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Slope Instability

View the Slope Instability Map

Why has Council identified areas of slope instability hazard in Nelson?

In 1996, the Nelson Resource Management Plan identified areas of 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; and the Tahunanui Fringe Slope Risk Overlay. These overlays define areas where there is a known or potential risk of slope movement or failure.

The Resource Management Act (1991) requires councils to manage Natural Hazards, including slope instability. As part of the Draft Whakamahere Whakatū Nelson Plan process the natural hazard information needed to be updated. This included taking a region-wide approach to identifying areas of slope instability.

Why has a LIM notation been placed on my property?

Under the Local Government Official Information and Meetings Act 1987 the Council has an obligation to issue Land Information Memoranda (LIMs) for information that includes the identification of any special features or characteristics of the land that relates to potential erosion, avulsion, falling debris, subsidence, slippage, alluvion, or inundation that is known to the Council but is not yet contained within the local Resource Management Plan.

Will the LIM notation be removed once Slope Instability Overlays are included in the Nelson Plan?

Yes, once the Nelson Plan is made operative and includes the Slope Instability Hazard Zone overlay, the LIM notation can be uplifted from the property. The notice can be removed because the information will be readily available through the District Plan.

How has Council identified slope instability and run-out hazard zone?

The Council commissioned further research into areas that could be potentially affected by Slope Instability Hazards to inform the Draft Nelson Plan Natural Hazards overlay.

Local geotechnical engineers carried out a Nelson –wide investigation to map land potentially susceptible to slope instability and any impacts from the run-out of slope instability.

The engineers report followed the Australian Geomechanics Society guidelines for ‘Landslide Risk Management’ which identify the following areas 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

Areas identified as susceptible to run-out where assessed by:

  • 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 Slope Instability only identifies land considered susceptible to slope instability and/or within the run-out zone of debris from the instability. The overlay does not consider risk to individuals and property. Assessing ‘risk’ considers the probability of a landslide or slip occurring and the potential consequence of it happening on people and property. ‘Risk’ assessment requires a high level of detailed information and is best assessed when all the specific details of a particular property, activity or land use are known, such as when a resource consent application is applied for, rather than at a broad region wide level.

How will the Council manage development in these areas?

Council and the community must make decisions together about how to best manage these areas. This mapping will be included within the Natural Hazards chapter of the Draft Nelson Plan.

The Nelson Plan and the Natural Hazards chapter, once finalised, will include specific rules relating to how development is managed in these areas.

The Natural Hazards chapter in the Draft Nelson Plan, that was consulted on in October-December 2020, contained objectives and policies that set out the overall approach to all Natural Hazards to ensure that development in these areas does not result in an increase in significant risk to people and property. However, the Draft version that was consulted on in 2020 did not contain the slope instability mapping or any specific rules relating to slope instability. We will carry out further engagement on these provisions once they have been drafted and before they come into effect, as these are decisions that need to be made with the input of the community.

There are a number of ways the Draft Nelson Plan could manage development within these areas. For example, in particularly hilly areas with less stable underlying geology it may require more management of activities, especially where those activities could cause erosion and sedimentation. In other areas, it may be a case of requiring geotechnical investigations.

The Draft Nelson Plan recognises the need to achieve a balance between enabling growth and development to meet people’s needs and making sure that future communities are not put at risk from potential hazards. Policies also recognise the importance of public awareness of risk, so that people can make informed decisions about where to develop into the future.

What does this mean for me?

As part of this Draft Nelson Plan process, the Council will discuss slope instability management options with the community. The way development is managed in the new areas of potential slope instability under the Nelson Plan has yet to be decided. We will be seeking feedback from the community during the next phase of engagement on the Nelson Plan, later in 2021. This may include discussion about new rules and restrictions that can be used to manage the type of development that is appropriate in potential locations of slope instability.

What if I am located within the existing Nelson Resource Management Plan Slope Risk Overlay?

Until the Draft Nelson Plan takes effect, the Nelson Resource Management Plan (NRMP) rules still apply to any property within the Grampians Slope Risk Overlay, the Tahunanui Core Slope risk Overlay and the Tahunanui Fringe Slope Risk Overlays.

This means a resource consent or Geotech assessment may be required for new development or redevelopment.

What if my property is within the new overlay?

The existing rules under the NRMP will continue to apply until the Nelson Plan comes into effect, so even if your property is within the new slope instability overlay, there will be no immediate changes to the way development is managed.

If I’m building?

When you apply for a Building Consent, the slope of the site, land stability and any natural hazard is taken into consideration(regardless of NRMP or Draft Nelson Plan hazard overlays) through the Building Act requirements.

Will I need to apply for Resource Consent?

You will only require a Resource Consent when your proposal contravenes a rule/s in the NRMP for example, if you exceed the building height standards.

If I’m subdividing?

The current rules of the NRMP will continue to apply until replaced by the Nelson Plan. The risk from natural hazards will be assessed as part of the application process.

If I’m carrying out earthworks or clearing vegetation?

There are district wide rules in the NRMP that manage the effects of earthworks and vegetation clearance where there is potential it may result in slope instability. You can find out more information on this in the NRMP

What next?

Before the Nelson Plan and any rules on slope instability become operative, there are a few more steps in the process. We expect engagement on slope instability to take place later in 2021. The community will have an opportunity to make submissions on the content of the Plan when it is publicly notified.

If you are located within the affected areas, expect correspondence regarding any future engagement stages. These will also be advertised on the Nelson Plan webpage