What is flood risk information?

It is part of normal Council business to know about the effects of natural hazards that could impact our community. One of the natural hazards that we are required to know about is potential flood risk. We need to understand the extent and effects of these flood risks so that we can work towards managing them across the city.

Flood risk maps for Tauranga City have been evolving since the early 1990s. These maps show which areas of the city might potentially be flooded in an extreme rainfall event, and to what extent.

The information they provide has always been available on request. It is noted in Land Information Memorandum (LIM) reports and used when reviewing building and resource consents. In recent years the flood risk information has been available on our electronic mapping systems.

We are required by several acts of Parliament to hold this information and to make it publicly available. These acts do not require us to inform you when the information is updated. This is something we choose to do.

Why are we updating the flood risk information?

As part of our response to flood events that have affected different parts of Tauranga City we now have more up to date information about the potential
flood risk for some areas. Over the next few years we will use new technology to update the flood risk information for the entire city.

There has also been major stormwater upgrade work in parts of the city which will have reduced the potential flood risk for those areas. Updates to the flood information will reflect those improvements. As custodians of this information we will always give you the most up to date information that we have available about your property.

What will Tauranga City Council do to fix the flood risk?

We will take the flood risk information into account when planning future stormwater improvements. The Mayor and elected members have the final say on the priority and timing of stormwater upgrade spending. They make these decisions every year through Annual Plan and Ten Year Plan processes.

What will Tauranga City Council do with this information?

The updated information has been stored on the files relating to your property and on our GIS mapping system.

This information will form part of our official records and adds to the overall property knowledge that will be made available for general enquiries
and for LIMs. We will make this information available to any other party upon request.

The information will be taken into account for new building consents and for resource consents for subdivisions. In these cases we will need to see
evidence of how the proposed building work or subdivision will address the potential flood risk.

Will this affect my insurance?

Council can not advise you about any effect that this information might have on insurance. In this matter you may wish to contact your insurance company.

What do we mean by an ‘extreme rainfall event’?

An extreme rainfall event is an unusual scenario,something that doesn’t happen very often. It is much, much more than a very rainy day.

The official term for the level of rainfall that we are talking about is ‘50 year rainfall event’. This is a very high amount of rain that, on average, would only occur once every 50 years. The term can sometimes be unhelpful because 50 year rainfall events could actually happen more frequently than once every 50 years. The 50 year rainfall is used as a benchmark for setting standards for new building floor levels and it is a useful figure against which to plan stormwater systems. The NZ Building Code sets this standard for all new buildings that no stormwater should enter the building in a 50 year rainfall event. The flood risk for Greerton was calculated using the amount of rain that is likely to fall in a 50 year rainfall event.

How do we assess the flood risk?

To assess flood risk we build a computer model of the area that we wish to study. The area of land being studied is called a catchment. We then use computer software to simulate different rainfall events on that catchment. This is the flood risk model.

The basis of the flood risk information is a system called LIDAR (Light Detection and Ranging) which measures the ground levels of the land using laser pulses. This generates an accurate contour map which we place into the three dimensional stormwater model. LIDAR is very reputable technology used by most councils in New Zealand.

Our first job is to work out which criteria will be entered into the flood risk model. The sorts of things that need to be considered include:

• How hard is it raining?
• How long has it rained for?
• What is the contour of the ground?
• Where will rain water soak into the ground (e.g.grass)?
• Where will rain water flow over the land (e.g. roofs, concrete)?
• How long will it take for rainwater to flow from one part of the catchment to another?

Technology today is able to take all of these variablesinto account. Once the criteria have been set we can run the flood risk model. There is a huge amount of information to be computed so it takes longer than real time to run each model. We run different rainfall scenarios and can then use the model to calculate how, when and where the rainwater flows away. The result will tell us which parts of the catchment are likely to remain covered by water, and to what depth.

When the model is complete it gets loaded into our GIS (Geographical Information System) mapping system for further analysis and processing. The information needs to be tidied up to the point where it can be presented as an accurate map. Once this is done the final map is peer reviewed. The flood risk information is now valid and active.


If you have any other questions... Please call Council on 577 7000