When they happen, landslides across the Tāmaki Makaurau Auckland region impact homes, infrastructure, roads and services and the effects of climate change can exacerbate these risks over time. Most recently, we experienced the scale of these impacts from the landslides that occurred during the severe weather events of early 2023.
Auckland Council has completed a region-wide study to map landslide susceptibility, to help Aucklanders understand and avoid these risks. This works deepens our understanding of the potential for landslides in the region and helps us plan better for the future.
Landslide susceptibility is an assessment of which factors may pre-dispose particular land to becoming unstable. It’s important to understand that these maps do not describe hazard or risk. This means that they do not show how many, or how often, landslides will occur (the hazard), or what the consequences could be (the risk). But rather, helps in our planning and understanding of landslides.
Councillor Richard Hills, Chair of the Policy and Planning Committee welcomes these maps and reiterates the importance of understanding the hazards in our region.
“We’ve seen the significant impacts that landslides can have in Tāmaki Makaurau and it’s important that we understand what this means as the region continues to grow. Landslides had devastating effects in both the 2023 Auckland Anniversary floods and Cyclone Gabriel on both private and public land. This included loss of life, and hundreds of millions of dollars of damage to property.”
“The maps provide vital information to planners, infrastructure authorities, owners, developers, and engineers so that they can understand and manage the risks from landslides,” says Cr Hills.
Two maps have been produced – one for shallow landslide susceptibility and one for large-scale susceptibility.
Auckland Council Chief Engineer Ross Roberts explains that the maps were prepared using data available at a regional scale and normally can’t be used as the only source of information at a property level.
“Because detailed, site-specific information is not available across the region, the maps can’t include some site-specific conditions and features. This means detailed assessments may still be required for accurate site-specific risk determination at an individual property level.
“Landslide risk assessments completed as part of the response to the 2023 storms are an example of a method we have used to provide more detailed risk information,” says Mr Roberts.
How will these maps be used?
The landslide susceptibility maps offer essential information to guide land use planning, urban growth strategies, helping to steer development away from potentially high-risk areas and toward lower risk, and to guide where more detailed assessments are needed to manage the risk.
Phill Reid, Manager Aucklandwide Planning, says the results of the study will have value in the process of strengthening the Auckland Unitary Plan to build resilience to natural hazards.
“Natural hazards, such as flooding and landslides, are some of the biggest challenges we face in Tāmaki Makaurau. The council is taking action to strengthen Auckland’s planning rules against the risks of these hazards, and the findings of this study are an important step towards this.
“Currently, new developments in areas exposed to natural hazards need to have an approved resource consent. The council introduced this rule under the Auckland Unitary Plan in 2016 to help manage development in areas with higher risks.
“Now, we’re looking to strengthen the rules and raise the standards that must be met when building in these areas. The law needs to change before we can do this. This could happen by the middle of 2025, and these maps will be used as part of this process.”
The regional maps and related landslide inventory data will be shown on Land Information Memorandum (LIM) documents to inform prospective and future owners of the potential for landslides in an area and will be used in decision-making for the council when identifying areas for development or for building council assets.
Each LIM will include two maps: one for shallow landslides affecting the top few metres of soil on steeper slopes or gullies, and one for large-scale landslides affecting deeper soil layers over a larger area.
LIMs will also contain a context statement noting that these maps are based on available data, some of which is not mapped to a high level of detail and so are not intended for property-level assessment.
The maps will be updated as required, for example, when significant new data is available.
For more information and to view the maps, please visit Knowledge Auckland for the full study.
You can also learn more here: Auckland Landslide Susceptibility Study Factsheet [PDF 1.43 MB] and the maps will be available online in the coming weeks.
Frequently asked questions
What is a landslide?
A landslide is the movement of a mass of rock, debris or earth (soil) down a slope.
Landslides across the Auckland region impact homes, infrastructure, roads and services and they typically occur:
- On steep slopes (natural or modified)
- On slopes comprising weak materials
- Due to stream or sea erosion
- Due to failure of structures like retaining walls.
Landslides can be triggered by:
- Heavy or prolonged rainfall, which may become more severe and frequent due to climate change
- Earthworks, where a landslide can be triggered by the weight of material added to a slope, or by loss of support where material is dug out below a slope.
- Strong earthquake shaking.
How is landslide susceptibility determined?
Landslide susceptibility has been assessed using high resolution topography data, land use, waterways, geomorphological and geological maps, and an extensive database of past landslides across the region.
Two types of rainfall induced landslides have been assessed and are presented on separate maps. This is because the two types require different approaches to manage them, and in some cases are controlled by different factors.
- Shallow landslides are relatively shallow and fast-moving. They occur on steep slopes or gullies, where the slope fails and debris runs down the slope. These are often controlled by land use, soil type and surface water runoff.
- Large scale landslides affect deeper layers of soil and rock, and can affect large parts of hillsides. They are often controlled by weaknesses in the underlying rock, and by deep groundwater.
What do the maps say about the impacts of landslides?
The landslide susceptibility maps show slopes that are potentially vulnerable to failure – or landslides.
The debris from failed slopes would move downwards and deposit the failed materials and run out further. Failed slopes can also regress further into land that would have otherwise not failed.
The areas of runout and regression are not shown on the maps because we do not yet have the tools to model this across the region, but all contribute to the risk from landslides. We intend to add these factors into future updates of the maps.
How should I use these maps?
The regional scale maps assist in land use planning and provide a starting point for landowners to engage specialists to assess the risk to their specific sites at a more detailed level, if they highlight a potential issue.
However, the maps shouldn’t be taken as showing a confirmed problem without more detailed assessment because they rely on data like geological maps which are not always accurate at a site-specific level.
