Scientific analysis of Cyclone Gabrielle’s flooding has underlined the extraordinary magnitude of the event.

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 Cyclone Gabrielle. 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 high as a one in 1,000-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 says 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.” 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 says.

Hawke’s Bay Regional Council Chair Hinewai Ormsby says she welcomes the report and its data is one piece of the puzzle to inform our flood review and scheme reviews.

“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 describes 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, Ministry of Business, Innovation and Employment (MBIE) says: “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.”

To read and download the report and infographics, go to https://www.hbrc.govt.nz/ourcouncil/cyclone-gabrielle-response/niwa-modelling-cyclone-gabrielle-flooding

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