On the back of the beast

FDL lobe flank
A view of frozen debris lobe -A’s flank: huge sediment slope, toppled trees / Image Laura Nielsen

We’ve joined scientists atop a frozen debris lobe, a slow-moving landslide in permafrost. They say we’re ‘on the back of the beast’. In the heavy rain and among fog-shrouded mountains, the scientists are making these uphill treks to record how temperature, water pressure, and local geological properties determine the slope movement of the massive lobes. These repeat measurements obtained at incredible accuracy can one day help us decode the secrets of the many massive frozen debris lobes (FDLs) currently approaching Alaska’s Dalton Highway.

FDL scientists
Trent Hubbard, Ronald Daanen, and Margaret Darrow (holding a differential GPS unit) / Image Laura Nielsen

Meet the three-person science team: Margaret M. Darrow, Ronald P. Daanen and Trent D. Hubbard. Darrow is an associate professor in the Mining and Geological Engineering Department at the University of Alaska Fairbanks. Daanen and Hubbard are geologists in the Engineering Geology section with the Alaska Department of Natural Resources’ Division of Geological & Geophysical Surveys.

They’re amicable and patient, despite that I know we’re slowing them down… all ‘What makes that happen?’ and ‘Do that again for the camera?’ Our hiking speeds can’t match the scientists’, either. I saw project lead Darrow put her boot through a vegetative mat over a crevice and sink to the thigh; she extricated herself like it was nothing. Afterwards, I probably stepped even more timidly.

No matter how much sopping wet underbrush and alder thickets the scientists must push through or how often they have to shout ‘Hey, animals! Ho, bears!’, these folks stay hard at work, inured by experience and dedication. “At some point you just don’t feel it anymore: there are 100 mosquitoes on you and they just eat you and you don’t notice. I think that being outdoors — it’s also nice,” Daanen says. “You have this offset. You turn around, and you stand still for a moment; you have these beautiful views.” Daanen and Darrow both seem enamored with the Alaskan field experience, no matter the trials:

”That’s what’s pretty fun about it: it’s fatiguing. It’s fun to be able to go to places where there are very few people and to see things that are striking, quite beautiful. And there aren’t a lot of distractions to take away from that. It’s a physical challenge, the elements are a challenge.” “We saw a bear today so… we see things that most people don’t have the opportunity ever to see.” ~Margaret Darrow

FDLs are lobe-shaped masses of debris, ancient erosion high on mountain slopes. They slide down permafrost-layered valleys, accumulating more debris into the lobes’ frozen bulk. While summer warmth lets big mud wallows called retrogressive thaw slumps send rapidly degrading groundcover, silt and liquefied permafrost ice sliding and glooping downhill, the lobes actually move year-round. Even during the winter FDLs slide as a rigid mass along a deep-seated ‘shear zone’.

“Now it is getting warmer in the Brooks Range and so this mass, this frozen mass is slowly starting to move faster. We know that because we compared historic photographic evidence rates with current rates that we measure with modern technology and it has been accelerating.” ~Ronald P. Daanen

Hiking today atop the lobe designated FDL-A, we hear the pervasive sound of rushing water; the rapidly degrading ice-rich silt of the lobe looses water, and that water is joined by water from rainfall. The unsteady ground of the shifting lobe creates ‘drunken’, ‘squirrely’ trees which topple to an angle in unquiet soil then struggle to correct themselves to vertical, then are toppled once more.

'Drunken trees' grow 'squirrely' on the unsteady ground of the FDLs / Image Laura Nielsen
‘Drunken trees’ grow ‘squirrely’ on the unsteady ground of the FDLs / Image Laura Nielsen

Everything is changing.

When they measured FDL-A during the fall of 2012, the scientists found it had accelerated its motion from 0.4 inches to 1 inch per day, yet the slope it grinds along is very consistent. They question the pulse of movement.

“This is really a dynamic system and it’s changing very rapidly. I think as a geologist that’s what really strikes me about them,” Hubbard says. When I asked him to describe his first experience with the FDLs he confessed “It was phenomenal, looking up and seeing it for the first time in person.”

“A lot of my job has been dealing with geologic hazards in infrastructure corridors, specifically: proposed natural gas pipeline corridors and potential hazards in areas of existing infrastructure. So that’s really why I became involved. And I was interested assessing the potential geologic hazard posed by these FDLs, you know, how do we look at them and determine which FDL poses more of a hazard to infrastructure, and what are the parameters that we use for that determination?” ~Trent Hubbard

FDL thaw slump
Margaret Darrow climbs out of a retrogressive thaw slump on FDL-A / Image Laura Nielsen

The two points I hear frequently which sum up FDLs are 1) ‘They are geohazards’ and 2) ‘We need more data’. Darrow says: “We need more data to put into the model, otherwise we’re just guessing. Educated guesses, but still guessing.” Traditional landslide slope stability models don’t work well because they have no information suitable to incorporate the effects of the frozen ground in and around FDLs. Without more data gathered right here in Alaska, we don’t have enough information to build a functional model or simulation of how the lobes might respond to different mitigation strategies. Yet figuring out a plan is important; FDL-A alone moves an estimated 22,000 tons of debris per year toward the highway and pipeline. And there are 22 more known lobes looming upslope and less than one mile from the road. The Dalton Highway corridor is state infrastructure — it’s Alaska’s road north, used by tourists, scientists and industry alike. The Trans-Alaska Pipeline, which runs near the highway, transports roughly 68 million dollars worth of oil every day.

FDL pipeline truck
The Dalton Highway and Trans-Alaskan Pipeline are important state infrastructure / Image Laura Nielsen

The Alaska Department of Transportation and Public Facilities currently plans to realign the highway near FDL-A (milepost 219), probably moving it the 400 feet closer to the pipeline which space allows.

”FDL-A is the closest lobe to the road. It’s our main study object, and it was 142 feet from the road as of June [2014]. We did a measurement today [August 21st, 2014] and it is closer but we don’t know exactly yet how much.” ~Ronald Daanen

FDL Lobe -D
Looking up at frozen debris lobe -D from the Dalton Highway / Image Laura Nielsen

Meanwhile, FDL-D (a lobe less massive than FDL-A) sits about 1500 feet from the highway but has been moving 150 feet per year.

The question now is whether FDL-A is destined to start moving like FDL-D does. Trends suggest it might. The scientists have a working theory: large cracks and crevices high on the lobe likely allow in extra water, forcing additional strain and decreasing cohesion, making the lobe more likely to pick up speed.

“So you get a crack from movement,” Daanen describes. “The snowmelt water – in early spring when the snow starts melting – runs in these cracks and freezes right there.” Before FDL-D accelerated and began moving 150 feet per year, the scientists spotted those cracks on -D. Now they’re seeing the ominous cracks on FDL-A, gaping catchments which collect water.

“Last year also when we were looking at the catchment of FDL-A we noticed similar cracks up high where we saw them on -D” “— Wide, full of water, they go all the way across the catchment. And here’s the kicker: FDL-A as of June of 2014 was 142 feet off the highway so, if it picks up and moves like -D did, it’s on the highway in a year. That’s an IF, that’s a big little word. But the data shows what -D has been doing is a possibility.” ~Margaret Darrow

So how do we deal with these geohazards? The answer so far is: get more data. When I asked Darrow what mitigation strategy she favors she answered: “What I would favor right now is more investigation. We need more geotechnical exploration to really be able to answer that appropriately.”

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