The Dart Landslide

Simon Cox   GNS Science

M. McSaveney GNS Science

Slip Stream is a tributary to the Dart River in the South Island of New Zealand. There has been an active landslide here for several thousand years, periodically sending down lobes of debris to gradually build up a large fan in the Dart Valley.







There was vegetation established right across the fan, but over the last few years the widespread cover of trees has been largely buried and killed off by a very active phase of erosion and deposition. Debris volumes of the order of 100 000 cubic metres have been coming down during heavy rains in the spring and summer periods.

Simon Cox  GNS Science

The debris gets mobilised into a wet mix of mud and boulders.  The latest large event occurred early in this month (4th January 2014), and the flows continued to build up over several days.

M. McSaveney GNS Science

The debris flows crossed right over the valley, blocking the Dart River with a low angled, shallow pile of soft sediment.

M. McSaveney GNS Science

A lake formed in the valley above the slip, becoming about 4 kilometres long. The river is cutting down into the debris, and it is expected that the depth of the lake will fluctuate during landslide activity. The Department of Conservation is diverting the affected part of the Dart Valley track so that trampers can continue to visit the area.

Photo DoC/Vladka Kennett

This image gives a good overview of the affected area.  It shows the fan with the darker coloured triangle of recent debris, as well as the length of the lake.

This is a graph from the Otago Regional Council website showing 7 days' rainfall recorded from the 9th to 16th January at the Hillocks, about 24 kilometres down the Dart Valley from Slip Stream.

The second graph shows how the river flow responded to the rain, with a sharp peak and a gradual tailing off after the rain stopped falling. The tail is not entirely smooth with a dip when the flow gets below 100 cubic metres per second. This suggests that when the river level drops, the continuing input of debris at the slip impedes the flow for a while, until the blockage is overcome and the flow rate increases again.

Mark Rattenbury (left), Simon Cox (right) and Mauri McSaveney (behind the camera) visited the area to assess the impact and any possible downstream hazard. Note that a special DOC permit is required to visit Slip Stream as it is in the sacred Te Koroka topuni area of Mount Aspiring National Park.  The slip is in a state of continual instability and the area is hazardous.

In this video Simon explains some of the interesting features of the slip, including some very strange bubbles that release dry dust when they burst:

The growth of Tasman Glacier Lake

The Tasman Glacier is the largest glacier in New Zealand. Its upper section is mostly white as you would expect of a river of ice. However, the lower half is covered with a layer of rock debris about a metre thick. This forms an insulating layer that slows down surface melting and allows the glacier to descend a long way down the valley to warmer elevations.

This photo, taken from the location of the the top of the moraine wall near the old Ball Shelter in 2007, shows what the debris covered surface of the lower Tasman Glacier looks like.
The moraine walls show how much the glacier surface has lowered in the last century. Before about 1912 the glacier surface was higher than the lateral moraine. New Zealand's other large valley glaciers have all been suffering a similar loss of ice.

Tasman Moraine Wall.      Julian Thomson GNS Science

This is what the moraine wall of the Tasman Glacier looks like close up. The unstable terrain is very hard to travel over, especially when you are carrying heavy gear like this group of glaciologists.



For more information about fascinating processes of glaciation check out this GNS Science web page.

On our flight up to the Grand Plateau for the height survey of Mount Cook recently, it was interesting to see the state of the terminal lake of the Tasman Glacier. This has been expanding rapidly in recent years.  Once the lake became well established, the water could undermine and erode the ice much more quickly.

This photo illustrates the process of melting of the ice. The surface water cuts away into the ice face to create a notch at water level. Once this notch is several metres deep, the overhanging ice collapses, leaving buoyant ice underwater that eventually breaks off in big pieces to float up to the surface as a new iceberg. The icebergs will continue to be eroded by the water in the same way. As they get lighter, they rise up in the water, lifting the ice notch up to give a mushroom like profile. The bergs often get top heavy by this process and can unexpectedly roll over.

This video that we made several years ago gives a dramatic illustration of this process seen from a boat at close quarters:

I have flown up the Tasman Glacier several times on various glacier field expeditions in recent years. This is a photo of the lower section in 2002, looking down valley. The glacier itself is about 2 kilometres wide and the lake is already extending up the east side of the glacier by about 5 kilometres.

Two years later (November 2004) you can see that the lake has continued to expand. The large ponds that can be seen near to the lake have grown and started to join together as more and more of the ice melts.

November 2007, after a large break out of ice bergs, the lake has greatly increased in size.

