17 Jul 2016

Dunedin land could be dipping faster

9:32 am on 17 July 2016

Emerging research from geology experts suggests South Dunedin's reclaimed land may be sinking faster than other areas.

Surrey Street, South Dunedin.

Dunedin City Council has admitted a faulty pumping station made the 2015 flooding of South Dunedin worse than it would have otherwise been. Photo: RNZ / Ian Telfer

Satellite data from three Otago University School of Surveying ground stations around Dunedin showed land may be sinking by about 1mm a year.

In addition, GNS Science's preliminary satellite radar data, covering the seven years from 2003 and 2010, suggested the sinking was not uniform, with reclaimed land around Otago harbour subsiding more quickly - by 2-5mm a year.

GNS Science head of regional geology Robert Smillie said the data was very preliminary.

"It's early days and we can't really draw any firm conclusions at this stage and really more work is required."

But he said the subsidence could be through compaction of near-surface sediment or from deeper tectonic processes.

"The data from South Dunedin is most likely [due] to settling from reclaimed land, so that's probably not too surprising."

Mr Smillie said it was not unusual for land to be sinking, and New Zealand was geologically a "very active country".

"Places like the Marlborough Sounds have been submerging for the last four million years. We're a country with land that goes up and land that goes down. The land in Christchurch has been going down for quite a few thousand years."

Mr Smillie said these factors were relatively well known and all part of ongoing research.

South Dunedin under water

Last month Dunedin City Council admitted a faulty pumping station made the damaging 2015 flooding of South Dunedin 20cm deeper than it would have otherwise been.

At the public meeting where the admission was made, the South Dunedin Action Group accused the council of having a secret plan to abandon the suburb and blame it on climate change.

GNS scientists have presented their data to Dunedin City Council and the Otago Regional Council, but Mr Smillie said this was as "another part of the technical side of the Dunedin story that we felt that the council needed to be aware of, rather than concerned about".

Stressing that the research was still in its early stages, Mr Smillie said the approach to the local bodies was just to make them aware of it.

"This is potentially important information that they'll need to take into account for planning purposes."

GNS Science said as a result of the potential subsidence, sea level rise may be faster than expected.

"You need to know what the sea level is doing relative to the land. So if the land is going down then obviously it will have an impact on the rate of sea level rise."

Mr Smillie said more data was being collected and further study was needed to know for sure if Dunedin was subsiding, and by how much. GNS Science has floated a project to drill at least two holes down to bedrock and has invited the city and regional councils to support it.

"If the subsidence is occurring we'd like to know if it's likely to carry on into the future and one of the best ways to do that is to look back into the geological past.

"So by drilling deep holes beneath South Dunedin, going into the subsurface, we hope we might be able to find geological markers... to give us past indications of subsidence rates."

Mr Smillie said discussions were continuing with the city council.

"But there's a whole range of factors and programmes that I guess the council needs to take into account, of which the drilling would just be one."

The cost of drilling would be in the tens of thousands, he said, but "certainly not looking in the millions".

Dunedin Mayor Dave Cull declined to comment on the GNS Science data, but said the council was working with Otago Regional Council on the area's natural hazards.

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