GNS Science report updates liquefaction risk in Hawke's Bay

HAZARD MAP: A new GNS Science report has updated the liquefaction risk Hawke's Bay and found much of the region remains largely unchanged.

A new report about the risk of earthquake-induced liquefaction in Hawke's Bay has given the population a clearer idea of how their properties may be affected.

The GNS Science report, commissioned by Hawke's Bay Civil Defence Emergency Management Group and the region's five councils, found much of the region's liquefaction hazard levels were largely unchanged or had improved since the last report carried out in 1999.

GNS Science engineering geologist and report author Sally Dellow said scientists were able to significantly refine and improve existing liquefaction susceptibility maps for the Hawke's Bay region in the wake of the 2010-2011 Canterbury earthquakes.

The reassessment report has just been released and builds on and updates the 1999 GNS liquefaction study that was carried out in Hawke's Bay.

It found little change in Wairoa and in the Napier and Hastings major urban areas, while parts of Taradale saw a reduction in the mapped liquefaction hazard.

Havelock North remained consistent with low liquefaction hazard.

"It is particularly pleasing to see a significant reduction in the hazard assessed for Central Hawke's Bay," Ms Dellow said.

Lisa Pearse from the Hawke's Bay Civil Defence Emergency Management Group said liquefaction hazard maps for the region were set to continually be revised as more data was collected, knowledge was built and science was advanced.

"What is important for people to remember is that earthquake-induced liquefaction, as seen in Canterbury, has been recorded in Hawke's Bay in the past, and is likely to happen here should a large (usually greater than magnitude 6) earthquake occur," she said.

Liquefaction effects had been reported in the Hawke's Bay region during historical
earthquakes with ground damage including sand boils, water ejection, subsidence and settlement and fissuring and lateral spreading, the report read.

It found Napier City surrounding Bluff Hill still had the highest liquefaction hazard in the region, which would require investigation to determine appropriate mitigation.

The area where the liquefaction hazard had changed significantly since the earlier work was the Central Hawkes Bay District.

Previously the Poukawa and Otane basins and the Tukituki River valley downstream of the Waipawa River forks were mapped as having very high liquefaction susceptibility.

However careful investigation of the reported liquefaction during historical earthquakes and an updated, more detailed geological map of the area showed a large reduction in the area mapped with a liquefaction hazard requiring investigation and/or mitigation.

The areas now mapped with a high liquefaction susceptibility in the District were small, associated with swamps and prone to flooding.

Ms Pearse said liquefaction hazard could be effectively managed and learnings from Canterbury used to ensure future development is designed and managed with improved knowledge about liquefaction.

She said because of this everyone should understand the risks to their properties, which can be found by visiting new maps on the website's hazard portal.

New national planning and engineering draft guidance for potentially liquefaction- prone land had just been released, which would complement the updated Hawke's Bay hazard mapping, she said.

"Local councils have only just received this new science information and draft guidance and will need to update their processes, which will take some time."

Those wishing to discover the liquefaction risk for their property can visit www.hbhazards.co.nz.