Liquefaction threat is ‘last straw’ for library’s river site, says climate group

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The Maitai River overflowed last week, flooding homes in Nelson's east.

Johny O’Donnell

The Maitai River overflowed last week, flooding homes in Nelson’s east. “Multiple risks” now add up for a plan to build a $46 million library along the river, according to the climate group.

Liquefaction risks revealed at the site of the planned $46 million library on the Maitai River in Nelson – including the building sliding – meant it no longer made sense to build there, according to a climate group.

Zero Carbon Nelson Tasman said the findings of a new geotech report added to the “multiple risks” of developing the site, and that Nelson City Council should now “close the door” on the project and consider alternative locations.

The group’s spokesperson geologist, Dr Aaron Stallard, said the report revealed that liquefaction – when the ground behaves like a fluid during an earthquake – was likely to be triggered at the site during moderate to strong shaking.

This shaking would occur at Nelson with any rupture of the Alpine Fault, which was due, he said.

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MARTIN DE RUYTER/STUFF

Avon Terrace, Nelson, houses are flooded by the Maitai River.

The liquefaction could cause the building, proposed at the northern end of the CBD, to have uneven floors, cracked structural elements, sheared pipe fittings and shift 80cm towards the river in the worst case, according to Tonkin and Taylor’s draft report.

Nelson City Council provided Stallard with the report as part of an Official Information Act request.

“The Alpine Fault ruptures roughly every 300 years, with the last rupture being 305 years ago,” he said.

“We are going to have a really big earthquake…that’s definitely something we have to keep in mind when planning new infrastructure. »

Construction options to protect the building from such risks were “expensive with higher carbon emissions than building on a more secure site”, he said.

And the report flagged potential problems with the construction of the building’s foundation due to high groundwater levels.

These tidal influenced water levels “can present a challenge for deep excavations for footings,” he said.

It has been estimated that liquefaction begins to initiate at the predicted site during earthquakes with a mean recurrence interval (ARI) of about 50–100 years.

Nelson City Council's preferred site for the town's redeveloped library is next to the present Elma Turner Library (right) on the banks of the Lower Maitai River.

Nelson City Council’s preferred site for the town’s redeveloped library is next to the present Elma Turner Library (right) on the banks of the Lower Maitai River.

A “permanent lateral displacement” towards the river of 8 cm was expected in an earthquake with an ARI of 100 years, and of 30 cm in an ARI of 500 years.

Much more liquefaction was predicted on the river side of the site.

Combined with the threat of sea level rise to the site – which has recently proven to be faster than previously thought – the construction of a library in the proposed area, adjacent to the present Elma Turner Library, resulted in unacceptable financial risk to taxpayers, Stallard said.

The site had not been flooded when the Maitai River overflowed last week – coastal flooding was the biggest risk, he said.

But there was a “real danger” of heavy rain occurring at the same time as a storm surge.

The swollen Maitai River on Thursday a day after prolonged heavy rain hit the city, seeing the river burst its banks, flooding homes in east Nelson.  The library site was not flooded.

ALDEN WILLIAMS

The swollen Maitai River on Thursday a day after prolonged heavy rain hit the city, seeing the river burst its banks, flooding homes in east Nelson. The library site was not flooded.

Frequently flooded buildings were at risk of being uninsured, with maintenance and repair costs piling up – something that could be seen at the site decades from now, he said.

“This [library plan] is not undertaken in the context of regional adaptation planning.

“The proposal to build expensive infrastructure on land prone to flooding and liquefaction makes no sense in times of climate emergency and when large insurers such as IAG are calling for construction on floodplains to stop.

Andrew White, community service manager for Nelson City Council, said the council’s preferred option of a new library on the corner of Halifax and Trafalgar Street was subject to an agreement with Wakatū Incorporation on a land swap, which n had not yet been concluded.

“Before making a decision, Council will need to be sure that the site is suitable for development – ​​a process requiring a thorough due diligence investigation.

“The draft geotechnical report forms part of this process and, although it has been given to an applicant under the Local Government Official Meetings and Information Act, as required, it has not yet been finalized and reviewed by the elected members of the Board.”

The council will meet next month to assess both that report and updated flood modeling information, White said.

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