Christchurch rockfall

By Grant Jacobs 16/09/2011

While I slave away, looking forward to the weekend when I might find time to write something (as opposed to pasting up other’s work*), two items on rockfall in, or from, the Port Hills.

The Port Hills lie to the south of the city, forming the northern portion of what locally is known as the Crater Rim – a circular ring of hills around the harbour that are the rim of an extinct volcano.

For me one of the not-foreseen hazards of the earthquakes in the Christchurch area was rockfall. While it might have been a known risk amongst geologists, I am under the impression many Christchurch residents (past and present) were, like me, caught by surprise on hearing reports of rockfall and the damage caused.

A PDF file** of the slides of a recent geotechnical presentation*** on rockfall hazard from the Port Hills are available on-line. This contains some interesting maps along with summaries of the situation, work being done, key messages and many slides illustrating examples of rockfall and land movement issues on the Port Hills.

Among the key messages these struck a chord with me, particularly the second:

  • Rockfall is a life safety issue more than a building issue and requires rigorous analysis
  • Rockfall hazard is a real and present risk without further earthquakes or aftershocks
  • Crucial to remember that dwellings in a geotechnical hazard zone will always carry a residual risk.
  • If hazard sign posted assume that recreational tracks are not safe, contractors can be working above!

Related to this is a short video that gives a few examples of the sort of rockfall issues that have been experienced:


I’m not a geologist or geo-hazards expert.

* Including, for that matter, writing about science happenings and whatnot from my own field!

** There are several other variations of this document released around the same time. I have to admit it makes it confusing to know which is the best to present. More information can be found on the CERA website and CERA on YouTube.

*** The presentation itself doesn’t appear to be on-line (yet).

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