15 July 2020

Cromwell: helicopter sluicing to manage a landslide

Posted by Dave Petley

Cromwell: helicopter sluicing to manage a landslide

An interesting landslide problem, and an unusual management approach, are under way at Cromwell on the South Island of New Zealand. The problem is a large rock slope failure that has developed at a critical road junction, between State Highway 8 and State Highway 8B, at Deadman’s Point Bridge. The image below, distributed by NZTA, shows the development of this slope failure and the obvious hazard to the road:-

Cromwell landslide

The landslide at Cromwell in New Zealand. Image provided by NZTA.


This appears to be a wedge failure with substantial displacement in the rear scarp.  The landslide is actively deforming. Clearly there is a high chance of a collapse event.  To manage the hazard the road is closed at night.

In the aftermath of the Kaikoura earthquake, the transportation authorities had some success in using helicopter sluicing to manage slopes.  This technique uses 1000 litre underslung buckets to deliver water onto the slope.  This video, posted to Youtube, shows this operation underway at Cromwell:-


The Otago Daily Times has a report on the helicopter sluicing operation at Cromwell:-

NZ Transport Agency (NZTA) said this morning that sluicing work, involving helicopters with monsoon buckets dropping water on the slip, would continue and traffic would be stopped for up to 30 minutes at a time.

“This sluicing work will continue for the next few days at least until we are sure the hillside is again stable.”

NZTA maintenance contract manager Mark Stewart said after dropping 150,000 litres of water to control and flush the landslip yesterday, helicopters and crew got to work again about 9.30am this morning.

I haven’t seen this approach used much outside of New Zealand – it is certainly an interesting way to manage a slope.

The Cromwell Gorge is one of the most famous landslide sites globally. Detailed monitoring of slope behaviour in the large creep landslides continues.

Thanks to John Davies and Scott Johnson for highlighting this one to me.  Much appreciated.


Quickslide 1: Cape Kidnappers risk assessment

Also in New Zealand, the Department of Conservation (DOC) has released the Quantitative Risk Assessments of the Cape Kidnappers site following the landslides there, which injured two people.   The documents are a very interesting exemplar of how to undertake this type of work.  Scoop has published some expert analyses of this work.


Quickslide 2: Mudslides and landslides in Indonesia

Heavy rainfall in North Luwu district of South Sulawesi province in Indonesia has killed 16 people and left 23 missing.  Whilst the reports blame flash floods and landslides, the images appear to show the aftermath of mudslides. Information is a little sketchy at this point.