Winter poses threat to Port Hills
MICHAEL WRIGHT
Last updated 05:00 12/03/2011
Geotechnical engineers are bracing for a long winter as aftershocks and
seasonal rain eat away the already unstable Port Hills.
Rock falls are a constant threat and landslips loom as the biggest problem in
the deteriorating weather. Engineering geologist Mark Yetton said the hillside
land would be susceptible to both for a long time. "Through to the end of
winter, and even in years to come, those cracks aren't going to go away," he
said.
Even though emergency evacuations were all but finished, more could come
later. "I absolutely can't rule it out. If I was anybody living on the hill areas I
would have a doggy bag of things near and dear to me handy." Yetton said
small landslips in the wetter winter months were almost inevitable. Generally,
rock falls were brought on by aftershocks and landslips by heavy rain. Each
could contribute to the other, but Yetton said the division was usually clear.
"Rain in open [rock] cracks drains pretty easily," he said. "But in the land you
can get damming of water and it exerts quite a high pressure."
The damage would only be known as aftershocks hit and rainfall increased.
"At the moment the hazard that's the most serious is the rock fall, because it's
easily triggered by aftershocks. "Soil slips are at a more moderate angle than
rocks. They've cracked but they haven't moved much. They stopped when the
shaking stopped."
Engineers were now putting in measuring systems to monitor land movement.
"Our intention is to map places where the most profound cracking has
occurred," Yetton said. The public would be kept informed and may be invited
to build backyard measuring systems. Engineers had three global positioning
system (GPS) monitors available to measure movement. They would be
placed on the areas of most concern, while other sites would be mapped with
pins or traditional surveying methods.
Yetton said speed of movement could be a warning sign, but total
displacement was the focus. Movement of more than 100 millimetres would
set off warning bells. "I could live with 20 millimetres," he said. "The further it
travels the weaker it gets. There's quite a big drop-off in strength." Yetton said
little attention had so far been given to the wider Port Hills area, including
parks, cycling and walking tracks.
"What is obvious are the little rock falls all over the bluffs and cliffs and people
who go there need to be careful for a long time yet." It could be months before
all tracks are open again. Christchurch City Council Port Hills area head
ranger Paul Devlin said staff were updating track conditions and access, but
progress was slow.