Many angles to decode frozen debris lobes


“Something chewed on the casing,” Margaret Darrow explained. “Probably a bear.” Blue chips were scattered from the cracked ABS pipe. Inside the casings that protect the holes drilled in and around frozen debris lobe -A there’s non-toxic propylene glycol. Propylene glycol, this brand a clear greenish liquid, prevents freezing – helpful for scientific instruments – but it also tastes slightly sweet. And bears are curious creatures.

Darrow with inclinometer, bear-chewed casing and instruments / Image Laura Nielsen
Darrow with inclinometer, bear-chewed casing and instruments / Image Laura Nielsen

Later, taking measurements, Darrow, geological engineer and associate professor at University of Alaska Fairbanks, exclaimed with jovial exasperation: “I touched the glycol! And now I’m gonna lick my fingers, I know it.”

The Dalton Highway corridor

“The Dalton Highway is the only road that goes north through the Brooks Range up to the oil fields, to Prudhoe Bay, to the North Slope. It is the road that allows the transportation of goods and services north and there are communities along the path that rely on it.” ~ Margaret Darrow

Who’s on the Alaska’s Dalton Highway? Oil field workers, big oil trucks and Ice Road Truckers – the industry is very reliant on the transport of heavy equipment. Subsistence hunters. Tourists. Scientists, heading to field sites at e.g. Toolik Field Station, an important long-term ecological research facility.

The Trans Alaska Pipeline System (TAPS) runs through the Dalton Highway corridor near the road. The pipeline transports crude oil. There’s a proposal to create a natural gas pipeline in the same corridor.

“It’s an important economic driver for the state. … Access to north of the Brooks Range is important, and the Dalton Highway is the only way to get there right now so therefore any threat to that highway is also important by association.” ~ Margaret Darrow

Slow landslides

23 frozen debris lobes have been identified which sit upslope and less than a mile from Alaska’s Dalton Highway.

Ronald Daanen, geohydrologist for the State of Alaska in the Alaska Geological Survey, spotted the frozen debris lobes in 2007 because the trees atop them weren’t all growing vertically. At that time the features had no official name of their own. They’d been noted in the 1970’s and 1980’s but were thought to be inactive. Daanen was headed to the North Slope along the Dalton Highway in 2007 when his interest was piqued.

“I was looking for drunken trees. Drunken trees are trees that kind of tip over when the ground thaws, right? You have ice, and the ground thaws, and it doesn’t settle evenly so you get drunken trees.” ~ Ronald Daanen

The trees partially tip, but some survive and continue to grow, slowly correcting their tops to grow vertically. They might tip over again and correct themselves again, leading to a sort of squiggle in the trunk shape. Daanen, at the time a permafrost researcher, spotted drunken trees on tongue-shaped features located on mountain slopes and took an interest in them.

“I took some photos and started asking people about these features but nobody really knew anything about it, until 2008 when I met Tom Hamilton who is a researcher geologist for the USGS. Well, he’s retired now but at the time he was still working there and we chatted about it. He remembered these features from his early days with the USGS when he mapped the corridor for the road and for the pipeline. He found these features very interesting and he told me a lot about it and encouraged me to do some more studying and so I submitted a proposal to Alaska EPSCoR.” ~ Ronald Daanen

EPSCoR is the National Science Foundation’s Experimental Program to Stimulate Competitive Research. EPSCoR funding allowed Daanen to do initial research on the frozen debris masses accumulating in the Dalton Highway corridor.

Comparing historic photographs with current data shows that some of these frozen masses are beginning to move faster.

Margaret Darrow theorizes about the origins of the frozen debris lobes:

“This area was glaciated during the Pleistocene and glaciers disappeared about 10 to 14 thousand years ago. … Perhaps there was debris left over from the glaciers. More debris is coming down the sides of the catchment through rockslide, landfall, solifluction – which is a slow process – and accumulating. And over time that debris reached a critical mass and then flowed over the edge of the catchment and is making its way downhill.” ~ Margaret Darrow

DGPS and drilling

We have little information about these mysteries save what scientists have recently uncovered.

Part of their work involves gathering elevation data. ‘Vivian’, the Differential GPS unit, gathers incredibly precise coordinates and elevation measurements. Of course, that only works when the scientists are actively on the lobe slope taking readings. Once they’re done in the field, they have to post-process the readings using software in the lab office.

Another part of the scientific work is accomplished by drilling into the lobe’s depths.

“I’m a geological engineer and I really like to see what’s underneath the surface. And to do that you need to drill with a big rig. … Boy, what comes out of there! I really like standing behind a drill rig because you just don’t know what’s going to come up; it’s exciting to watch what shoots out of the hole.” ~ Margaret Darrow

FDL-A onslope drilling site / Image Laura Nielsen
FDL-A onslope drilling site / Image Laura Nielsen

They learned there was water pressure below. They studied the composition of the soil inside the lobe. A piece of wood which emerged from the hole during drilling was radiocarbon dated; it was 1330 years old. A vibrating wire piezometer helped gauge the water pressure. And an inclinometer was used to measure the horizontal displacement until the casing inserted into the drill hole sheared – it broke as the lobe pushed its way downhill. Measurements showed the bulk of frozen debris lobe -A’s motion came from a shear zone 66 to 74 feet below the surface.

