More than two years of technical investigation and review has produced new and valuable information about where water might pond around Hamilton in heavy rain and extreme events.

The new update adds ‘flood depressions’ to the city map, identifying areas around Hamilton Kirikiriroa that have the potential to fill up with water when pipes and culverts are blocked, where there is simply too much water, or where there are no stormwater pipes to carry water away.

Hamilton City Council is making the data available on its public Floodviewer tool; a city map which helps residents understand rain effects on their property.

“Think of them like bathtubs,” said Council’s Stormwater Infrastructure Engineer, Andrea Phillips. “If a pipe or culvert gets plugged by debris or other items, or there is simply too much water for the systems to cope with, the water will pond until it can be released or fills the depression.”

This scenario, although rare, was seen in the Auckland Anniversary Weekend floods of 2023, as torrential rain resulted in blockages to pipes and culverts. Understanding what might happen in a similar rare event in Hamilton helps Council, and homeowners, plan for the future.

Around a third of properties in Hamilton have some of their land within a flood depression area (mostly properties around the city’s extensive gully system) and for most the flooding in the modelled scenario is minor or restricted to only a part of the land.

Most of the properties with higher potential flooding will have previously been advised by Council as part of its wider flood mapping work. For added assurance, Council is writing to about 650 properties which have a potential average of a metre of flooding, or more, at a building footprint) in the modelled rare event.

This will inform those potentially-affected properties and means there will be cases where nearby properties, even in the same street, don’t receive a letter because the modelling is under the one metre average. Regardless of the flooding depth chosen, there will always be properties which fall just outside the benchmark, but all properties have individual data available on Floodviewer.

“The numbers are big, but knowledge is power,” said Phillips. “This new information identifies areas of the city we should be more cautious about when planning projects. Between flood hazards and extents, which assume water is draining away, and depressions, which assume pipes and culverts are blocked, we’re building a better understanding of what could happen in our city during heavy rain.”

Hamiltonians can visit Floodviewer to find out how their property or home could be impacted by flooding. Tips to prepare their property can be found at getready.govt.nz but simple steps include keeping their home’s drains free from debris, leaves and rubbish (if safe to do so). Securing loose items when heavy rainfall is predicted can also help to minimise the chance of these items being washed downstream to block a pipe or culvert.

“While we’re experiencing a hot summer, large rainfall events can still occur suddenly and without warning. Flooding can happen at any time of the year,” Phillips says.

Council uses flood information as part of its building and consenting process, to make sure new development happens in a way that won’t make flooding worse or put people at risk.

This starts with making changes to the District Plan, the rule book for development, to make sure Hamilton is growing in a way that protects people and property in areas that are prone to flooding. A Flood Hazards Plan Change (Plan Change 14) is being drafted and will include a formal public consultation process.

“This isn’t something for one person or organisation to tackle,” said Phillips. “This is a chance for us to work together – Council, Civil Defence and the community – to learn from the experiences of other regions around the country and do what we can to protect each other and our city.

“The data is what it is – it’s now how we use it that counts.”

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