Slope instability hazard, which includes landslides, is the movement of a mass of rock, debris or soil down a slope. Slope failures can be triggered by natural processes (e.g. rainfall events, earthquakes) or activities such as land development and earthworks. Debris run-out areas are land that may be impacted from soil and rock debris from upslope failures.
In our district some areas are subject to elevated erosion and/or slope instability hazard reflecting the characteristics of soils and underlying geology and the degree of physical and/or chemical weathering of the underlying rock. Separation Point Granites are typically deeply weathered at the surface and are particularly susceptible to severe erosion during heavy rainfall when exposed through vegetation removal and/or earthworks.
Slope failures cause damage by direct impact and burial, the formation of landslide dams, and slides falling into water bodies causing localised large waves (e.g. tsunami or seiches). Much of the damage done by slope failure is permanent. They can cause significant damage to land, buildings, infrastructure (e.g. road, power, telecommunications, drinking water supplies) and the environment, as well as economic and social disruption. Loss of life and injury is also a real possibility and has previously occurred in Tasman.
December 2024 Update: Both Nelson City Council (NCC) and Tasman District Council (TDC) have sought independent technical advice on slope instability hazards in our districts to inform our resource management plan reviews. As part of NCC's recent Plan Change 29 hearing process the mapping applied to the slope instability susceptibility study for Nelson is being revised. TDC staff are currently working with our consultants to understand the implications of this and review the mapping outputs for Tasman. Consequently, the maps below should be treated as indicative only as they may be subject to change following this review process. If you have any questions about this study please contact the Natural Hazards team at TDC..
The existing Slope Instability Risk Area (SIRA) overlay in the Tasman Resource Management Plan (TRMP) and related provisions are not affected by this mapping review.
We have started work on a natural hazards plan change(external link) to the Tasman Resource Management Plan (TRMP). As part of the process, we are improving our knowledge about local hazards and sharing that information with affected landowners and communities.
The TRMP identifies selected key locations with a higher likelihood of slope failure occurring through the ‘Slope Instability Risk Area’ (SIRA) overlay as shown on our planning maps. The associated planning rules seek to control the location of habitable buildings and earthwork activities in these areas to protect people and property. The overlay and rules act as a flag to pay extra attention to slope instability hazards when developing a site or undertaking new building work. The rules require a geotechnical assessment of the ground conditions and site stability and recommend appropriate mitigation measures (if necessary). Currently a SIRA has not been developed across the entire district, rather it has been developed for key areas where there is development pressure. There remain areas, such as remote rural and back country areas, where slope instability hazards have not been assessed.
We have sought expert technical advice to review slope instability hazards at three key locations within the existing SIRA, being:
A ‘study area’ was identified for each location which considered the existing SIRA boundary, adjacent areas where there is known or potential slope instability, current or anticipated future development pressure, and topographical boundaries such as ridgelines.
The outputs of the assessment identified:
The outputs of the assessment will be used to inform the natural hazards plan change.
The methodology, maps and recommendations can be viewed in the Review of TDC Slope Instability Risk Areas (July 2021) technical report. We have broken the report into sections so it's easier to download.
The existing SIRA overlay in the TRMP and associated provision are not affected by the mapping review.
Appendix A: Instability and Run-Out Susceptibility Areas:
Appendix B: Summary of Mapping Methodology (pdf 125 KB)
Appendix C: Comparison of Instability and Run-out Susceptibility Areas with SIRA:
Please note that the boundaries of the Clifton - Pōhara - Ligar Bay SIRA study area were limited by the coverage of existing LiDAR data and therefore part of the SIRA was not able to be included in the review. The area includes the north-east area of the SIRA which is mountainous and has very little, if any, development. The review may be extended to this area in the future.
The Council has not included the Ruby Bay Sea Cliffs Slope Instability Risk Area (SIRA) in this current review, as we largely understand slope instability hazards in this area and there is minimal development pressure. We can undertake a review of the boundary of this area if required in the future.
It is recognised that there will be areas in our district outside the existing SIRA that are susceptible to slope instability hazards and debris run-out that were not included in the review. Such hazards are considered on a case-by-case basis with building consent or resource consent applications. Further research will be undertaken as part of our ongoing natural hazards work programme. Currently, this includes work to better understand the slope instability hazards on the Separation Point Granites. Additional areas may be selected based on future development pressure.
What is slope instability and debris run-out?
