A Changing Risk Landscape
The frequency and intensity of extreme weather events in New Zealand — as evidenced by the Auckland Anniversary Weekend floods and Cyclone Gabrielle in early 2023 — has prompted a fundamental rethink of how buildings and infrastructure should be designed and located in flood-prone areas. What was previously considered a 1-in-100-year flood event is occurring more frequently, and the planning and construction implications of this changing risk landscape are significant.
For construction professionals, flood resilience is increasingly not just a design aspiration but a practical necessity — both to meet evolving regulatory requirements and to protect clients from losses that can be devastating and in some cases uninsurable.
Site Selection and Floor Levels
The most effective flood resilience measure is building above flood levels. Floor level requirements — set by councils in flood-prone areas based on flood modelling — specify minimum floor levels above which habitable spaces must be constructed. These requirements have been reviewed and in many cases tightened following recent flood events, and builders and developers need to check current council requirements for flood-prone sites.
In some areas, previously consented subdivisions and building platforms are being reviewed in light of updated flood modelling, potentially affecting sites that were developed before the updated risk assessments were available. Understanding the current flood status of a site — through council flood mapping and, for high-value projects, site-specific flood modelling — is essential due diligence before committing to building.
Resilient Design Strategies
Where building above flood level is not practicable — in existing buildings, or in areas where some flood risk must be accepted — design strategies can reduce the impact of flood events. Flood-resilient materials (concrete, masonry, tiles rather than timber framing and carpet in flood-vulnerable areas) reduce damage when flooding does occur. Electrical systems positioned above likely flood levels reduce the hazard from energised systems in floodwater. Entry and exit provisions for water (weep holes, sacrificial vents) allow water to drain quickly after flooding without the hydraulic pressure damage that a sealed structure can experience.