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 engineering characteristics of soils and underlying geology and the degree of physical and/or chemical weathering of the underlying rock. In particular, Separation Point Granites are susceptible to severe erosion during heavy rainfall, particularly where they have been 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.
Both Nelson City Council (NCC) and Tasman District Council (TDC) commissioned BECA to provide technical advice on slope instability in our districts to inform our resource management plan reviews. Through NCC's Plan Change 29 hearing process a concern has been raised regarding the methodology applied to the slope instability study for Nelson. TDC staff are currently working with our consultants to understand the nature of this concern and if there are implications that may require an update to our SIRA review as detailed below. If you have any questions about this study please contact the Natural Hazards team at TDC.
We are reviewing the natural hazards provisions in the Tasman Resource Management Plan (TRMP).
The Tasman Resource Management Plan identifies 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 stability 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 locations in 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 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.
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 when LiDAR becomes available.
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.
Other land outside of the identified study areas was also not considered in the current review. It is recognized that there will be areas in our district, outside the SIRA that are potentially susceptible to slope instability and debris run-out. Such hazards will be 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 and as additional LiDAR data becomes available.
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. The overlay and rules act as a flag to pay extra attention slope instability hazards 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 and/or debris run-out, what does this mean?
This new 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 and run-out susceptibility areas are not considered a replacement for site-specific investigations, nor does it consider risk 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 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 slope of the site, land stability and 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 hazard was 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 geotechnical assessment. 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 a further site-specific assessment. The inclusion of properties in the SIRA and/or this new mapping ensures that both current and future landowners are aware of slope instability hazards and people and property remain 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 in reviewing the natural hazards provisions in the TRMP will be to consider this technical information to inform the identification of ‘issues and options’ for managing our natural hazards. Over 2024 this work will be progressed along with other technical background work.
Council currently takes a risk-based approach to natural hazards management, and this will continue with improvements to the TRMP framework where necessary. 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.
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