To meet regional council requirements, Hamilton City Council has an ongoing programme to understand the effects of a very rare, and very large, rain event across the city.
This means looking at what would happen if we had the amount of rain predicted in a storm which would occur, on average, once in 100 years. The chance of this happening in any given year is 1%. Even though it's very rare, It's important to plan for this so we can invest efficiently for future stormwater planning and ensure development doesn't create new problems or impact nearby properties.
Mapping of potential flooding in a one-in-100-year event is developed with computer modelling that uses landform information (land contours), climate change information, and known and predicted rainfall and water flow patterns.
When Council receives new or updated data about these potential flood effects on properties in the city we are required by law to make this publicly available upon request. Council uses an online tool, Floodviewer, to host existing and updated data and make it more accessible to the public.
It's important to note this work does not increase the likelihood of flooding within your property boundary during a one-in-100-year rain event, it simply produces data and makes it publicly available. This information can assist landowners in making decisions about future development on their property.
You can search Floodview by property address and this webpage carries more detailed information and a comprehensive list of frequently asked questions.
Floodviewer holds a lot of information and can seem quite complex for people not used to working with Council mapping systems. We recommend reading through the FAQs thoroughly before entering the Floodviewer site and attempting to understand the layers of mapping within Floodviewer.
If you have more detailed queries regarding your property or future plans for development we are happy to help. Please contact one of our team to discuss it either by email at City.Development@hcc.govt.nz or by calling 07 838 6699 during business hours.
What does this mean?
What is flood mapping?
Hamilton City Council has an ongoing programme to understand what would happen in areas of Hamilton if we had a very large, and rare, amount of rain. The measure we use is an event which would happen, on average, once in 100 years. Statistically, this means there is a 1% chance of this happening in any given year.
We call this a 'one-in-100-year event' and it provides a consistent measure for Councils to use across the district. Mapping of potential flooding in a one-in-100-year event is developed with computer modelling that uses land contours climate change information and known and predicted rainfall and water flow patterns. This information has been identified for around one third of Hamilton so far.
Why is it important?
Our information shows whether affected properties would have a low, medium or high flood hazard if we have a one-in-100-year rain event. It means people can be prepared for an event like this and can also inform what residents do on their property.
For example, owners of properties with a flood hazard showing should consider where flood water would go if there were changes to the property such as a raised garden, a shed or playhouse. It helps people avoid inadvertently directing water where it is not wanted, for example, towards the door of a building.
When was the last one-in-100-year flood in Hamilton?
The last widespread 100-year flood event in Hamilton occurred in 1958. The Waikato River was last at a 100-year flood level in 1998. Waikato River flooding is affected by rainfall upstream of Hamilton. Flooding in Hamilton, other than in the river corridor, is affected by rainfall directly on the city.
What does the mapping mean for my property?
The flood mapping shows the effect of water on the land, not whether it would enter the building if there was a one-in-100-year event. Whether flood water gets into a building depends on how high off the ground the floors are compared with the depth of the surrounding flood water.
Council can supply you a detailed map showing maximum flood depth and velocity and design flood level for individual properties in a 100-year flood for flood-modelled areas.
These maps may be requested by phoning 07 838 6699 during normal business hours or emailing firstname.lastname@example.org
Why is this information on LIMs?
Council is required to produce the best available flood information and make it available to the public on request. A LIM outlines all information a council holds on a property, including permitted land use, consents, stormwater and sewer drains, rating information and potential natural hazards including erosion and flood hazards.