Tahunanui's shifting sands

Time and tide take toll on Nelson's coastline

Last updated 08:02 13/11/2012
Tahunanui beach

SANDS OF TIME: An aerial photo of Tahunanui Beach, showing erosion since 1948

Mike Johnston
ON THE ROCKS: Dr Mike Johnston, geologist measuring the trend of the rocks at Magazine Point.
SIGNING ON: Names carved into the sandstone at the base of the graded bed at Magazine Point near Rocks Rd.

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Tahunanui Beach could one day lie at a different angle and adjacent rock formations buried if the sandspits building up each side of the Blind Channel collapse.

Nelson geotechnical consultant Mike Johnston said a confluence of Tasman Bay tidal currents and the effects of water flowing out from the Waimea Inlet made the area very dynamic, which in geological terms led to rapid change.

The beach which currently lies in a west-east direction could one day be aligned more north-east and become more of a shoaling beach if the sandspits gave way. Dr Johnston said that could happen suddenly, given the right conditions such as a large storm combined with a large tide which would lead to the sea breaking through the sandbacks slowly building each side of the Blind Channel.

He said predicting when they might collapse was a bit like predicting earthquakes: "The longer you go without one, the closer to the event they become."

The predicted build up of sand in the north-eastern corner of Tahunanui Beach could partially bury ancient rock formations at Magazine Point below Rocks Rd.

"That sand could be released relatively quickly and is likely to cover the rocks," Dr Johnston said.

He said any considerable shoaling in the area would mean that Tahunanui Beach would "not quite be the same beach it is today".

The shifting sands threaten to bury rock formations which reveal the geological history of this part of New Zealand from 23 million years ago.

"Most rock around Nelson is much older but this is the only remnant of this tertiary era.

"It gives us an insight into what was going on then. It's an important link in understanding what New Zealand was doing as it was getting near the end of a tectonically stable period, and the transition to another mountain-building period."

Dr Johnston said the Magazine Point rock formation was once sediment lying beneath deep water and had been uplifted during a time of violent upheaval.

He said earthquakes and volcanoes made New Zealand a "very dynamic place".

"If we didn't have these things we would be under water. It's these situations which give New Zealand its unique landscape of fiords, sounds, mountains and volcanoes, and its biodiversity."

Dr Johnston said challenges lay in attempts to prevent or alter the course of natural change.

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- Nelson


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