Port Hills investigation report findings released

Christchurch City Council today released the findings of the first of a series of reports looking at slope stability in the Port Hills after the earthquakes.

The Council commissioned the Institute of Geological and Nuclear Sciences Ltd (GNS Science) to complete the reports in response to changes in the stability of slopes in the area following the 2010/2011 earthquakes.

The Canterbury Earthquakes 2010/11 Port Hills Slope Stability: mass movement Stage One Report released today details investigations of 36 areas in the Port Hills where mass land movement has occurred. The report assesses the nature of hazards and if these hazards pose a risk to life, homes and critical infrastructure. 'Mass movement' describes slope instability from the movement of significant volumes of soil and/or rock.

"Hill areas are susceptible to instability based on a range of environmental factors. Like other councils around the country, managing and mitigating the risks around land changes brought about by weather conditions and natural hazards is an ongoing job for us," says Mike Theelen, General Manager, Strategy and Planning Group.

"These reports are about giving us a better understanding of the extent of that risk in some areas, to try to avoid any loss of life, prevent damage to homes and infrastructure, and make sound planning decisions about future land use."

The Stage One Report will provide affected Port Hill land owners with a preliminary level of knowledge on the slope stability of the area in which their property is sited. The Council this week sent a letter to landowners in the 36 areas affected, to let them know their property is located either completely, or partly, within the areas of slope instability identified in the report.

This is a separate process from the Government's zoning review.

"The information in this GNS Science 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," says Mr Theelen.

"These areas will need to be managed in a different way to reduce the likelihood of causing additional instability. This could include stricter control of earthworks, surface and subsurface water control, vegetation clearance and retaining walls in the Port Hills. These will be addressed by changes to the District Plan.

"For property and landowners looking to repair and rebuild, we want to assure them the Council will continue to process consents in these areas, but for future development we are likely to need them to provide more site-specific geotechnical information than we do at present."

The Council has been working with the Engineering Advisory Group and the Ministry of Business, Innovation and Employment (MBIE) to produce guidance for engineers working on foundation solutions for some of these areas.

Key findings of the Stage One Report: * The report gives a preliminary understanding of the nature and significance of slope instability in 36 main mass movement areas across the Port Hills and prioritises the areas for further study. * Four of the 36 mass movement areas are subdivided into two or more sub areas as there are different levels of hazard exposure within each of these four areas. This brings the total number of areas and sub areas to 46. * The 46 areas are grouped into three classes: - There are 15 preliminary Class I sub areas where more research is a top priority as further mass movement could see lives lost. Homes and/or critical infrastructure may be severely damaged. - There are 18 preliminary Class II and 13 preliminary Class III sub areas where more study is planned but is not a priority, as any further mass movement is unlikely to see lives lost. * The report recommends the need for further assessment, emergency management planning, and land use and infrastructure planning. * The classes given to areas are preliminary, as they may change when more information becomes available in further reports.

"We understand that Port Hills property owners face ongoing uncertainty and delay, but decisions involving people's lives and homes need to be well informed, and based on good quality and sufficient scientific information and advice," says Mr Theelen.

"The Port Hills is a dynamic landscape, and this investigation process following the earthquakes has, and continues to be, complex and time consuming. We are grateful to people in the Port Hills for their ongoing patience," he says.

Mayor of Christchurch Lianne Dalziel says she appreciates it has been an extremely difficult time for residents in the Port Hills since the earthquakes.

"After waiting so long, people in the Port Hills are understandably seeking certainty on the status of their land. This report provides further information which will help residents understand what has happened to land stability on the Port Hills as a result of the earthquakes.

"We have had this GNS Science report since 1 August this year, and hoped to release it alongside CERA's Port Hills zoning review announcement. However, the CERA announcement has been delayed. From the Council's perspective, it's important that people have all the information they can get about the status of their property as soon as possible."

"Many residents will want further information. We are planning public meetings for people most affected, and Council staff will be available to meet with people one on one to discuss what the report findings mean for them. We are working with CERA and MBIE to reduce uncertainty for people in the Port Hills as quickly as we can."

A full copy of the GNS Science Stage One Report and more information is available on the Council's website at www.ccc.govt.nz/porthillsgeotech

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