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Eroding . . . Wave action continues to wear away Motunau’s cliffs. Photo: Supplied

By ROBYN BRISTOW

Coastal erosion is behind cliff instability in the Motunau Beach area, not seismic activity.

The Hurunui District Council’s regulatory services manager, Judith Batchelor, says that while seismic activity increases the likelihood of cliff collapse, coastal erosion from wave action is considered the main cause of long-term cliff instability at Motunau.

Her comments come following the release of a report by Kirk Roberts Consulting on the extent of cliff instability in the area.

The report, commissioned by the council, says that while the November 2016 earthquake sequence had some short-term impact on cliff stability, coastal erosion was the main driver.

The long-term predicted conservative rate of erosion is 0.8m per year, consistent with predictions before the earthquake.

Ms Batchelor says there are still “clearly risks” posed to coastal properties due to cliff instability, but the good news is the earthquake sequence had not affected long-term erosion rates.

“Coastal erosion is hard to predict as the coastline does not erode at the same pace in all places. It is likely to erode more during stormy years and less in other years.

“This report supports the existing coastal hazard lines, which is reassuring,” she says.

The report recommends ongoing monitoring of the cliff face and beach area.

Environment Canterbury is looking to use drone technology to improve its existing beach and clifftop monitoring at Motunau.