Decadal to seasonal coastal change observed at Barter Island, Alaska

Barter Island, the gateway to the Arctic National Wildlife Reserve (ANWR), is located about 120 km west of the U.S. Canadian border on the Beaufort Sea coast. The island has a long history of Inupiat occupation and was a major trade center until the late 19th century. In the 1950s, the U.S. Air Force built an airstrip and Defense Early Warning radar station on the island. The village of Kaktovik was incorporated in 1971 and currently has a population of 315. The island has broad, low-lying sand spits extending both east and west from the topographically higher tundra hinterland of the island. Coastal permafrost bluffs range in height from a few meters to more than ten meters and consist of a very complex sequence of material ranging from dense clay, sand, gravel, and both massive and segregated ice. Coastal bluff and shoreline positions were delineated from maps, aerial photographs, and satellite imagery acquired between 1947 and 2016. Open-ocean facing coastal bluffs show an average long-term (69 year) erosion rate of about 1.5 m/yr, punctuated by individual years with erosion rates of up to 5 times higher. Evaluation of shorter-term (13-30 year) bluff change rates show a steady increase in retreat over the last 3 decades. Long-term mean shoreline change rates show similar, but slightly lower, trends compared to bluff erosion rates, which can be explained by bluffs only eroding, whereas the beaches can both accrete and erode annually. Net long-term bluff retreat rates are higher to the east, as manifested by a westerly, 500-m migration of a node of maximum bluff retreat, possibly caused by a divergence in longshore transport due to changes in incident wave directions. In the short term (1- 5 years) both bluff and shoreline change rates show large variations, both temporally and spatially, which is indicative of their dynamic behavior. Primary failure modes of Barter Island bluffs appear to be a combination of thermal degradation and thawing of permafrost in the exposed bluff face, mechanical and thermal niching at the bluff toe, followed by rotational slumping of the higher bluffs near the central portion of the island and block collapse of the lower bluffs at the flanks of the island.