Vulnerability values for death “if struck by a rockfall” are summarized in Finlay et al.
(1999) and typically range between 0.1 and 0.7. They recommend a value of 0.5, assuming
that it may be possible to avoid a moving boulder (notwithstanding that this relates to
the probability of a person being present, which appears elsewhere in the model). In the
Port Hills, for people in homes, it is likely that their ability to avoid a boulder would be
significantly reduced. This is counterbalanced by the level of protection a house may
provide by stopping a boulder from entering it, which is dependent upon the size and
velocity of the boulder. Data from houses hit by falling boulders on 22 February
2011 indicate only about 45% were penetrated by boulders. Based on these discussions,
we adopted a vulnerability value of 0.5 for our rockfall risk assessment, notwithstanding
that only one of the 2011 rockfall deaths was indoors, and all documented 2011 Port Hills
rockfall hits were fatal.