Following the Canterbury Earthquakes, GNS Science has completed several reports on land stability for the Council, on mass movement, rockfall and cliff collapse.

GNS Slope Stability Reports (mass movement)

The Council commissioned these reports in response to the 2010/2011 Canterbury Earthquakes.

The reports follow the first GNS Science mass movement report released in November 2013, which gave mass movement areas in the Port Hills a preliminary Class I, II or III category. Class I areas are where any further mass movement could see lives lost, and homes and/or critical infrastructure severely damaged.

Final Reports: Mass movement

1.

Risk assessment for Maffeys Road [PDF, 12 MB]

2.

Risk assessment for Quarry Road [PDF, 8.4 MB]

3.

Risk assessment for Cliff Street [PDF, 9.8 MB]

4.

Earth/debris flow risk assessment for Defender Lane [PDF, 11 MB]

5.

Debris avalanche risk assessment for Richmond Hill [PDF, 15 MB]

6.

Risk assessment for Redcliffs [PDF, 16 MB]

7.

Risk assessment for Deans Head [PDF, 8.9 MB]
8. Risk assessment for Clifton Terrace [PDF, 16 MB]

GNS Science Port Hills rockfall and cliff collapse reports

In response to the Canterbury Earthquakes, a series of seven detailed technical reports on slope instability on the Port Hills were prepared by GNS Science for the Christchurch City Council. The reports 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.

A series of three summary brochures below explain key technical terms and concepts:

  1. Understanding life safety risk concepts for rockfall and cliff collapse in the Port Hills [PDF, 292 KB]
  2. Life-safety risk from cliff collapse on the Port Hills [PDF, 611 KB]
  3. Life-safety risk from rockfall on the Port Hills [PDF, 1.8 MB]

Geomorphology mapping for rockfall risk assessment

This report provides information on the nature of surface geological materials (soils) and the processes that formed them.

Principles and criteria for the assessment of risk from slope instability in the Port Hills

This report gives an overview of the considerations for completing a risk-based approach to the management of slope instability hazards in the Port Hills. The report recommendations include levels of annual individual fatality risk that the 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.

Pilot study for assessing life-safety risk from cliff collapse

The purpose of this pilot study was to develop a methodology to assess the 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.

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 Council geotechnical consultants. 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 reissued reports are below.

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. Important technical aspects of the report are summarised in Life-safety risk from cliff collapse on the Port Hills.

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. The rock roll zoning decisions were based on maps associated with this report.

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 report explains the field verification process used and the required changes made to the risk zone boundaries.

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 areas of the Port Hills not assessed in the pilot study. Important technical aspects of the report are summarised in Life-safety risk from rockfall on the Port Hills.

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 [PDF, 17 MB] - Risk Model Yr 1 2012
  • Maps C [PDF, 17 MB] - Risk Model Yr 5 2016
    (In a very small number of cases some discretion was applied)

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.