Flood Hazard Mapping FAQ's

We’re doing this to help prepare for the future. Waipā District Council is responsible for helping reduce the impact of natural hazards, like flooding. As weather patterns change and extreme storms become more common, it's important that we understand the flood risks in our rural areas.

This mapping project will show us where water is likely to flow and pond during big weather events. It’ll also help us keep rural property information up to date on LIM reports, and allow us to review the key rural drains we maintain.

While the words hazard and risk are related, they are different. In this instance, flood water is the hazard. Modelling identifies hazards only.

The risk is the effects of the hazard combined with the impact it may have on people, property and the environment.

Waipā District Council flood hazard modelling has been completed in four urban areas and the rural parts of our district. The four urban areas are Te Awamutu (includes Kihikihi), Cambridge (includes Leamington), Ōhaupō and Pirongia.

The urban flood mapping is more detailed because towns have complex stormwater systems and more infrastructure.

It comes down to data. In rural areas, many culverts and drains are privately owned, and we don’t always have records of them. So, we base the mapping on the land’s shape (using aerial surveys) and the road culverts we do know about.

Because we don’t have a full picture, especially when it comes to private systems, the maps are more of an indication where water is likely to go during major storms. Water might flow differently if there's a hidden culvert or a blocked drain.

The urban flood hazard mapping used modelling based on stormwater running directly into the rivers in Te Awamutu. To help us better understand how this could impact future development, we carried out targeted modelling on the Mangapiko and Mangaohoi Rivers.

Waikato Regional Council’s data wasn’t as detailed as we needed, and they weren’t in a position to carry out a review at the time. So, we completed the mapping ourselves. They now hold the data and are responsible for it, as it falls within their area of responsibility.

No other river flood hazard mapping was done.

Waikato Regional Council’s Waikato Regional Hazards Portal includes river flood information for Waipā District.

We use computer modelling, because there is not enough historic and observational information available.

We start by dividing the Waipā District into smaller areas called catchments. These are natural drainage areas — like big bowls — where rainwater flows downhill to a certain point.

Then we look at several key factors:

• The shape of the land – Using high-tech aerial survey data (called LiDAR), we can map the contours of the land. Since water flows downhill, this helps us see where it’s most likely to go.
• Rainfall patterns – We simulate what would happen in a 1-in-100-year storm, factoring in how much rain falls, how fast it falls (intensity), and how long it lasts. We also run models both with and without climate change impacts, since changing weather patterns can affect future storm strength.
• Ground conditions – Rain behaves differently depending on the surface it hits. Grass and soil absorb more water, while hard surfaces like concrete cause rain to run off faster. This affects how much water ends up flooding an area.
• Stormwater infrastructure – We include known stormwater features like culverts, which carry water under roads and through properties, to see how much water they can handle during a big storm.

The final output shows us which areas are likely to flood, how deep the water might get, and how fast it could be moving.

This helps us (and the community) plan for future flood risks and make smart decisions about land use, drainage, and infrastructure.

Note: Any flood mapping done before 2019 used older LiDAR data collected in 2008. We updated our models in 2020 using new LiDAR data to make them more accurate.

We use the best available flood data and modelling to manage and plan stormwater upgrades. Decisions on what gets done and when we are made through the Annual Plan and the Long-Term Plan.

Flood information in the District Plan and online viewer also helps guide how new land use and development are managed to reduce future flood risk.

Most stormwater in rural areas is handled through private drainage systems — open and closed drains that move water off land. Some of these drains go back generations and were created to make the land more usable.

Council helps make sure the overall land drainage system across Waipā is effective, especially where water needs to move beyond just a single property. We also offer advice and support when needed, but in most cases, maintaining drains on rural properties is the landowner’s responsibility.

You can find out more about stormwater and land drainage on our website: Stormwater

Flood hazard mapping is a snapshot and shows the best information we have at a certain time. We know that things can change, and the maps will be updated as more information comes to hand. Our next rural flood mapping update is currently planned for the 2030-31 year (as part of the 2025-34 Long Term Plan).

Before then, we will invite landowners to share any relevant information – like changes to land drainage – that we can consider for further updates.

If you’d like to talk to us about your property or the maps, please call 0800 924 723 or email info@Waipadc.govt.nz

If you have any questions about flooding on your property, you can call us on 0800 924 723 and ask to speak with our district planning team, building team or duty planner.

You can also email your enquiry to info@waipadc.govt.nz and we’ll get back to you.