GIS modelling in support of earthquake-induced rockfall and cliff collapse risk assessment in the Port Hills, Christchurch

Abstract

Ground shaking associated with the 22 February 2011 M_w 6.2 Christchurch earthquake exceeded MM10 and dislodged boulders from cliffs on the upper slopes of the Port Hills, southeast of Christchurch City. Boulders rolled into the urban areas below. Cliffs on the lower slopes collapsed and debris avalanches inundated homes built at their bases and undercut homes built at their crests. Boulders and debris avalanches impacted over 200 buildings and killed five people, resulting in widespread evacuations. Large aftershocks caused further boulder rolls and cliff collapses. Before buildings could be reoccupied a life-safety risk assessment was required. This study included pilot investigations of the 19 worst affected areas and these were used to develop hazard and risk models. Once ground verified, the models were extended to the entire Port Hills area. GIS was the main tool used in the development of the models, but other tools and techniques were also utilised.

Keywords::

• GIS
• earthquake
• slope failure
• rockfall
• boulder rolls
• cliff collapse
• debris avalanche
• cliff recession
• risk assessment
• Port Hills
• Christchurch

Acknowledgements

Many people were involved in this project, including staff from GNS Science, Christchurch City Council, the Canterbury Earthquake Recovery Authority, the New Zealand Natural Hazard Research Council, staff and students from University of Canterbury, and the members of the Port Hills Geotech Group including staff from OPUS International, URS NZ, GHD NZ, Aurecon NZ, Geotech Consulting, and Bell Geoconsulting. Peer review of the data and models was by Drs Fred Barnes, Tony Taig and Laurie Richards. The authors thank those who assisted in the preparation and review of this paper, particularly Phil Glassey, John Begg and Eileen McSaveney from GNS Science, the anonymous JOSS reviewers, and Dr Antoni Moore, Otago University.