Slope instability hazard, which includes landslides, is the movement of a mass of rock, debris or soil down a slope. Slope failures can be triggered by natural processes (e.g. rainfall events, earthquakes) or activities such as land development and earthworks. Debris run-out areas are land that may be impacted from soil and rock debris from upslope failures.
My property is located in the Slope Instability Risk Area, what does this mean?
The Tasman Resource Management Plan identifies a ‘Slope Instability Risk Area’ (SIRA) overlay and associated planning rules which seek to control the location of habitable buildings and earthwork activities (e.g. tracks, building platforms) in these areas.
The SIRA provides high-level guidance to landowners and the wider community on areas that could be subject to slope instability hazards. The overlay and rules act as a flag to pay extra attention to slope stability when developing a site or undertaking new building work in these areas. The SIRA rules require geotechnical assessment of the ground conditions and site stability and may include recommended mitigation measures (if necessary).
My property is located in an area potentially susceptible to slope instability hazards and/or debris run-out, what does this mean?
This information provides high-level guidance to landowners and the wider community on areas that could be potentially susceptible to slope instability and/or debris run-out hazards. In many instances, the properties identified are already included in the Tasman Resource Management Plan’s SIRA.
The slope instability susceptibility and run-out susceptibility areas are not considered a replacement for site-specific investigations, nor does it consider the potential impact to individuals or property. Landowners may hold site specific information, such as a geotechnical assessment, which will provide more accurate detail than what is shown on the report’s maps.
Can I subdivide or build if my property is located in an area potentially susceptible to slope instability hazards and/or debris run-out?
Subdivision requires a resource consent. All natural hazards, including slope instability, are already taken into account when considering an application to subdivide land under the requirements of the Resource Management Act 1991.
When you apply for a building consent, the land stability at the site and its exposure to natural hazards are taken into consideration through the Building Act requirements.
The identification of the slope instability and run-out susceptibility areas act as a flag to pay extra attention when subdividing or undertaking building and associated earthwork activities. This may require geotechnical assessment of the ground conditions and site stability and recommended mitigation measures (if necessary).
I’ve already had a geotechnical report completed for my property, so why am I still included in an area identified as potentially susceptible to slope instability and/or debris run-out?
Landowners may hold a geotechnical assessment for their property as part of a historical subdivision and/or building consent process. This assessment will provide details on slope instability hazards for the property and how the hazards were mitigated in association with the development. However, the property still remains included within the area potentially susceptible to slope instability and/or debris run-out. This is because any future development (e.g. habitable buildings or associated earthworks activities) still requires the potential for slope instability hazards to be considered. The extent an existing geotechnical report can be relied upon for such future development will depend on the age and scope of the original report. It may be necessary to undertake further site and project specific assessment. The inclusion of properties in the SIRA and/or this new mapping ensures that both current and future landowners are aware of the potential for slope instability hazards and to ensure people and property are protected.
What is the assessment methodology?
The assessment methodology generally followed the Australian Geomechanics Society’s (2007) and GNS’s (2007) guidelines for landslide susceptibility mapping. This included creating an inventory of slope instability features supplemented with a literature review on the locations and mechanisms of previous slope failures. The locations and types of existing features were identified from historical records, previous mapping, and/or geomorphic evidence observed in aerial photographs, satellite imagery, and/or LiDAR elevation models and supplemented with local engineering knowledge.
The assessment methodology was peer reviewed by CGW Ltd (consulting engineers) to ensure that the methodology was consistent with the guidelines and best practice.
How is the Council using this slope instability information?
Consideration of slope instability hazards have been embedded in the Council’s work programmes for a number of years. We use this information in:
Next Steps
The next steps for the TRMP natural hazards plan change will be to engage with the community on the ‘issues and options’ for managing our natural hazards, including slope instability. Engagement is proposed to begin early to mid-2025 and will seek the community’s views on the options for updating the TRMP provisions that relate to natural hazards.
Landowners, iwi, stakeholders and the wider community will have opportunities to input into the natural hazards TRMP plan change as we progress through the plan drafting stages.
Related Pages
Where do I find information about what to do following a natural hazard event?
Information, advice on getting prepared and what to do during a natural hazard event can be found on the Nelson Tasman Emergency Management Group website.
The Natural Hazards Commission Toka Tū Ake also has information and advice for home owners, tenants and home buyers to ensure your home is safer for natural hazards.
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