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Slope Instability

Source: Walter Bluff Reserve, August 2022 (Stuff)

Source: Nelson House - landslide - August 2022 (NCC)

Slope instability is the movement of rock, debris, or earth (soil and rock) down a slope in areas where the topography enables sliding to occur. Many hillsides in the Nelson region are potentially susceptible to slope instability.

Introduction

What is slope instability?

Slope instability is the movement of rock, debris, or earth (soil and rock) down a slope in areas where the topography enables sliding to occur. Slope instability features can range from large landslides several square kilometres in area, to shallow slope failures only a few metres wide. When slopes fail, debris can move downhill onto other areas.

Slope instability can happen due to a mix of natural and human factors. Natural causes include the type of ground underneath, how steep the slope is, heavy rain, earthquakes, and weathering. Human activities like building roads or platforms, removing vegetation, or even forest fires can also trigger instability.

Slope instability can also affect land below the area where the actual landslide has occurred (i.e. land at the bottom of the hill over which debris can run out onto).

Slope instability in Nelson

Many hillsides in the Nelson region are potentially susceptible to slope instability. This has been confirmed through years of studies and observations following severe weather events.

Slope instability assessments

Since the 1990s, Nelson City Council has commissioned several studies on slope instability that have improved our knowledge of this natural hazard. These studies help to better understand this natural hazard and where it could potentially occur. Recent studies have looked at new areas or taken a closer look at known areas using updated methods. These studies do not yet cover all parts of the Nelson region and there may be areas where slope instability could occur that have not been assessed or mapped.

These are regional-scale assessments that give an indication of potential susceptibility to slope instability. They do not provide detailed information for individual properties. If detailed information about a specific site is required, a qualified professional will need to carry out a site-specific assessment.

The studies are listed below, starting with the oldest to the most recent.

Report

Geotechnical Assessment Tāhunanui Slump, Soil and Foundations - October 1995

Geotechnical Assessment Grampian Foothills - December 1995

In 1996, the proposed Nelson Resource Management Plan (NRMP) identified three areas of potential or existing development where erosion or slope instability was known or could potentially occur. These were mapped as Slope Risk Overlays:

  • Grampians Slope Risk Overlay
  • Tahunanui Core Slope Risk Overlay
  • Tahunanui Fringe Slope Risk Overlay

These overlays are still part of the NRMP today, with rules that regulate activities and development within them.

Report

Slope Risk Overlays Review - February 2015

In 2015, the Slope Risk Overlays in the NRMP were re-evaluated along with a broader assessment of slope instability hazard in hilly parts of Nelson city. These overlays were not incorporated into the Nelson Resource Management Plan (NRMP).

Report

Slope Instability Overlay Report - 6 November 2020

In 2020, Council commissioned a new assessment of slope instability susceptibility and debris run-out impacts in the Nelson region to update our information and help with future resource management planning.

The assessment followed the Australian Geomechanics Society guidelines for 'Landslide Risk Management' (2007a) and identified the following areas as potentially susceptible to slope instability:

  • Cliffs
  • Natural slopes steeper than 35° (rapid landslides may occur)
  • Natural slopes 20-35° (landslide travel possible)
  • Slopes where geologic and geomorphic conditions are such that sliding is possible
  • Slopes with a history of instability, including large, currently inactive landslides subject to undercutting of the toe or reactivation by development
  • Land on a ridgeline with a slope greater than 35 degrees if the ridgeline is less than 30 meters wide
  • Land within 100m downslope of a mapped earthquake fault line.

The report also identified areas susceptible to run-out of debris from upstream landslides, including:

  • Areas of land downslope of mapped instability, where there is evidence the land was previously affected by run-out.
  • Areas where the steepness of the slope profile beneath the mapped area of instability may result in debris material (soil and rock) extending into this area.

The 2020 Slope Instability Overlay has not been incorporated into the Nelson Resource Management Plan (NRMP).

Peer review

Slope Instability Overlay Report - Peer Review - 10 November 2020

Report

Slope Instability Report - 9 December 2021

After the 2020 assessment was released, Council received feedback on the Slope Instability Overlay. In response, a technical advisory group was set up, made up of local engineering geologists and geotechnical engineers.

The advisory group recommended dividing the overlay into three tiers of slope instability to better reflect the different levels of susceptibility to slope instability. The debris run-out zone remained unchanged.

The revised Slope Instability Overlay has three tiers and a debris run-out zone, 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 zone - Areas identified as being potentially susceptible to the runout of debris from up-slope instability.

The 2021 revised Slope Instability Overlay has not been incorporated into the Nelson Resource Management Plan (NRMP).

Report

Slope Instability Overlay post August 2022 - updated May 2023

From 16 to 19 August 2022, Nelson was hit by a severe storm. The [then] National Institute of Water & Atmospheric Research (NIWA) classified it as a 1-in-120-year rain event, meaning a storm of this intensity has a 0.83% chance of occurring in any given year.

The prolonged heavy rainfall caused landslides and slips on many hillsides across Nelson. Some of these resulted in damage to homes and local infrastructure.

After the August 2022 event, Council commissioned a report to compare the actual slope failures with the Slope Instability Overlay mapping done in 2021 (i.e.the 2021 report above).

The findings showed that the damage observed during the storm generally matched the susceptibility areas indicated in the 2021 mapped tiers and run-out zones. However, some refinements were made to the assessment method to address the observed damage, and the mapping was updated in some areas.

This 2023 report should be read together with the 2020 and 2021 reports as well as the Updates to Slope Instability Susceptibility Overlay report below.

The revised Slope Instability Overlay has not been incorporated into the Nelson Resource Management Plan (NRMP).

