Hillside collapses to form New Zealand's newest lake

Saturday, 21 July 2018

Explosives were used on the earth and boulder dam formed by the slip that created Mangapōike Lake.

'Um, I think you might want to have a look at this new slip,' the top-dressing pilot told Gisborne farmer Dan Jex-Blake on February 25.

'Yeah, I know about that one. Been there forever,' Jex-Blake said.

'Nah, I don't think so. You need to see this,' the top-dressing pilot replied.

Lake Mangapoike covers about 31 hectares. It was threatening to get much bigger before a channel was cut in the earth dam by explosives.

So the fourth-generation owner of Mangapoike farm, about 55km southwest of Gisborne, jumped on the plane.

**READ MORE:

Mangapoike Farm owner Dan Jex-Blake with the newly formed Lake Mangapoike in the distance.

* Station owner hopes for 'permanent lake' after landslide stabilised

Gisborne District Council principal scientist Murry Cave and Mangapōike Farm owner Dan Jex-Blake and the new lake a few months after it formed.

* Huge slip creates large lake and 'significant hazard'**

He couldn't quite believe what he was seeing.

Where there was once a grass-covered bluff was now a vertical wall, a massive scar of debris and mud, and where there was once the clear-flowing Mangapoike River was a fast growing lake.

The slip, believed to have occurred overnight on February 24/25, saw nearly 3,000 cubic metres of rocks and earth move down the hill, and is what scientists have called a significant national event - as big as the biggest to come down in the Kaikoura quake, and has created the country's newest lake.

Gisborne District Council principal scientist Dr Murry Cave at the face of slip at Mangapoike.

Last week Jex-Blake took Stuff and Gisborne District Council scientist Murry Cave to the site.

Standing on crumpled hillside and peering into the steep precipice was enough to leave anyone in awe.

About 11.5million tonnes of rock and earth came down in the landslide.

'We knew it was significant in a national sense the moment we saw it,' Cave said.

He said about 11.5 million tonnes of material was displaced in the event, the cause of which is still unknown.

Mangapoike landslide in Gisborne formed the nation's newest lake.

'We had a few small earthquakes before it, but not so big that you'd expect this type of failure, and there was no rain before it. We suspect that the Te Araroa earthquake [in September, 2016] caused it to weaken and over time it just sheared off,' he said.

The rocks and earth created a large dam on the Mangapoike River.

New Zealand dabchicks have made the new lake their home.

'We thought we'd have a month to sort something out. Then we got heavy rain. The lake filled rapidly and it was clear we had to do something,' he said.

A dam failure would have seen a destructive wave of water wash some 15km down the Mangapoike Valley and into the Wairoa River, past the township of Wairoa and out to sea.

The lake was backing up the valley, threatening several buildings on Jex-Blake's farm as well as bridges and the road.

'Had it continued to expand it would have been terminal for us as a farming business, basically,' Jex-Blake said.

In late March, when the lake had reached 33hectares and still growing, Cave set off several hundred kilograms of explosives to blow a trench in the dam.

That, and a second blast the next month, did the trick, and the Mangapoike River began flowing again, creating a second, smaller lake in the process.

Over time the lake has settled at a size of around 31 hectares and is about 50metres deep at its deepest point.

The lake won't serve any practical use for Jex-Blake. The farm gets about 2.1m of rain a year, so doesn't really need more water. But it could make a great wildlife refuge. It's become home to numerous birds, including native dabchicks, and it makes for quite a nice kayak trip, he said.

'It's pretty cool. It's pretty amazing that we're here in this space and time for a once in a millenium type event,' Jex-Blake said.

He said his wife Tam recalled a loud noise the evening that the slip was believed to have occurred, but obviously had no idea what it was.

Cave

said similar dams had formed after the Kaikōura quake, but they had not lasted long-term due to the material that formed them being less resilient.

'The closest equivalent we've got to this is Lake Waikaremoana, which is obviously a lot bigger and a lot older but was formed by the same type of mechanism. That one lasted thousands of years. We hope this one will too,' Cave said.