Landslide susceptibility describes the relative likelihood (i.e. is one area more or less susceptible than another area to landsliding) of future landsliding in an area based on underlying properties, such as local terrain, geological and hydrological conditions.
It does not include the frequency of landslides, or the consequences (e.g. areas that may be inundated by landslides are not identified).
We have undertaken a study that represents a city-wide assessment of landslide susceptibility that is aligned to the current best practice (AGS, 2017; de Vilder et al. 2023) and regulatory framework (BoPRC, 2016), and accounts for observations of landslides in heavy rainfall events in Tauranga and in similar volcanic terrain in Japan that post-date many of the previous studies.
The term 'susceptibility' should not be confused with 'hazard' and 'risk'. Landslide hazard is the potential for a landslide to cause damage and landslide risk shows the vulnerability or the potential for loss an area has due to a landslide.
Landslide Susceptibility Assessment
The landslide susceptibility was evaluated by identifying factors that influence or contribute to landslide occurrence in Tauranga from desktop and field studies. These factors include slope angle, relief and profile, geomorphology and geology, overland flow path and distance to streams. Each factor was weighted according to their relative importance in landslide occurrence e.g larger weightings were applied to factors that have a greater influence on landsliding.
The weighted factors were combined in GIS to provide a susceptibility score. Susceptibility classes were defined based on comparison of the susceptibility scores to the landslide inventory and the geology and geomorphology of the study area.
Landslide susceptibility scores were classified into four categories to represent Very Low, Low, Moderate and High landslide susceptibility.
A summary of the process is presented below and more information provided in the technical report.
Landslide susceptibility mapping process
Tauranga Landslide Susceptibility Study Technical Report 2023
Landslide susceptibility maps
Two landslide susceptibility maps have been prepared, with susceptibility to rainfall-induced landsliding and earthquake-induced landsliding assessed separately.
Areas with a high susceptibility are more prone to landslides, while those with low to very low susceptibility are less likely to experience them.
A specific online viewer has been created to allow a general view of an area (not property-scale) relative to landslide susceptibility.
Landslide Susceptibility Mapping
There is some inherent uncertainty in undertaking landslide susceptibility mapping due to the scale and quality of the input data, and the assumptions made. As a result, the mapping is not recommended to be used at property-scale.
Commonly asked questions
Landslide susceptibility describes the relative likelihood (i.e. is one area more or less susceptible than another area to landsliding) of future landsliding in an area based on underlying properties, such as local terrain, geological and hydrological conditions. It does not include the frequency of landslides, or the consequences (e.g. areas that may be inundated by landslides are not identified).
The term 'susceptibility' should not be confused with 'hazard' and 'risk'. Landslide hazard is the potential for a landslide to cause damage and landslide risk shows how vulnerable or the potential for loss an area has due to a landslide.
The Bay of Plenty Regional Policy Statement (RPS) provides a framework for managing the Bay of Plenty region’s natural and physical resources. The RPS highlights significant issues which relate to land which must be considered by council when developing district plans. It also sets out what should be achieved and how it will be achieved through policies and methods.
Policy NH 3B of the RPS outlines the long-term strategic direction and the outcomes for the way natural hazard risk is managed throughout the Bay of Plenty region. Council must give effect to the RPS and identify areas that are susceptible to landslides from rainfall and earthquakes, evaluate potential likelihood and consequences of the hazards and assign appropriate hazard risk levels.
Landslide susceptibility mapping is the first step in a process to define landslide risk, and is intended to be used in the following council processes:
- District and city plans, and plan changes.
- City-wide natural hazard risk assessments.
- Future land-use planning, to aid in planning for and understanding the effects of natural hazards on existing land-uses, potential subdivisions and growth management.
- Current and future infrastructure planning, in the consideration of upgrades to existing infrastructure, provision of infrastructure and understanding the long-term effects on the operation and performance of infrastructure.
The avoidance or mitigation of natural hazards is one of the key functions of a council in giving effect to the Resource Management Act 1991 (RMA). A similar requirement also exists for local councils under the Building Act, Local Government Act and Civil Defence Emergency Management Act.
As a result, we are required to have a good understanding of the areas potentially at risk from natural hazards to support appropriate land-use planning and development decisions. Mapping is the most accepted method to identify hazard-prone areas and provide the greatest level of certainty to the public.
A specific online viewer has been created, to allow a general view of an area (not property-scale) relative to landslide susceptibility. The landslide susceptibility layers are also available on Mapi under natural hazards, slopes and relic slips’ .
There is some inherent uncertainty in undertaking landslide susceptibility mapping due to the scale and quality of the input data, and the assumptions made (albeit made by professionals proficient in landslide assessments). As a result, the mapping is not recommended to be used at property-scale, and as a guide for next steps in landslide risk assessments.
Landslide susceptibility mapping is the first step in a process to define landslide risk, and is intended to be used in the following council processes:
- District and city plans, and plan changes.
- City-wide natural hazard risk assessments.
- Future land-use planning, to aid in planning for and understanding the effects of natural hazards on existing land-uses, potential subdivisions and growth management.
- Current and future infrastructure planning, in the consideration of upgrades to existing infrastructure, provision of infrastructure and understanding the long-term effects on the operation and performance of infrastructure.
Yes, we have a legal obligation to provide information identifying natural hazards that we hold to the public under the Local Government Official Information and Meetings Act 1987. This includes noting such information on LIM reports for properties.
What is a Land Information Memorandum (LIM)?
We are required to make information, such as the technical assessments relevant to hazards, available upon request. We cannot advise about any effect this information may have on property value or insurance. It is recommended that professional advice is sought from a property valuation or insurance expert about any concerns you may have regarding these matters.
We continually review data we hold about natural hazards to ensure we have the latest information available for our community, this includes natural hazards such as flooding, liquefaction, tsunami and coastal erosion.
More information on Natural Hazards