Report

Updates to Slope Instability Susceptibility Overlay made during the Plan Change 29 Hearing process - 14 October 2025

Maps - Updates to Slope Instability Susceptibility Overlay made during the Plan Change 29 Hearing process - 14 October 2025

The 2023 revised Slope Instability Overlay mapping was proposed to be incorporated into the Nelson Resource Management Plan (NRMP) via Plan Change 29. During the plan change process, a data smoothing error was identified in the slope angle dataset used in the 2020, 2021, and 2023 assessments. The error was the result of a data processing issue which caused the slope angle to be overestimated. The slope angle dataset has been reprocessed and corrected. As a result, the mapped slope instability susceptibility areas have been revised and reduced in size. The re-assessed overlay used the same methodology applied in the 2021 and 2023 assessments. This re-assessment is described in a 2025 report with related revised mapping, with the report recording and explaining the changes that were made to the Slope Instability Susceptibility Overlay as a result of the Plan Change 29 process in 2024. This report should be read together with the 2020, 2021 and 2023 reports above.

The revised mapping was not incorporated into the NRMP as part of Plan Change 29, but it is the most up to date information that Council holds on slope instability susceptibility and is used to inform LIM notations for slope instability.

The natural hazard maps

You can view your property on our interactive Slope Instability Map. Check this map to see the most recent mapping of areas potentially susceptible to slope instability - i.e. based on 2020 report, as updated in 2021, 2023 and 2025 as above.

Frequently asked questions

Please note that these FAQs specifically relate to slope instability hazards. For general FAQs that relate to all hazard types, please see our Natural Hazards Landing Page.

Visit the Slope Instability Map to find out whether the latest information the Council holds indicates your property may be susceptible to a natural hazard.

Properties affected by the following mapping overlays are considered to be potentially susceptible to slope instability:

  • Slope Instability - Tier I
  • Slope Instability - Tier II
  • Slope Instability - Tier III
  • Run-out zone.

Please note that the studies on which the mapping is based are of regional scale and are indicative only of potential slope instability affecting any individual site or property. If detailed information about a specific property is required, a qualified professional will need to carry out a site-specific assessment.

Natural hazard maps change as our environment changes and our knowledge evolves. The map reflects our most up-to-date understanding of slope instability, based on the latest information available to us.

The map (and tiers) identifies areas that are potentially susceptible to slope instability, based on a range of factors. These include, but are not limited to, observations of where slope instability has occurred in the past, and characteristics of the land that may make it potentially susceptible to slope instability in the future, such as steepness of the slope, and the underlying geological conditions.

Local engineering geologists carried out a region–wide investigation to identify areas of Nelson that are potentially susceptible to slope instability or affected by debris from slope movement (known as run-out).

This region–wide assessment followed the Australian Geomechanics Society guidelines for 'Landslide Risk Management' (2007a) and identified the following area as potentially susceptible to slope instability:

  • Cliffs
  • Natural slopes steeper than 35 degrees (rapid landslides may occur)
  • Natural slopes 20-35 degrees (landslide travel possible)
  • Slopes where geologic and geomorphic conditions are such that sliding is possible
  • Land on a ridgeline with a slope greater than 35 degrees if the ridgeline is less than 30 metres wide
  • Slopes with a history of instability, including large currently inactive landslides subject to undercutting of the toe or reactivation by development
  • Land within 100m downslope of a mapped earthquake fault line.

The report also identified areas susceptible to run-out, including:

  • Areas of land downslope of mapped instability, where there is evidence it was previously affected by instability run-out
  • Areas where the steepness of the slope profile beneath the mapped area of instability may result in debris material (soil and rock) extending into this area.

The region-wide assessment and mapping undertaken in 2020 identified a map of potential slope instability and a debris run-out zone. As a result of feedback, in 2021 the map of potential slope instability was divided into three tiers (described below). The total area mapped as potentially susceptible to slope instability did not change, and no changes were made to the debris run-out zones.

The revised tiers of slope instability are:

  • Tier I – Area of known active instability with previous slope failures impacting residential properties which warrants specific planning regulations.
  • 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.
  • Areas potentially susceptible to debris run-out.

In 2023, the 2021 region-wide assessment was updated after the 2022 extreme weather event in Nelson. Some refinements were made to the methodology used to assess the tiers, and there were some updates to the mapping.

Then, the revised mapping of the Slope Instability Overlay was proposed to form part of Plan Change 29 to the NRMP. As further outlined above, this mapping was further revised during the Hearing of Plan Change 29 to account for an error in data smoothing. This error has been corrected, as summarised in a 2025 report (linked above). No changes to the methodology to identify the hazard were made.

The natural hazards webmap shows the most recent mapping of slope instability that the Council holds. This relates to the slope instability assessment undertaken in 2020, with subsequent updates in 2021, 2023 and 2025.

In comparison, the NRMP mapping shows the Slope Risk Overlays identified in 1996. Plan Change 29 proposed to update mapping and provisions for slope instability, however during the plan change process a data smoothing error was identified (which affected the maps produced from the 2020, 2021 and 2023 reports). In short, this error caused the slope angle data to be overestimated. The ‘slope angle error’ has now been corrected, however the new (corrected) mapping of slope instability was not carried through as part of the final Plan Change 29.

The most recent mapping of slope instability that the Council holds is the mapping that was corrected through the Plan Change 29 process. The spatial extent of the area affected by slope instability was reduced compared to the 2020, 2021 and 2023 assessments. This most recent mapping, as outlined in the 2025 report linked above, is now used to inform LIM notations for slope instability. You can see the most recent mapping for slope instability on the Slope Instability Map.

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