November 2013 from our recent flight up to the Grand Plateau on Mount Cook. It is inevitable that the lake will continue to expand. Due to the overdeepening effect of the glacier on its bed, the deepest point of the lake will be some distance up from the terminus, probably below the  area in the foreground of the image. After expanding past the deepest point, the lake will get shallower and shallower as it progresses up the valley, potentially to the point where the bed of the glacier meets the lake surface. It has a long way still to go.

This image (added as an update in early March 2015) shows that the basic shape of the lake hasn't changed substantially since the previous photo was taken over a year ago. However, if you look at the position of scree slopes on the right of the photo you can see that the glacier's retreat is continuing.

In this last photo you can see that as the lake erodes further up the glacier, the terminal ice cliff at the edge of the lake is getting higher due to the increasing surface elevation of the ice. There is a very good view of the lateral moraine wall in the background, that used to be below the level of the glacier surface back in the nineteenth century.  The glacier ice in this area has thinned vertically by roughly 200 metres since that time.

The Changing Height of Mount Cook

Mount Cook  rock avalanche 1991. Lloyd Homer, GNS Science

On 14th  December 1991 a massive rock avalanche occurred from the East Face of Aoraki /Mount Cook, sending an estimated 14 million cubic metres of rock in a 1.5 kilometre wide cascade across the grand plateau and down onto the Tasman Glacier. It is thought that the avalanche travelled at speeds of 400 to 600 km per hour, and the resulting seismic recording at Twizel, 75 km away, lasted well over a minute, registering the equivalent of a magnitude 3.9 earthquake.

Mt Cook Dec. 1991.  M. McSaveney, GNS Science

Prior to the avalanche the surveyed height of New Zealand's highest peak was 3764m. The Department of Survey and Land Information (now LINZ) calculated that this was reduced by 10 metres after the summit fell off with the rock fall.

As you can see from the photo, the peak became extremely narrow and unstable. In the image taken by GNS geomorphologist Mauri McSaveney a few days after the event. A lone climber that can be seen as a tiny dot inside the red circle  gives some idea of the scale. The "new" summit was obviously highly unstable, and would be subject to quite rapid erosion following the rockfall.

Since 1991, there has been no re-calculation of the revised elevation of 3754m until recently.

At the end of November 2013, I flew up to Plateau Hut with a climbing team who planned to take direct GPS measurements of the summit ridge of the mountain, a short distance from and a few metres below the highest point. The measurement would then be used to validate a computer model made from recent aerial photos to give a precise calculation of the present height of the peak itself.

The team was made up of (left to right): Geoff Wayatt, mountain guide; Nicolas Cullen from Otago University; Brian Weedon, mountain guide; Pascal Sirguey (project leader) from the National School of Surveying at the University of Otago; Jim Anderson from Survey Waitaki and myself. Geoff, Brian, Nicolas and Jim made up the climbing team.

GNS Science provided support in terms of the helicopter flights.  I was able to accompany the team to Plateau Hut where I spent two days gathering a visual record whilst they were involved in their climb.

Mount Cook East Face   Julian Thomson, GNS Science

The plateau of Mount Cook is arguably the most spectacular alpine setting in New Zealand. This image shows the 1500m high East Face of Mount Cook in the early morning light seen from Plateau Hut. The normal route up the mountain follows the Linda Glacier, starting on the right hand side of the image and following into the shadow behind the long low angled rock ridge (Bowie Ridge) up to the summit rocks.

As well as Mount Cook itself, the Grand Plateau has views of many other summits along the main divide, including Silberhorn, Tasman and Dixon. This image shows the top section of Syme Ridge on Mount Tasman. There are three climbers just visible on the ridge just above the centre of the photo, about 10 hours into their climb from the hut.

This image shows the patterns of crevasses on the grand plateau, just above the Hochstetter Icefall.

Plateau Hut at night.  Julian Thomson, GNS Science

The climbing party left Plateau Hut just after midnight with clear, cold weather conditions that were perfect for the climb.

Mt Cook Summit Rocks, Photo Geoff Wayatt

Aoraki / Mount Cook is a challenging peak to climb, with very dynamic glaciers and steep rock and ice faces to negotiate. In this photo, the climbers are in the ice gullies that run through the summit rocks.

Photo Nicolas Cullen

View from the summit, with Mount Tasman in the background

Photo Nicolas Cullen

Looking along the summit ridge of Aoraki / Mount Cook, with the two GPS units in place. The very highest point is about 45 metres distant. The GPS units measured a height of 3719 metres at their position. This measurement was consistent with the height from the computer model which then allowed the height of the high peak to be calculated as 3724 metres above mean sea level.

This means that Aoraki / Mount Cook is a full 30 metres lower than the 1991 estimate of its height, showing that the mountain peak has continued to erode significantly during the last 22 years.


There is more information about the project at the Otago University School of Surveying website.

Here is our video of the expedition :