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.
Canterbury Earthquakes 2010/11 Port Hills Slope Stability Reports - released August 2014
Stage One Mass Movement GNS Science Report [PDF, 18 MB] released in November 2013