Coastal erosion of ice-rich permafrost bluffs

Along the coast of Alaska’s north slope, a record decrease in 2007 of the extent of the Arctic sea ice and an annual increase to 14 meters of coastal erosion spawned research funded by NOPP (the National Oceanographic Partnership Program) that is just now coming to a close. Dr. Irina Overseem of the University of Colorado and the Institute of Arctic and Alpine Research presented at Mines on her team’s findings.

Sea ice naturally recedes and expands annually as a function of wind, temperature, and radiation. NASA GISS data indicates that the arctic region is warming and Overseem worked with other scientists to determine whether and how the arctic warming influences the Alaskan bluff coastline.

Alaskan permafrost bluff surfaces are extremely flat in the area studied, with melt and heave polygon formation of slightly contoured boundaries all over the surface. These polygon boundaries are preferential failure planes, or weak planes where melting and erosion occurs. Height-wise, the unexposed mass far exceeds the exposed bluff face. About 35 centimeters of concentrated sediment comprises the very top layer of the bluff followed by about five meters of exposed ice, and approximately 250 meters of solid permafrost ice submerged beneath the exposed face.

Overseem described the bluff coastline as a “dirty iceberg coast.” The bluff’s dirt layer composition is mostly fine muds on the north slope where erosion is greatest, a result of the area’s deposition in alluvial fan and delta formation from tertiary sediments. The absence of sand in these muds is critical because sand plays an important role in coastal protection and retention. It is interesting to note that other areas of the coast experience less drastic bluff erosion due to the presence of sand particles in the sediment layer.

The research team was able to show a correlation between the decrease in the Arctic sea ice extent and increase in coastal bluff erosion rate. Variables such as the distance the wind blows, wind shear stress, wave height, along shore water velocity, torque balance, and temperature change are all sources of coastal erosion. The two logical suspects, those that seem to provide the most intense impacts on the bluff, are temperature rise and wave impact.

The Arctic sea does not produce significant waves consistently around Alaska’s northern slope. Waves do cause some coastal erosion, but they are not the main cause. Water level rises by an insignificant amount except when wind blows water into the coast from farther out at sea.

Video footage of the coast erosion taken by continuously running cameras showed that at times when the water level does not visibly change, coastal erosion is still quite dramatic. Huge chunks of coast broke off the bluffs and dissipated into the Arctic Sea in only a week’s worth of footage. Temperature was then investigated as the primary source of most bluff erosion.

The team used the minimal data available from their collection at Drew Point, Alaska and found it could be combined with meteorological records collected at Barrow, Alaska between 1979 and 2001. The data indicated that as temperature increased, retreat of the sea ice caused more waves with greater height to hit the coast in the exposed water phase of the annual sea ice cycle. In the 1980s, the high end of wave height was 250 meters. Now, that maximum is at 500 meters. The increase in wave size has enabled decrease of the ice extent. The smaller the ice extent, the farther away the wind can blow water towards the coast. Wave height and force increases the more time and distance the wave has to build up before hitting the coast.

Despite this increase in wave height, the team did not find that greater wave impact was directly correlated to increase in coastal erosion. What they did find was a seasonal variation in the number of big erosion events. In the summer, a large amount of radiation hits the sea and the bluffs, warming them. In the fall, sea ice has not yet expanded to cover the waters and reduce wave size, but coastal erosion is reduced. If the ice is present, one might think that temperature is quite variant. Overseem’s team supposes that less radiation in the fall causes the sea temperature to fall significantly. If more funding were available, a more advanced camera that could take readings of thermal radiation could prove the hypothesis.

Ramifications of coastal erosion are extreme. First, coastal erosion is destroying towns such as Shismaref, Alaska. In Shismaref, 565 people need to be relocated and have requested government support without much response as their houses are washed into the Arctic Sea. Additionally, the area is a breeding ground for many species of rare birds. As wave height increases, freshwater bodies on shore are being salinized by the sea. Humans, deer, birds, and other beings living in the area are at risk of running out of potable water. There are also unmitigated oil wells that were abandoned many years ago. Because they were never properly sealed off, the Arctic Sea is eroding the land around the wells and poisonous chemicals are being leached into the sea. Engineers are working save the bluffs before the northern slope of Alaska’s coast is washed into the sea.

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