The site at Mt Herbert Road, Waipukurau sits on the edge of the Tukituki River floodplain. The client needed a flood assessment to confirm safe building levels for a proposed subdivision. The regional council's existing flood study was old and had limited coverage at the site, so a new assessment was needed from scratch.
The key question was which flood modelling method to use. Three accepted methods give different results on the same river - with flows varying by up to 25%. Choosing the wrong one could either leave building sites at risk or unnecessarily restrict the development.
We ran all three methods side by side rather than picking one upfront. This gave a clear picture of the range of possible flood flows. We adopted the most conservative result for all design decisions, including setting floor levels for every lot. The approach was documented and accepted by Hawke's Bay Regional Council, and the consent proceeded without dispute.
In February 2023, Cyclone Gabrielle put the assessment to a real-world test. Tukituki stopbanks breached at seven locations upstream of the site. Surrounding properties flooded. The assessed building sites stayed dry - exactly as the modelling predicted. The conservative approach was the difference between flooding and not flooding.
A 25% difference in design flow between the standard SCS method and the TP108 variant is not a minor adjustment - it's the difference between a site being flood-safe and a site being exposed. Cyclone Gabrielle was the independent test. The conservative method was correct.
