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Flood hazard modelling – extreme rainfall

Flood hazard maps have been updated

Submissions are now closed on proposed changes to the city plan, that look at how we can enable future development and redevelopment in Tauranga without increasing flood risk. As part of this work we developed a rule framework to manage the effects of flooding in intense rainfall events on people, properties and infrastructure, and updated our flood maps. Find out more at the link below.

Plan Change 27: Flooding from intense rainfall events

Flood hazard modelling shows which areas of the city might be flooded in an extreme rainfall event and to what extent.

Because the information is used for building consents, subdivisions and infrastructure planning, it needs to be accurate as possible.

View current flood hazard maps

Flood hazards are modelled on a 100-year rainfall event; this is extreme rainfall that you might experience at least once in your lifetime.

To generate a flood hazard map we build a computer model of the area (also known as a catchment) and use specialised software to show the effects of different intensity of rainfall.

We use 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 stormwater computer model. LIDAR is very reliable technology used by most councils in New Zealand.

We then use a range of criteria to create the modelled maps. The sort of things we consider are:

  • 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 hard surfaces (e.g. roofs, concrete)?
  • How long will it take for rain water to flow from one part of the catchment to another?
  • What stormwater systems are already in place?

The technology looks at all the criteria then runs the flood hazard model. Each model calculates how, when and where the rainwater flows and tells us which areas are likely to be covered by water and to what depth. Flooding less than 100mm isn’t shown.

When the modelling is complete, it’s reviewed to make sure it’s correct and uploaded into our GIS mapping system. The updated maps are sent to homeowners and kept on the property file. The information is also included in Land Information Memorandum (LIM) reports and used when reviewing building and resource consents. We will always provide the most up to date information that we have available about your property.

Overland flowpaths

As well as modelling for depth, we also model the speed and direction of floodwater. This gives us important information about where flood waters flow during a heavy rain event.

Last Reviewed: 02/02/2021

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