Land formation's dangers highlighted

People thinking about building or developing on any of Otago's alluvial fans need to be aware of the risks they are taking and use the information recently released by the Otago Regional Council, councillors say.

In recent months, the council has released two reports on alluvial fan hazards in Otago, completed on its behalf by Opus International Consultants and Geological and Nuclear Sciences Ltd (GNS).

Alluvial fans are sediment accumulations deposited at the base of valley slopes shaped like an open fan or segment of flattish cone.

They are formed by intense heavy rainfall over many decades.

Council natural hazards analyst Richard Woods said in a report to the engineering and hazards committee, the form and setting of the fans in Otago often made them attractive for residential development such as at Stoney Creek in Wanaka or Pipson Creek in Makarora.

"Identifying major fans and understanding the significance of the hazards . . . is a necessary part of managing the risks associated with human occupation of alluvial fans and avoiding inappropriate development."

The reports identified 2029 areas containing alluvial fans - 1329 classified as active and 700 inactive.

Consultation with territorial authorities identified 27 areas across the region which warranted further investigation because of their hazard exposure in developed areas or the demand for intensified land use, Mr Woods said.

Cr Louise Croot said the reports were valuable as they highlighted how the fans put communities at risk and how people needed to be careful where new developments took place.

The information needed to be more widely circulated rather than just distributed to local authorities and the New Zealand Transport Agency, she said.

Cr David Shepherd said it was important the information was used in a practical way to warn communities.

It needed to be included on land information memorandum reports so people knew the risks they were taking.

"It needs to be well and truly embedded in people's minds."

Cr Gerry Eckhoff said it was not necessary to alarm people whose risk was low, for example those living in Roxburgh who had lived on a fan for hundreds of years with only the odd event causing distress.

However, in some areas such as Pipson Creek, education was crucial so people understood the risks they were taking.

"I'd not like to see exclusion zones all over Otago but people should be able to look and make a balanced judgement."

Engineering and natural hazards director Gavin Palmer said the information would be included in the council's natural hazards database, which was in development.

Local authorities were also required to make the information available through reports like land information memorandum reports.

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