City landslip risk mapped in GNS report

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 says.

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 said.

The six areas were identified due to past identified landslips and to existing development or development potential.

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 said.

''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 landslides.''

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.''

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