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Last reviewed: Thu, 05 Dec 2013

Port Hills GNS reports



Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Stage One Report, August 2013

The Institute of Geological and Nuclear Sciences Ltd (GNS Science) recently completed the first of four planned reports on slope stability: mass movement in the Port Hills for Christchurch City Council. The Canterbury Earthquakes 2010/11 Port Hills Slope Stability Stage One Report details investigations of 36 areas in the Port Hills where mass movement has taken place. ‘Mass movement’ is slope instability from significant volumes of soil and/or rock having moved together as a single mass.

The report assesses hazards, and considers if these hazards pose a risk to people, homes or critical infrastructure. Like many other hill areas in New Zealand, the Port Hills have always had the risk of slope instability. By having a better understanding of the extent of that risk in areas, we want to avoid any loss of life, prevent damage to dwellings and infrastructure, and make sound planning decisions about future land use.

The report will provide affected Port Hill land owners with preliminary knowledge on the slope stability status of the area on which their land is sited. The information in this GNS Science Stage One Report, and further investigations in subsequent stages of the project, will provide the Council with more information about these areas when making decisions on consenting, land use and infrastructure planning and development.

View the full copy of GNS Science Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Stage 1 report on the findings from investigations into areas of significant ground damage (mass movement). [PDF 18MB]

Appendix 1 – Stage One Mass Movement Hazard Exposure Category Maps [PDF 5MB]

Appendix 2 – Stage One Mass Movement Maps [PDF 16MB]

View more information on this GNS Science Stage One report, including FAQs, timelines and case studies.

Rockfall risk model review 2012

Since issuing the GNS Science reports in 2012, some additional rockfall and landslide related fieldwork was done in the Port Hills. From this it became apparent that in a few isolated areas the rockfall risk models (and subsequent maps generated from these) did not appear to reflect the actual ground conditions in these localised areas.

A review of the affected areas was carried out by GNS Science and Christchurch City Council geotechnical consultants. It is important to remember that models are not exact and that judgement is required when using them. The review included field inspections to verify:

  1. the key input parameters used in the risk model; and
  2. the results from the original ground truthing.

The review resulted in some localised adjustment to the models. Three of the detailed technical reports below have been reissued to incorporate the changes:

These changes can be reviewed in the links below. In the section below titled Separate maps associated with the reports, the Maps series A to G have also been re-issued to incorporate the changes described above.

GNS Science Port Hills Slope Stability - September Reports 2012

A series of seven detailed technical reports on slope instability on the Port Hills have been prepared by GNS Science for the Christchurch City Council. The reports were prepared in response to the 2010-2011 Canterbury Earthquakes and contain recommendations for the Council on how to address the risk from rockfall/boulder roll and cliff collapse. The findings of the reports have been used by CERA and the Christchurch City Council to assist with decisions on zoning, planning and infrastructure development, and regulatory matters. In addition to the reports there are about 400 separate maps to assist the reader of the reports in their interpretation of the report findings.

A series of three summary brochures have also been prepared. The brochures explain with the use of graphics key technical terms and concepts.

1. Understanding life safety risk concepts for rockfall and cliff collapse in the Port Hills [PDF 300KB]

2. Life-safety risk from cliff collapse on the Port Hills [PDF 600KB]

3. Life-safety risk from rockfall on the Port Hills [PDF 1.7MB]

Canterbury Earthquakes 2010/2011 Port Hills Slope Stability: Geomorphology mapping for rockfall risk assessment

This report provides information on the nature of surface geological materials (soils) and the processes that formed them. The information is compiled as geomorphic maps at a scale of 1:500. The nature and extent of the various surface materials on the Port Hills helps explain the erosion and deposition processes that have occurred in the past, including earthquake initiated large scale boulder roll and cliff collapse, and high intensity/long duration rainfall initiated landsliding. The maps also illustrate those active processes that we can expect in the future. Aerial photograph interpretation, satellite images and digital terrain models, with limited field work, were used to create the maps.

Geomorphology mapping for rockfall risk assessment - reduced resolution whole report [PDF 17MB]
Parts: Geomorphology mapping for rockfall risk assessment - pp 1-25 [PDF 4MB], Appendix 4 maps A6-D16 [PDF 10MB], Appendix 4 maps E2-J6 [6MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Principles and criteria for the assessment of risk from slope instability in the Port Hills, Christchurch

This report gives an overview of the issues that need to be considered in completing a risk-based approach to the management of slope instability hazards in the Port Hills. A framework and process for managing risk is described, international and New Zealand policy is outlined, and the risks faced by New Zealanders is summarised. Assessment criteria that could be adopted by the City Council are discussed along with options for treating the risk. The report recommendations include levels of annual individual fatality risk that the City Council should consider. Important technical aspects of the report are summarised in Understanding life-safety risk concepts for rockfall and cliff collapse in the Port Hills (Summary Series 1/3).

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Principles and Criteria for the Assessment of Risk from Slope Instability in the Port Hills, Christchurch [PDF 1.4MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from cliff collapse

The purpose of this pilot study was to develop a methodology to assess risk to life from cliff collapse. The report considers cliff collapse triggered by earthquakes and other causes, and expresses risk as annual individual fatality risk. The risk to life, between Redcliffs and Scarborough, is calculated for both cliff top recession and cliff base debris deposits. The report recommendations include the need for ongoing investigations to better determine the nature and extent of damage to the ground at the top of cliffs.

