NIWA modelling of Cyclone Gabrielle flooding

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.