To minimise risks of landslips in some parts of Dunedin,
residents and developers need to avoid developing, modifying
or discharging water to those areas, a GNS Science report
The report, done on behalf of the Otago Regional Council,
details the hazard significance of landslips in central
Dunedin, Otago Peninsula, Warrington to Karitane, Waikouaiti,
Saddle Hill to Brighton and Green Island to Blackhead.
It will be presented to the council's technical committee
today and go on to the council's online Otago natural hazards
database in the future.
The work was done to gain a better understanding of the
landslip risk in Dunedin and would also provide additional
information to support the Dunedin City Council's district
plan review of its natural hazard provisions, the report
The six areas were identified due to past identified
landslips and to existing development or development
GNS identified two types of landslip affecting the city:
deep-seated bedrock landslips which were usually prehistoric,
and shallow, surficial landslips, which mostly occurred on
moderate to steep slopes.
It found across the six suburbs, only 13% of the overall area
had landslips classified as definite or likely.
While Otago Peninsula's geological character and widespread
moderate to steep slopes meant surficial landslips were a
hazard, it had relatively few large landslips, the report
''Residents and developers alike should be mindful that in
many places on the peninsula, moderately to steeply sloping
ground can generate relatively fast-moving surficial
Those risks could be avoided by not making adverse
modifications or discharging water to slopes, it said.
''One practical way of improving the safety of existing
buildings or other assets in potential landslide run-out
zones is by planting a line of trees or a strong hedge to
catch or deflect debris.''
Most of the 13% of the Saddle Hill to Brighton area was
classified as definite or likely to have landslips and was on
the slopes of Saddle Hill, Jaffray Hill and Scroggs Hill.
Those slips were considered to have high sensitivity to
erosion and modification.
''The slopes developed on those rocks are clearly very
susceptible to landsliding and the easiest solution to that
is to minimise the placement of high-value infrastructure and
assets in those areas.''
The study found instability around Church Hill Rd and
District Rd in Green Island indicated the slopes were highly
sensitive to small changes in stability, such as groundwater
fluctuations due to rainfall or slope modification.
Small landslips in central Dunedin in Cargill, Albany and
Duke Sts over the years were judged as having high
sensitivity to changes in stability but not considered prone
to undergo future rapid movement.
''These small localised failures indicate that conditions
exist in places that are conducive to localised instability
on slopes in the central Dunedin area.''
About 41% of Warrington to Karitane had definite or likely
landslips and future movements were likely to continue at the
slow rates seen historically, similarly disrupting
infrastructure, it said.
''There is no reason to expect any large-scale rapid
reactivations, although this cannot be ruled out, especially
along parts of the coast where the sea is actively eroding
the toes of some landslides.''