The extreme flooding that forced a weeks-long closure of Alaska’s only highway to the North Slope in the spring of 2015 also had longer-term effects on permafrost thaw, according to newly published research.
Radar measurements made through satellite imagery revealed that four years after the Dalton Highway flooded at its north end, some areas of the permafrost terrain in the area had sunk up to 3 inches.
Most of the changes happened during the first two years after the flooding, with an overall general stabilization after that, said lead author Simon Zwieback, an assistant professor at the University of Alaska Fairbanks’ Geophysical Institute.
The most dramatic thaw and ground-sinking effects were in ice-rich areas of the permafrost, where the freeze exists in ice wedges that are spread in a network a few feet beneath the ground’s surface, Zwieback’s study found.
Effects on the permafrost could be found 10 miles away from the road, he said. Most of the major impacts were within the Sagavanirktok River Delta, he said.
Along with the sinking — also known as subsidence — the flood resulted in some new plant growth, with what’s known as greening also tracked through satellite imagery.
But there was a lot of variability in the subsidence and emergence of new plants.
“It was pretty patchy and localized. There were several places that were flooded where there was a lot of subsidence and where there were also other changes that we tracked from space, whereas in other places that were flooded we didn’t see any strong response after the flood,” Zwieback said.
Thaw is causing the sinking across the North Slope in general, as documented in research by Zwieback and some of his UAF colleagues, but the thaw-induced sinking triggered by the 2015 floodwaters was more extreme.
However, the flooding impacts on the terrain were not all bad for permafrost, Zwieback’s research found. Some spots gained sediment that nourished vegetation, which can serve as a buffer against heat in the air, thus helping preserve ground freeze, the study found.
A proximate cause of the 2015 flood was an unusual buildup of aufeis, the term for ice that is formed when groundwater and river water swells to the surface and freezes. The large aufeis accumulation forced spring melt to escape the channels of the Sagavanirktok River, Zwieback said.
“Once you get the snowmelt, then the channel is blocked by all this ice and it has to make it to somewhere else, and that’s what contributed to the magnitude of the flooding,” he said.
There is some debate among his coauthors, who include some of the world’s top permafrost scientists, as to whether human activity exacerbated the ice buildup, Zwieback said. One theory is that seismic exploration in the area, by compacting the snow and thus removing some of its insulation properties, diverted some groundwater to the surface, he said. But others argue that the aufeis formation is an annual event in that area, and human activity there had no particular effect, he said.
The newly published study does not explore that debate, and it does not much discuss the impact of the flood on infrastructure – though the presence of the Dalton Highway itself likely contributed to the pooling of water on the western side of the road, he said.
For Zwieback, the 2015 flood was a subject of fascination even before he moved to Alaska. “There was a big flood that caused a lot of damage and attracted a lot of attention and yet we had almost no idea of what went on looking a little bit beyond the infrastructure,” he said.
For North Slope oil operators, the flood was extremely disruptive. It forced a closure that extended to 18 days, interrupting critical delivery supplies. It made necessary some immediate repairs, followed by a two-year rebuilding project that raised sections of the road substantially.
But it was not the only flood closure on that far-north section of highway. Last June there was another, albeit shorter, closure forced by spring flooding.
Although the 2015 flood was considered a 50-year event, likely to happen only once every half-century, Zwieback said he expects more big floods in the future. “With increased precipitation we know that may actually be an underestimate, so they may happen more frequently,” he said.
Climate warming is resulting in more precipitation in the Arctic, as warmer air holds more moisture.
For the Alaska Department of Transportation and Public Facilities, permafrost thaw along the 414-mile Dalton Highway is a persistent problem requiring regular maintenance and reconstruction work.
The most recent project addressing thaw beneath and along the highway is a rebuild of a section located 108 miles south of Deadhorse. A multiyear project that started in 2021 is raising and evening out a 16-mile road section that had sunk because of permafrost thaw. The sinking had put sections of the road almost flush with the adjacent tundra terrain, making the area vulnerable to severe snow drifting “that has helped earn this section of the Dalton Highway the nickname ‘No Man’s Land,’” the department’s website says. The work is expected to be completed in 2024, a department spokesperson said.
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