All hill areas face ongoing slope stability risks, but earthquake damage has increased this risk to some land in the Port Hills.
As part of the Port Hills land damage assessment programme, in November 2013 Christchurch City Council released the GNS Science Stage One Report [PDF, 18 MB] which identified potential areas in the Port Hills that were at risk from mass movement (sometimes called landslides).
The Report 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. In a Class II area, any further mass movement could damage homes and affect critical infrastructure, and in Class III could cause damage to homes. Lives are not thought to be at risk from the mass movement hazard in Class II and Class III areas.
In August 2014, the Council released eight further GNS Science Reports looking further at the level of risk from mass movement in eight areas between Mount Pleasant and Sumner. The reports showed 37-green zoned homes were in areas where the risk to life from mass movement is considered ‘intolerable’ by the Council. This means the risk to life for a resident from mass movement in any one year is equal to or greater than one in 10,000.
The Council and Crown have purchased properties, where there was no cost-effective engineering solution to reduce the risk to them to an acceptable level, or their properties are needed to carry out engineering works to reduce or remove the hazard.
Engineering concepts have been developed for each of these areas to reduce the risk to the other homes to an acceptable level. The approach has been to remove the hazard wherever possible.
Mass movement remediation work
- Quarry Road - Main Road: Work here has removed the hazard through benching the slope. This work was completed in early 2015.
- Quarry Road - The Brae: Remediation work will comprise earthworks to remove unstable soil.
- Maffeys Road: A bund is being constructed here to contain debris from a slope failure, and protect green zoned houses in this area.
- Defender Lane - Egnot Heights/Taupata Street: Works here were completed in mid-2015 and are a combination of risk mitigation measures, including slope excavation at Egnot Heights, and the construction of a debris barrier (or bund) at Taupata Street.
- Deans Head: This area is affected by mass movement as well as cliff collapse and rockfall affecting the road corridor. Remediation works were carried out as part of the Sumner Lyttelton Corridor programme.
Reports
Canterbury Earthquakes 2010/11 Port Hills Slope Stability Reports - released August 2014
- Risk assessment for Maffeys Road [PDF, 12 MB]
- Risk assessment for Quarry Road [PDF, 8.4 MB]
- Risk assessment for Cliff Street [PDF, 9.8 MB]
- Earth/debris flow risk assessment for Defender Lane [PDF, 11 MB]
- Debris avalanche risk assessment for Richmond Hill [PDF, 15 MB]
- Risk assessment for Redcliffs [PDF, 16 MB]
- Risk assessment for Deans Head [PDF, 8.9 MB]
- Risk assessment for Clifton Terrace [PDF, 24 MB]
Stage One Mass Movement GNS Science Report released in November 2013