What is liquefaction?
Liquefaction happens when saturated soils lose strength and stiffness (behaving as a liquid rather than a solid) in response to earthquake shaking. Only some soil types are prone to liquefaction and only some earthquakes are strong enough to cause liquefaction.
Where are liquefaction areas identified?
The District Plan maps do not currently identify any liquefaction areas.
Council does however hold a report which maps liquefaction hazards across the District. This was prepared for the Western Bay of Plenty Engineering Lifelines Group in 2002 to assist with the management of risks to lifelines such as roads, rail and water reticulation infrastructure.
While the report indicates where liquefaction may occur in the future, there are limitations to note. The broad level regional scale assessment presented in the report was based on information available at the time (which means the maps may now be out of date) and was not for the purpose of identifying liquefaction hazards that would be accurate at a property level.
You can view these maps below.
Search your property - current natural hazard maps
To show the natural hazard map for your property please enter a valid Address or Parcel ID below and click the launch button. Or you can browse the natural hazard maps.
Tips for finding your property:
- Type the address in full such as 10 Example Road or 10 Example Street, instead of using street name abbreviations (Rd or St etc)
- If there are identical addresses in other parts of the District, you'll need to type in community abbreviations e.g. Beach Road (KK), Beach Road (MAK) or Beach Road (WB). Help with identical addresses
- If searching by Parcel ID, recent letters from us about natural hazards show your Parcel ID number.
How is the mapping done?
To generate liquefaction maps, the following things are taken into account:
- Soil type
- Groundwater levels
- Strength of earthquake
Here is the technical report supporting the liquefaction maps:
What further work is coming up?
Council and the Regional Council will be modelling liquefaction in the near future. It will show us what areas would be affected by a 0.1% Annual Exceedance Probability (AEP), 0.2% AEP, 0.033% AEP and maximum credible event. The modelling will be based on both the current situation and in 100 years' time taking into the account the effects of climate change on groundwater. When the mapping is completed, risk assessments and changes to the District Plan will also be required.
Frequently asked questions about liquefaction
What is an Annual Exceedance Probability (AEP)
This is a statistical term referring to the likelihood of a natural hazard event occurring in any year. As a generic example, the event might be 300mm of rainfall in 24 hours. If such an event has a 2% chance of occurring in any year it would be described as a 2% AEP rainfall event. This means it would be expected to occur on average once in every 50 years. Although this is sometimes referred to as a 1 in 50 year flood, it could actually happen at any time. For example, it might not occur again for 80 years or it could even occur twice in one year.
AEP terms associated with earthquakes causing liquefaction include:
- 0.2% AEP event = expected to occur on average once in every 500 years
- 0.1% AEP event = expected to occur on average once in every 1,000 years
- 0.033% AEP event = expected to occur on average once in every 3,030 years
What are the requirements if I am constructing a habitable building?
Any identified liquefaction risk (minor, moderate or widespread) will be addressed through the building consent process. A liquefaction assessment must be prepared by a qualified expert for any new habitable building or any additions (25m2 or larger) to any existing habitable building.
This could be a Category 1 Geo-professional, a Category 2 Geo-professional or a CPeng registered engineer with a geotechnical practice field.
What are the requirements if I am constructing an addition (under 25m2) to a habitable building?
No liquefaction assessment is required.
What are the requirements if I am constructing a shed/garage or a minor structure such as a fence/wall?
No liquefaction assessment is required.
If I have a building site approved from a previous subdivision can I go ahead and build?
Yes, but only if the subdivision has addressed all identified natural hazards, and building works are in accordance with the requirements of the subdivision. If new natural hazard information has become available since the subdivision, this will need to be addressed when you go to build.
Is new subdivision allowed?
Yes it is, but you'll need to provide a geotechnical report with the resource consent. This will help identify suitable building sites and will explain what foundations are required. If there are no suitable building sites, a subdivision can be declined.
Who can I ask to prepare a geotechnical report?
Got a different question?
If you have any further questions, please contact Council on 0800 926 732 or at email@example.com.