment and performance at the MBIE, said the science sector “has expert knowledge to share on the impact of extreme weather events such as Cyclone Gabrielle. The funding provided by MBIE on behalf of the government will enable us to better prepare for future events.”

More: Read and download the report and infographics here.

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