Matata: The perfect disaster

Wayne Maloney calculates that this stream went from 10cm to 10m deep in 30 seconds. Picture / Alan Gibson

A Matata man has labelled the chaos caused in his town a month ago as "The Perfect Disaster".

Engineer Wayne Maloney has spent a quarter of a century as custodian of a stream behind the coastal settlement that, during heavy rain on May 18, became a raging torrent of mud, trees and boulders.

"You know the movie The Perfect Storm? Well, this was The Perfect Disaster," he says.

Mr Maloney, whose home was destroyed by the debris that swept down the usually placid stream, says a variety of natural forces combined to devastating effect that day, destroying 27 homes and threatening the entire future of the town.

And he believes a swarm of earthquakes plaguing Matata since early this year set off the tragedy.

This week, Mr Maloney took the Herald up the stream, deep into a gorge bearing the scars of boulders as big as 6m in diameter being tossed between its cliffs.

"Something very, very dramatic happened here," he says.

The rock cliff faces are covered in deep scratches. Tree trunks up to 20m long are wedged between the cliffs.

The trunks are stripped of bark and shredded, like they have been through a grater, showing the force inflicted on them.

Native vegetation is stripped 15m above the stream in places.

"Hard to imagine being out here. It would've been like Huka Falls," says Mr Maloney.

The 59-year-old says the whole geography of the stream has changed.

The stream bed is now 3-4m higher because of massive silt deposits carried in the storm. The water's course has shifted 10m in parts. About 500m up the stream, where the gorge narrows to just a metre of two, a boulder sits lodged between the cliffs, 2m high.

Water cascades over the top. Neither boulder nor waterfall were there before, Mr Maloney says.

He says the boulders come from the hills, which rise quickly to 300m behind the town. Only earthquakes could unsettle such huge pieces of rock, says Mr Maloney. His theory becomes more plausible when a short-lived but violent tremor rattles the ground.

Such quakes are an almost daily occurrence, say Mr Maloney and other Matata residents.

He grew bananas, macadamias, avocados and taro on his land before the disaster.

He calculates the stream went from 10cm to 10m deep in 30 seconds.

He also tried to gauge the speed of the flow. "I counted one, two, three. It moved 80 metres."

He saw trees and boulders fired out of the ferocious mass "like cannonballs".

Mr Maloney hopes experts compiling a report into whether destroyed areas of the town can be rebuilt do not label the disaster a flood.

Matata has experienced floods before - this was something altogether different, he says.

Brad Scott, a volcanologist at the Institute of Geological and Nuclear Sciences, says the idea of a "perfect disaster" at Matata is plausible, "but the primary issue was high-intensity rainfall".

Rain, rather than earthquakes, probably triggered the landslides that dislodged the boulders from the hills.

Earthquakes need to be about 4.5 on the Richter scale before they cause landslides, but the largest quake before the disaster was 3.8, says Mr Scott.

The movement of the boulders down the stream could be explained by a phenomenon known as "debris flow" containing high levels of mud and gravel.

"If the density of the debris flow is higher than the boulder, it will then pick it up and carry it."

The size of the boulder is irrelevant, he says.

A deluge of the sort that hit Matata - 94.5mm fell in one hour at nearby Awaponga and 347mm in two days at Tauranga - provided the mechanism for debris flows, which strengthen in narrow gorges.

Debris flows are capable of moving houses, as happened in three cases at Matata.

Mr Scott says the disaster was unusual but not unheard of in New Zealand.

Experts plan to submit their report on the cause of the disaster to Whakatane District Council on July 12.