Flood modelling uses scientific data and computer simulations to predict flooding patterns.

These models are essential for understanding flood risk, designing protective infrastructure, and assessing potential impacts on people and property.

For Te Karaka, a detailed hydraulic model has been developed to simulate flooding across the township and surrounding floodplain.

This model was calibrated using real flood height data collected from the community after Cyclone Gabrielle - ensuring it reflects actual conditions on the ground.

The model was then used to simulate a 5,625 cumec “design flood” - a major event larger than Cyclone Gabrielle.

It tested how each of the 2 proposed stopbank alignments would perform, helping determine the required stopbank heights and assess wider impacts across the floodplain.

Glossary

• Flood modelling – the use of scientific data and computer simulations to predict how flooding might occur in certain areas based on river flows, rainfall, land elevation, and more.
• Hydraulic model – a computer-based tool that simulates how water behaves across land, rivers, and infrastructure during a flood.
• Cumec – short for cubic metres per second (m³/s). It measures the volume of water flowing through a river per second. For example, 5,625 cumecs means 5,625 cubic metres of water passes a point in the river every second.
• Stopbank – a raised earth embankment built alongside a river to help contain floodwaters and protect nearby land.
• Design flood – a theoretical flood event used for planning. It’s based on statistical analysis and represents a flood of a certain size and probability. In this case, the design flood is larger than Cyclone Bola or Gabrielle events.
• Floodplain – flat land around a river that's prone to flooding when the river overflows its banks.

In June 2025, Council held a workshop with iwi, community representatives, and consultants to select 2 stopbank alignment options for further consultation. All 4 options were considered, including raising and strengthening the existing stopbank.

Both preferred options propose moving the stopbank closer to the township. This “retreat” approach creates more space for floodwaters, reduces the required stopbank height, and improves long-term resilience by lowering the risk of overtopping or breaching. It also better accommodates future changes in the riverbed (known as aggradation).

However, this change means that a small number of properties currently within the protected area would fall outside it, and some may experience deeper flooding during major events.

Council is committed to working directly with affected property owners. This may involve compensation, voluntary property purchase at market value, or other mitigation measures. All associated costs, including valuations, will be covered by Council.

The proposed retreat may also lead to changes in flood levels across the wider floodplain, which have been assessed through detailed modelling.

Flood risk impacts - wider floodplain

Modelling shows that, contrary to what some might expect, retreating and raising the Te Karaka stopbank will not significantly increase flood risk downstream.

This is because the Kanakanaia Bridge naturally slows the flow of water heading downstream, helping prevent any major increase in flood risk beyond the scheme.

In a flood the size of Cyclone Gabrielle, additional downstream flood depths are minimal - less than 200mm immediately below the scheme, and dropping to under 50mm just a few hundred metres further downstream.

Some properties south of State Highway 2, including near Takipu Marae, may actually benefit from reduced flood depths. This is due to floodwaters no longer overtopping from the township via the highway, as they did during Cyclone Gabrielle.

For the larger design flood event, downstream flood depths are expected to decrease by up to 200 mm, thanks to reduced flow conveyance through the township.

Flood Risk Impacts - wider floodplain

For flood events similar in size to Cyclone Gabrielle, the proposed stopbank retreat provides a minor benefit upstream.

However, under the larger design flood scenario, some upstream properties may experience slightly higher flood depths. This is because floodwaters will no longer spill through the township, causing a “backwater effect” that’s made worse by the flow restriction at Kanakanaia Bridge. That said, the impact is less severe than if the stopbanks were raised in their current location.

These effects vary by location and are minimal further upstream.

Near Kanakanaia Bridge

Modelling shows that properties near Kanakanaia Bridge are already at risk of flooding during a major event like the 5,625 cumec design flood - even with the current stopback in place.

Raising the stopbank will slightly increase flood depths in this area, due to reduced flow through the township.

Council will work directly with each affected property owner once a preferred option is selected, to understand and address individual impacts.

As part of the project, State Highway 2 will be raised in 2 locations, and the approach to Rangatira Bridge will be elevated to match the new stopbank crest - which will sit higher than the existing bridge deck.

Historically, the 2 low-lying sections of SH2 at either end of the scheme were managed reactively using sandbags, guided by river telemetry from Kanakanaia Bridge.

This approach proved inadequate during Cyclone Gabrielle. Since then, several upgrades have been made:

• Onsite batteries now support up to 5 days of continuous webcam footage from the Waipaoa River.
• Satellite-based communication has been added to ensure telemetry continues even if radio systems fail.