Coastal geomorphology of southwest Banks Island, Northwest Territories: Historical and recent shoreline changes and implications for the future.

摘要

Predicted changes in Arctic climate include decreased sea-ice extent, increased storm frequency, and sea-level rise. The western Canadian Arctic is highly sensitive to sea-level rise and climate change due to, high ground ice concentrations in unlithified sediments, increased storm effectiveness, decreased sea-ice extent, and high erosion rates. The southwest coast of Banks Island has similar climate conditions, substrates and ground ice contents. Publicized community concern about changing coastal conditions led to Sachs Harbour, NWT being chosen as one of the first coastal sites for detailed study as part of an ArcticNet project.;Thirty-one coastal surveys, nearshore bathymetric surveys, and sediment samples were used to determine present coastal processes and rates change. Suspended particulate matter before and after a precipitation event were used to determine the effectiveness of runoff. Aerial photographs and satellite images were used to determine historical change along the coastline and within the community. Sea ice and storm records were analysed from the 1950s to present in order to determine the frequency of events and the associated sea-ice conditions.;Coastal bluffs along the southwest coast are undergoing retreat at variable rates, dependant on ice content within sediments. Two main areas of retreat occur, to the west of Martha Point in an area of exposed ground ice with retreat rates of 5.9 m since 2003, and to the southeast of the community where fine grained sediment and narrow beaches leave the bluffs exposed. Within Sachs Harbour, most coastal bluffs are not presently retreating with the exception Line 2 which is retreating approximately 0.40 m/a.;Storm records for Sachs Harbour indicate that event frequency throughout the record varies, with declining storm frequency since 2000. Due to the presence of sea ice in the region during the open-water season, storm events often have limited fetch which minimizes the impact on the southwest coast.;Sachs Harbour and the southwest coastline of Banks Island is dominated by low unlithified coastal bluffs containing segregated ice lenses and ice-rich silty sand horizons. Initial investigations identified three possible mechanisms for coastal erosion: wave activity and storm events, rising sea level and decreasing sea-ice extent, and increased thermal ablation coupled with regional submergence.;Aerial photograph and satellite image analysis have indicated four major depositional areas, Sachs Spit, Martha Point Spit, Sachs Landing Beach, and Cape Kellett Spit. Areas of erosion include, west of the Martha Point Spit, within the community, and to the southeast of the community. Sediment transport is a complex. There are two major sediment transport cells, a large cell toward the west and a smaller cell moving sediment towards the community.;As thermal erosion is the dominant mechanism of coastal change in Sachs Harbour and throughout the study area, armouring or other anthropogenic measures to prevent erosion within the community will be ineffective. As the community is fronted by a large beach, community expansion on the coastal bluffs is not advisable. With warming conditions in the region, thermal retreat will continue and as sea ice extent decreases, the effectiveness of storm events of eroding this coastline will increase.