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from cliff collapse [PDF 24MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Life-safety risk from cliff collapse in the Port Hills

This report is based on the methodology established in Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from cliff collapse and assesses the risk to life at the top and base of cliffs. Only the cliffs that meet specific potential failure criteria are included in the assessment. The risk to life is calculated as the annual individual fatality risk and is applied to an individual’s occupation of their residence. The report details the methodology used and the inherent uncertainties. Important technical aspects of the report are summarised in Life-safety risk from cliff collapse on the Port Hills (Summary Series 2/3).

Reissue July 2013 - Canterbury earthquakes 2010/11 Port Hills Slope Stability: Life-safety risk from cliff collapse in the Port
Hills
[PDF 15MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Additional assessment of the life-safety risk from rockfalls (boulder rolls)

This report assesses the sensitivity of the rockfall modelling used in Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from rockfalls (boulder roll) and Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Life-safety risk from rockfalls (boulder roll) in the Port Hills. A new set of maps show how the reduction in earthquake activity with time changes the risk zone boundaries. The report includes a range of recommendations including the need for the City Council to consider both the original rockfall modelling results and the results from the sensitivity analysis in its decision making. The rock roll zoning decisions were based on maps associated with this report.

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Additional assessment of the life-safety risk from rockfalls (boulder rolls) [PDF 1.5MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from rockfalls (boulder roll)

The purpose of this pilot study was to develop a methodology to assess risk to life from rockfall (boulder roll). The risk is expressed as the annual individual fatality risk. Rockfall triggered by earthquakes and other mechanisms such as rainfall is considered. The assessment includes an analysis of historic and potential future events. The report explains the field verification process used and the required changes made to the risk zone boundaries. Maps showing four classes of annual individual fatality risk are included in the report for select locations on the Port Hills.

Reissue July 2013 - Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from
rockfalls (boulder rolls)
[PDF 29MB]

Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Life-safety risk from rockfalls (boulder roll) in the Port Hills

This report is based on the methodology established in Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from rockfalls (boulder roll) and includes those areas of the Port Hills not assessed in the pilot study. These areas are Hillsborough, Richmond Hill Road, McCormacks Bay, Mt Cavendish, Taylors Mistake, Moncks Bay, Cass Bay, Governors Bay, and some parts of Lyttelton, Bowenvale, Sumner, Vernon Terrace and Avoca Valley. The report provides recommendations to the City Council including the need to continue monitoring the rockfall source areas as further instability is likely to occur. As for the pilot study report, this report contains maps showing the zones of risk, potential rockfall source areas and building locations. Important technical aspects of the report are summarised in Life-safety risk from rockfall on the Port Hills (Summary Series 3/3).

Reissue July 2013 - Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Life-safety risk from rockfalls (boulder rolls) in the Port Hills [low res PDF 14MB]

Separate maps associated with the reports

Latest Maps series A to G are associated with how the reduction in earthquake activity with time changes the risk zone boundaries - August 2013.

Maps used for CERA Zoning decisions:
Maps A - Risk Model Yr 1 2012 [PDF 15MB]
Maps C - Risk Model Yr 5 2016 [PDF 15MB]
(In a very small number of cases some discretion was applied)
Maps showing sensitivity of the rockfall modelling:
Maps B - Risk Model Yr 3 2014 [PDF 8MB]
Maps D - Risk Model Yr 7 2018 [PDF 8MB]
Maps E - Risk Model Yr 8 2019 [PDF 8MB]
Maps F - Risk Model Yr 9 2020 [PDF 8MB]
Maps G - Risk Model Yr 10 2021 [PDF 15MB]

As at August 2012 the following maps series A to G have been superseded and remain here for reference.

Maps used for CERA Zoning decisions:
Maps A - Risk Model Yr 1 2012 [PDF 15MB]
Maps C - Risk Model Yr 5 2016 [PDF 15MB]

(In a very small number of cases some discretion was applied)
Maps showing sensitivity of the rockfall modelling:
Maps B - Risk Model Yr 3 2014 [PDF 8MB]
Maps D - Risk Model Yr 7 2018 [PDF 8MB]
Maps E - Risk Model Yr 8 2019 [PDF 8MB]
Maps F - Risk Model Yr 9 2020 [PDF 8MB]
Maps G - Risk Model Yr 10 2021 [PDF 15MB]

Frequently Asked Questions and enquiries for rockfall, cliff collapse and life risk

We have compiled a series of FAQs and answers. [PDF 80KB]

 

Canterbury Support Line Ph 0800777846Got a question?
For Port Hills residents seeking more information contact

Please note:

The GNS Science reports published by the Council on this website have been prepared and provided by GNS Science at the request of the Council, and CERA (in some cases) for those agencies. The Reports are now published by Council to assist the public understanding of the complex issues.
The Council makes no warranty or representation regarding the accuracy or completeness of the GNS reports. The Council is not qualified to have any view on the accuracy of the information contained in the GNS reports. The GNS reports should not be relied upon at a site specific level without advice from a qualified professional.
 

Authorising Unit: Project Management

Last reviewed: Thursday, 5 December 2013

Next review: Friday, 5 December 2014

Keywords: damage, land, port hills