Darrow stresses that to understand the lobe, we need more geotechnical exploration. It’s important to get as much data as possible from inside the lobe as well as from atop it.

“It helps to piece together a puzzle of what you’re seeing below the ground surface in addition to the soil samples that come up. I like to look at those too and describe what they are. Drilling’s great.” ~ Margaret Darrow

LiDAR and repeat photography

In trying to understand features like this, it’s useful to use multiple approaches. Different components combine to form a more complete picture.

“I’m looking at LiDAR data and any other remotely sensed imagery I can acquire and trying to incorporate that. to quantify slope and other characteristics for each of the frozen debris lobes and compare them, because if we know rates of movement and we know slopes and we know how much area is contributing to the frozen debris lobes and what kinds of bedrock are present… We’re trying to put all of this together and so the more remotely sensed imagery we can get our hands on to gather information from and then combine it with field data, the better our understanding is going to be. And then of course we can present this information to the public and hopefully it helps those people making planning decisions. I mean, that’s the ultimate goal: we want to provide information that’s useful to the folks in the state of Alaska.” ~ Trent Hubbard

Trent Hubbard, geomorphologist in the Engineering Geology section with the Alaska Division of Geology & Geophysical Surveys, incorporates LiDAR data to help understand FDLs. LiDAR is Light Detection and Ranging Data. An airplane or other aircraft can carry LiDAR equipment. The equipment sends out laser beams that bounce off Earth’s surfaces: the ground surface, and even treetops and vegetation, then records the returning data. Combined with aircraft and on the ground location data it can be used to create a variety of elevation maps.

“You catch the return signal. And through mathematical computations you can calculate the elevation of the ground surface or whatever object that that laser hit. … What happens is you have literally thousands and thousands and thousands of points, and they’ll all be at different locations and elevations. And it kind of gives you a 3-Dimensional view of the Earth’s surface.” ~ Trent Hubbard

Photo compilations are another part of the puzzle. With camera units mounted on helicopters, they’re taking lines of overlapping photos that can be mosaiced to create an overall model by combining the image with elevation data. Images can help the scientists document changes in the frozen debris lobes over time.

“The idea is that we’ll be able to tie these into some coordinate system and we’ll be able to create a digital elevation model and that will be useful in determining changes in the frozen debris lobes’ surface elevation over time. That’s the ultimate goal.” ~ Trent Hubbard

The digital elevation model is essentially an image created from a series of points, each of which has its own, elevation and location data. Since the frozen debris lobes are moving, elevations and consequently the images change over time.

“How do we look at them and determine which frozen debris lobe poses more of a hazard to infrastructure, and what are the parameters that we use for that determination?” ~ Trent Hubbard

“How much of an area is contributing material to the frozen debris lobe? What is the aspect, what is the slope angle? And so we went around the frozen debris lobe and we collected bedrock samples and we actually ran some strength tests on that. We’re trying to determine: is there a spatial relationship between the bedrock and the frozen debris lobes? What’s the relationship? Are they all located in a certain kind of bedrock, or are they related to faults?” ~ Trent Hubbard

It’s a remote area, and not that much information is available. They need truly detailed information in order to better understand frozen debris lobes as geohazards.


“You can do different mitigation techniques within these slope stability models to see what would stop the landslide from moving. But in the case of these frozen debris lobes 1) we don’t have enough data to really constrain a model and 2) there’s the temperature dependency that just doesn’t exist in traditional slope stability models because they’re traditionally used in places where it’s not frozen. We need to evaluate what would be the best approach for the modeling.” ~ Margaret Darrow

There’s a wide range of information required to craft a model (a simulation of the FDL run on a supercomputer). Topography and elevation. Bedrock location and strength. Shear zone characteristics. Lobe shape, location, velocity, composition and soil strength. Temperature. Water pressure levels at different depths and locations. With a robust model, the scientists could simulate how the lobe would respond to different mitigation techniques.

To slow or stop the lobes, Alaska could attempt a number of things. Try to build a buttress in front of the lobes or drill through them and use soil nails to stabilize them. Try to drain the water from the lobes. Try to lower the lobes’ temperature. The solutions are costly and might not work; there’s no model capable of simulating precisely what will happen, and at current the scientists lack funding to continue their research.

“We need more data to put into the model. Otherwise we’re just guessing. Educated guesses, but still guessing.” ~ Margaret Darrow

Laura Nielsen

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

Frozen Debris Lobes project

  • Interviews with the scientists during their field work season, 2014