The Nelson region contains hillslopes that may potentially be susceptible to slope instability. Slope instabilities range from large landslides several square kilometres in area to local areas of instability in the order of a few metres wide. The occurrence, type, and location of slope instability is influenced by the underlying geology, steepness, and human use of the slope (for example logging, forest fire, construction of roads).
Our understanding of slope instability in Nelson continues to evolve as new areas are assessed and we undertake more detailed studies of areas we are already aware of. Methodologies for assessing slope instability also change over time. Studies of slope instability commissioned by Nelson City Council are described below.
Slope instability assessments and mapping in Nelson
In 1996, the Nelson Resource Management Plan (NRMP) identified areas of known or potential erosion or slope risk through the Slope Risk Overlays. There are currently three areas that are covered by Slope Risk Overlays.
- The Grampians Slope Risk Overlay
- The Tahunanui Core Slope Risk Overlay
- Tahunanui Fringe Slope Risk Overlay
In 2015, the Slope Risk overlays in the NRMP were re-evaluated along with a more broad assessment of slope instability hazard generally in hill areas of Nelson city.
A review of the Slope Risk Instability Overlays defined in the Nelson Resource Management Plan - Dr Mike Johnston, 2015
The Council commissioned further research into areas that could be potentially affected by slope instability. A region–wide assessment identified land potentially susceptible to slope instability and the impacts from run-out of slope instability. The assessment followed the Australian Geomechanics Society guidelines for ‘Landslide Risk Management’.
The report can be downloaded here:
- Slope Instability Overlay Report – BECA 6 November 2020 (31MB PDF)
- Peer Review report – CGW 10 November 2020 (704KB PDF)
After the release of the 2020 assessment, the Council established a technical advisory group of local engineering geologists and geotechnical engineers to address community feedback on the slope instability overlay. The advisory group recommended that the overlay be subdivided into three tiers to reflect varied susceptibilities to slope instability. No changes were made to the run-out zone. The revised slope instability overlay was prepared by Beca and presented in the 2022 report. The revised tiers are as follows:
- Tier I – Area of known active instability with previous slope failures impacting residential properties.
- Tier II – Areas identified as having elevated susceptibility to slope instability including areas with existing deep seated or earthflow instabilities and/or geologic units known to have an elevated susceptibility to instability.
- Tier III – Areas identified as susceptible to slope instability based on the geologic and geomorphic setting and/or with previous records of slope instability failure.
- Run-out zones.
The report can be downloaded here:
To see mapping associated with this report please go to the map below:
View the Geotechnical Hazards Map
Please note that the data contained within the NCC Slope Instability Overlay Report has been generated at a regional scale (1:5,000). It is therefore indicative only of potential slope instability hazard affecting any individual site or property. To determine the specific slope instability affecting any individual site or property, a site-specific assessment may need to be undertaken.
How do I know if my property is located within an area potentially susceptible to slope instability?
Use the link above to the natural hazards mapping webmap to find out whether the latest information we hold indicates that your property may be susceptible to slope instability.
What if my property is within an area potentially susceptible to slope instability?
Council is required to update Land Information Memoranda (LIMs) notations with new and updated information relating to natural hazards, including slope instability.
Slope instability hazard information may be used in building, subdivision, or resource consent processes.
In the future, areas of Nelson that may be susceptible to slope instability are expected to be included into the NRMP, or into a new resource management plan.