Flood hazard modelling shows which areas of the city might be flooded in an extreme rainfall event and to what extent.
Updated Matua flood hazard map
We recently updated our flood map data for Matua using the latest topography and infrastructure so we can now better show the role of minor overland flow-paths in flood risks. This means the flood hazard map has changed, allowing us to:
- Indicate where flood risk levels may be higher or lower than previously indicated for some properties.
- Identify new properties at risk of flooding and exclude some properties previously included in flood zones.
Owners of affected properties will receive a letter from us with more detail and an invitation to discuss this change.
View updated Matua flood hazard mapi
Because flood hazard information is used for building consents, subdivisions and infrastructure planning, it needs to be accurate as possible.
View current flood hazard maps
Plan Change 27: Flooding from intense rainfall events
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.
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.