Swapping luck for flood resilience in Northland

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The exit point for six major tributaries, the Northern Wairoa River captures water from a vast area – 3,650 square kilometres – more than one quarter of Northland. Its levels rise and fall under the combined influence of multiple riverine inflows, storm surges and wind, along with the Kaipara Harbour tides, which reach tens of kilometres inland. 

In the past, the river system’s complexity has made flood management and forecasting extremely difficult. Multiple tributaries converge with differing travel times and magnitudes, and interact with tidal influences, making peak levels hard to predict. 

The heavy rains brought by Cyclone Gabrielle in February 2023 pushed the system to the limit, and it was luck that peak flooding occurred during a neap tide. That experience underscored the need for an integrated model that could capture the combined effects of tides, rainfall and storm surges. 

Better information, better preparedness 

As a part of the recovery from the cyclone, Northland Regional Council received funding from the central government for projects to tackle climate change impacts and improve the region’s flood protection, including modelling the Northern Wairoa.  

Submergent won the tender for the modelling, awarded for the team’s long-standing reputation. Though a new company, Submergent’s experienced modellers were already familiar with the area and its challenges, having undertaken stormwater modelling for Dargaville.  

Working to tight deadlines 

Submergent developed a robust rainfall, tidal and storm-surge model for the Northern Wairoa catchment, one of Northland’s most complex and flood-prone systems. 

Not just experts, the Submergent team also worked collaboratively and flexibly to meet the tight deadlines set by the central government as a condition of the funding. 

Real-world validation  

Using an agile, iterative approach, the team delivered an early working model that was calibrated and refined against major storm events, with results validated under varied flow conditions. The model outputs are being ground-truthed by NRC staff, so the community can see that it reflects their experiences of past major events. This public input will allow Submergent to further refine the modelling and build acceptance within the community.   

Aiding collaboration between councils 

The new models will underpin flood forecasting for the river system, and will feed into updated hazard maps and Civil Defence planning. The region may also use the modelling to improve flood resilience in Dargaville, and inform joint projects with Whangarei and Kaipara District Councils.  

With a large harbour, multiple streams and stormwater all interacting, a single consolidated model will be essential for councils to assess options for future resilience building. 

Efficient work adds more value   

Despite being outside the initial scope, the Submergent team also modelled all of the Kaihu River, one of Northern Wairoa’s major tributaries. This is the first time the council has fully modelled how the river interacts with the surrounding system.  

This more comprehensive understanding wouldn’t have been possible without Submergent’s pragmatic efficiency. Its streamlined workflows meant it could extend the project’s scope without delaying delivery. 

A pleasure to work with 

Submergent took a collaborative approach from the start, prioritising successful outcomes over ego. Working with the council, the team identified gaps in existing data, particularly in understanding the river channel in the lower reaches, and how it was impacted during and after Cyclone Gabrielle.  

Throughout the process, Submergent worked hard to help council staff understand the technicalities and strengthen their data. 

Confidence, clarity and clear communication  

For the Northland Regional Council, the Northern Wairoa River system modelling has replaced uncertainty with evidence-backed flood intelligence. Submergent’s combination of advanced hydrologic and hydraulic modelling, local knowledge and open collaboration produced results that are strengthening preparedness, resilience and cross-council cooperation. 

This balance of technical rigour and clear communication is why Submergent and the council’s relationship is only just beginning. 

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