Slips in hills above Kaikoura highway could keep coming down

Saturday, 8 April 2017

Cleaning up yet another slip on the highway south of Kaikoura.

The struggle to reopen the highway south of Kaikoura illustrates the challenge to get the more heavily damaged road north of the town ready to be used by Christmas.

The problem is not just the hundreds of thousands of cubic metres of rock brought down over the highway and railway line by the 7.8-magnitude Kaikoura Earthquake in November.

The slip at Ohau Pt, where an estimated 110,000 cubic metres of rock and debris came down across the highway.

It's also the vast quantities of other material loosened by the quake still sitting in the hills above, and at risk of being brought down by heavy rain or in a strong shake.

The NZ Transport Agency (NZTA) has had to repeatedly close State Highway 1 south of Kaikoura as new slips keep coming down along the route. On the night of March 26 it was closed again - for a planned eight days – after heavy rain made slip face stabilisation work critical.

A 150-metre high dam on the Hapuku River was caused by the earthquakes.

* New SCIRT-like alliance for Kaikoura rebuild

* Work begins clearing large slip covering SH1 at Ohau Point, 'very visible progress' promised**

The highway did reopen as scheduled at 7am on Tuesday – but only for three hours. Despite the removal of 5000 tonnes of rock from 17 slip sites during the previous eight days, heavy rain had once again destabilised some areas.

In some places along the Kaikoura coast, 'quite substantial volumes of material' were sitting on the slopes above the highway, GNS Science engineering geologist Sally Dellow said.

An example was about 1 kilometre to the south of Ohau Point, where 'some big landslides have hung up on the slope'.

As a result of an aftershock or a rainstorm, loose material still sitting on the slopes above the road could come down.

'So landslides blocking the road from the Kaikoura Earthquake hasn't stopped,' Dellow said.

It wasn't just debris moved by the earthquake that could cause future problems. There was also loose rock in some of the source areas where landslides had originated.

'It's going to be an ongoing problem.'

NZTA earthquake recovery manager Steve Mutton said a large team of geotechnical engineers was making sure all the risks and issues along the Kaikoura coast were identified: 'It's a massive job.'

As well as the 10 'massive' slips over the highway north of Kaikoura, other sites where there may be a risk had been identified, Mutton said. Some of those sites were being sluiced – helicopters with monsoon buckets drop water on slip faces to bring down loose material – and others were being monitored.

'It's a very fragile environment through there. As part of our reinstatement work, we're creating areas to ensure that the road users remain safe,' Mutton said.

In places the highway and railway would be moved away from the slips, so if any rocks did fall the road and rail would not be affected.

'It's a very constrained environment. We have a beautiful coastline on one side and sheer cliffs on the other. Over 100 designers are working on this project to make sure we deliver the best possible outcome,' Mutton said.

Access tracks had now been created around nine of the 10 landslides on the highway north of Kaikoura, improving access to Ohau Point – the 'most complex and most difficult' of the slips to clear.

While the landslides blocking the highway have attracted most of the attention, they are just a few of around 10,000 landslides brought down by the Kaikoura Earthquake in an area of about 10,000sq km between Cheviot and Seddon.

A large landslide on the Hapuku River in the Kaikoura Range probably contained 12 million cubic metres of material, and landslides had formed about 200 dams, Dellow said.

Landslide dams could fail rapidly. Eleven were identified as having the potential to cause problems. More detailed work was being done on those.

'If a landslide dam fails catastrophically or rapidly, it sends a flood wave full of sediment and debris rushing down the river valley.

'So if there are people in the river valley, if there are bridges or other infrastructure, if there's a school close to the riverbank, they could potentially be hit by a flood wave. It's probably not survivable,' Dellow said.

Most of the landslide dams were likely to disappear in a few years. The convention was that about 50 per cent of landslide dams disappeared in the first year after being formed.

Some lasted for a long time. For example Lake Waikaremoana formed behind a very large landslide that happened about 2000 years ago.