Little Pigeon Bay gets largest tsunami

It may only be ‘‘little’’ but it probably got the largest tsunami generated by the magnitude7.8 Kaikoura earthquake.

By a quirk of geography, Banks Peninsula’s Little Pigeon Bay was directly in the firing line of the tsunami as it ran southwest down the South Island’s east coast from offshore of Kaikoura.

The funnel-shape of the bay forced the agitated sea higher as it approached the beach, where it swamped and wrecked a historic weatherboard cottage by the shore.

Researchers say what happened in Little Pigeon Bay should be a reminder to the public that local effects can make even a small tsunami a damaging event.

Niwa tsunami scientist Dr Emily Lane said the long-period tsunami waves arrived close to high tide, around 4am on Monday last week, and peaked about 2.8 metres above high-tide level in the bay, based on the Sumner tide gauge.

The first wave pushed about 140m inland, mostly running up a dry stream bed. A second, smaller, wave reached about 100m inland.

In terms of the waves’ heights above mean sea-level, the first was 4.1m above and the second 3.5m above.

The force of the water and pounding from debris, including tree trunks, badly damaged the cottage, pushing its front wall in and lifting part of it off its foundations. Its deck was discovered deposited on the beach, she said.

Scientists from Niwa, GNS Science, the University of Auckland, the University of Canterbury and consulting firm eCoast last week visited several bays on the northern side of Banks Peninsula to see where the tsunami had the greatest effect.

‘‘We looked at Pigeon Bay. There was a little bit of inundation on the road, low-lying by the bridge, we could see a bit of evidence from seaweed, and there were surges up the river.

‘‘Talking to people from Port Levy, they reported very similar there,’’ Lane said.

It was unlikely anywhere along the Kaikoura coast experienced a tsunami on the scale of that in Little Pigeon Bay, Lane said.