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NIWA modelling of Cyclone Gabrielle flooding

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 findings

NIWA’s analysis underlines the extraordinary magnitude of the event. At 13 of the 20 sites, it was deemed the largest flood on record.

Pre-Cyclone 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 one river site, according to NIWA’s modelling.

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

The modelling is valuable for the Regional Council in furthering knowledge of the region’s rivers and managing flood risk.

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.

NIWA's full report is available here

The table below shows a pre-Gabrielle ARI and a post-Gabrielle ARI. These are different because an event the size of Cyclone Gabrielle increases understanding of the probability of a flood this size occurring in a given year.

In the table, there are two ARIs for Mangaone River Rissington, and two ARIs for Ngaruroro River at Fernhilll. The lower ARI is based off the recorded peak river flow, before the river gauges at those sites failed, due to the severity of flooding. The higher ARI is based off a modelled estimate of the peak flood flow.

See the report for what methodology was used.

  Annual Return Interval (ARI)
Site name  Pre Gabrielle  Post Gabrielle 
Tūtaekurī River at Puketapu  980  400
Mangaone River at Rissington  >1,000^ 
Ngaruroro River at Fernhill  710^ 

Ngaruroro River at Whanawhana  120  70
Waipawa River at RDS  >1,000  120
Tukituki River at Tapairu Rd  160  70
Tukituki River at Red Bridge  80  60
Esk River at Waipunga Bridge  220  180
Esk River at Berry Rd  550  120
Wairoa River at Marumaru  250  120
Waiau River at Ardkeen  50  40
Waiau River at Otoi  30  30
Hangaroa River at Doneraille Park  420  220
Ruakituri River at Tauwharetoi  50  40
Taurekaitai Stream at Wallingford  60   50
Mangaorapa Stream at Mangaorapa Rd  >1,000  110
Pōrangahau River at Saleyards  >1,000  80
Tukipo River at SH50 (Punawai)  170  90
Kopuawhara Stream at Railway Bridge  4  4
Awanui Stream at Flume  50  30

Peak river flow rates per year

These infographics show the peak flow rate in cubic metres per second, for each year where records are available, at four of our sites. Click to see the full PDF file.

What is a 1-in-100 year weather event?

This video from NIWA explains what a 1-in-100 year weather event means, and corrects some common misconceptions.


An Annual Recurrence Interval is the average number of years that is predicted will pass before an event of a given magnitude occurs. For example, a 40-year ARI event would on average happen every 40 years. 

This is the probability a flood of a certain magnitude this year will occur in a given year (i.e. a 100-year ARI flood has an 1% AEP). 

The occurrence of a rare flood does not reduce the chances of another rare flood within a short time period. The probability of occurrence is the same every year. 

The pre-Gabrielle ARI expresses how likely we thought this magnitude flood was before it happened (based on available data). The post-Gabrielle ARI tells us, now that we know Cyclone Gabrielle has happened, our best estimate of having a similar sized flood in the future. 

To understand the risk of floods, the Regional Council uses modelling techniques to understand the probability of a flood event occurring. 

This NIWA work provides critical modelling about river flows that occurred during Cyclone Gabrielle and means that agencies are better able to judge areas at risk from future flooding and better able to predict water depths and velocities should flooding occur. 

Understanding the magnitude and frequency of floods is vital in managing flood risk.  

At 13 of the 20 measurement sites, it was deemed the largest flood on record. The sites where this was not the largest on record are: 

  • Ngaruroro River at Whanawhana 
  • Tukituki River at Redbridge 
  • Waiau River at Ardkeen 
  • Waiau River at Otoi 
  • Hangaroa River at Doneraille Park 
  • Ruakituri River at Tuwharetoi 
  • Kopuawhara Stream at Railway Bridge 

The Regional Council will assess the impact for all flood infrastructure. In two areas in particular where the NIWA data is less certain, Waipawa and Esk Valley, further work is required to understand the flows and implications for flood design mitigations.  

This is a regional scale report and will inform all current scheme reviews and planning for future schemes. Including proposed schemes under the land categorisation. 

The report was commissioned by MBIE and prepared by NIWA as a state of the environment document. The main purpose of the NIWA modelling was to understand Cyclone Gabrielle’s flooding in the context of what was previously known about flood flow values in the region.  

The NIWA data is not meant to be definitive, but a useful starting point for agencies to assess the impact on such things as building consents and bridge designs. 

Cyclone Gabrielle showed that we are already beginning to see the impacts of climate change in Hawke’s Bay, and modelling shows that the impacts will become stronger and more frequent over time. We know that rainfall is more intense and storms more frequent under increasing degrees of global warming.  

The flood schemes are not designed for a Cyclone Gabrielle-sized event. Initial testing at key locations indicates we are still within the 1 in 100-year standard. 

This NIWA report shouldn’t cause alarm for areas where there is still some further work to do. We are still providing 1:100-year level of protection until we find otherwise. The report is an important step in understanding if further upgrades might be needed in places like Waipawa.

The level of 1:100-year protection in the Upper Tukituki/Waipawa is the current standard. NIWA’s report indicates a level of uncertainty about the return period. Further work is required and HBRC has already commenced this work with urgency. 

It was always intended that the NIWA work is a starting point for the detailed analysis of the flood review schemes. In Upper Tukituki we commenced a holistic review of the scheme last July and have made great progress, and now the NIWA data has fed into to the model to fully understand the service level.

This report was not commissioned to inform insurance.


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