Geologic Controls on Erosion Mechanism on the Alaska Beaufort Coast

Two prominent arctic coastal erosion mechanisms affect the coastal bluffs along the North Slope of Alaska. These include the niche erosion/block collapse mechanism and the bluff face thaw/slump mechanism. The niche erosion/block collapse erosion mechanism is dominant where there are few coarse sediments in the coastal bluffs, the elevation of the beach below the bluff is low, and there is frequent contact between the sea and the base of the bluff. In contrast, the bluff face thaw/slump mechanism is dominant where significant amounts of coarse sediment are present, the elevation of the beach is high, and contact between the sea and the bluff is infrequent. We show that a single geologic parameter, coarse sediment areal density, is predictive of the dominant erosion mechanism and is somewhat predictive of coastal erosion rates. The coarse sediment areal density is the dry mass (g) of coarse sediment (sand and gravel) per horizontal area (cm2) in the coastal bluff. It accounts for bluff height and the density of coarse material in the bluff. When the areal density exceeds 120 g cm−2, the bluff face thaw/slump mechanism is dominant. When the areal density is below 80 g cm−2, niche erosion/block collapse is dominant. Coarse sediment areal density also controls the coastal erosion rate to some extent. For the sites studied and using erosion rates for the 1980–2000 period, when the sediment areal density exceeds 120 g cm−2, the average erosion rate is low or 0.34 ±± 0.92 m/yr. For sediment areal density values less than 80 g cm−2, the average erosion rate is higher or 2.1 ±± 1.5 m/yr.