Study finds 2015 Alaska Flood caused widespread ground subsidence

Assistant professor Simon Zwieback from the University of Alaska Fairbanks Geophysical Institute, along with a team of researchers, conducted a study published on September 27 in the journal Permafrost and Periglacial Processes, which examined the 2015 flooding of the Sagavanirktok River in northern Alaska. This flood had immediate impacts, such as temporarily closing the Dalton Highway, but it also led to longer-term consequences in the permafrost-rich region.

Zwieback, the lead author of the paper, specializes in using space-borne remote sensing to study the Arctic. The study revealed that following the flood, there were significant and varied changes in the tundra and permafrost landscape. They observed subsidence in areas that were flooded, with some locations experiencing over three inches of subsidence in just a few years. Additionally, there were many areas with less pronounced but still measurable changes in ground level. They also noted a greening of the landscape and increased wetness, which varied across the affected areas, providing insights into how these landscapes react to floods.

The flooding, which occurred in mid-May 2015 and affected the Dalton Highway and Deadhorse airport, was exacerbated by a buildup of aufeis, layered ice formed from the freezing of river water. This ice diverted the thawing river’s water away from natural channels. The study highlights the complex relationship between rivers and their floodplains in regions with continuous permafrost. Human activities related to the expansion of the Prudhoe Bay oilfield and the presence of the Dalton Highway have interfered with natural drainage.

The authors suggest that the flood may have contributed to subsidence by warming the ground, causing ground ice to melt. This warming could result from increased wetness, disturbances to the protective layer of organic matter, or sediment deposition, which allows more heat to penetrate the ground.

To assess ground deformation over the years following the flood, Zwieback analyzed satellite data from 2015 to 2019. The study found widespread but variable subsidence, with the most significant changes occurring in the two years immediately after the flood. This subsidence was largely attributed to the melting of ground ice in the soil, which contains ice in the form of ice wedges and segregated ice.

The presence of ice wedges, typically around 30 feet apart, can lead to surface changes, such as water ponding, darkening, and warming, which, in turn, intensify thawing below the surface.

Subsidence was observed in areas with high ice content but not in others, indicating multiple factors at play, including organic layer disturbance and sediment deposition. The researchers noted fine-grain sediment in the top two inches of soil samples but couldn’t definitively link it to the flood, despite its proximity to the highway.

The study also found that ice-rich locations experiencing subsidence showed increased greenness and wetness, while ice-poor floodplains became greener without deforming. The research suggests that flooding, despite its initial impact, can have long-term benefits, such as sediment deposition, which promotes insulating vegetation cover and organic matter. This, in turn, reduces flood frequency and elevates the landscape.

This research gains significance in the context of the changing climate and increasing stresses in the Arctic. As the region becomes wetter and the flood regime evolves, understanding how riverine landscapes respond to these changes becomes crucial.

Source: University of Alaska Fairbanks

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