Erosion of permafrost affected coasts: rates, mechanisms and modelling
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The coastal zone is the geographical and geomorphological interface between land and ocean and is an important gateway for a region’s commercial and cultural exchange with the rest of the world. Arctic coastlines erode at rates 3 to 4 times higher than temperate coasts (Aré, 1988). In Canada from 1971 to 1987, erosion rates averaged 10 m per year in certain locations (Hequette and Barnes, 1990) while rates of erosion varying from 6 to 55 m per year are reported in northern Russia during the same time span (Aré, 1988). Furthermore, despite long-term monitoring at some sites, data on Arctic coastal recession are still sparse and sporadic over the vast Arctic area. The high erosion rates reported on Arctic coasts present difficult engineering challenges for coastal infrastructures (harbors, buildings and pipelines). A limited number of morphodynamic models have been proposed to forecast the erosive responses of Arctic coasts. Arctic coasts are characterized by the presence of both onshore and offshore permafrost as well as the presence of sea ice. While it is commonly accepted that both thermal and mechanical action simultaneously contribute to erosional processes of frozen shores, most of the existing studies (Southgate and Nairn, 1993; Nairn et al., 1998, Hoque and Pollard, 2009) address erosion from a coastal engineering approach and consider wave action as the main eroding force. These studies limit the erosion process to the sea-ice free period of the year. The study done during this PhD framework offers an interdisciplinary consideration to Arctic coastal erosion, combining geology with geotechnical engineering. Three main field investigation sites have been studied: Vestpynten (Svalbard), Varandey and Baydaratskaya Bay (Northern Russia). To determine both current and historical rates of coastal recession, geomorphological surveys were conducted and satellite images and aerial photographs were studied for each site. Ground temperatures were measured along transects perpendicular to the shorelines. The mean annual ground temperatures (MAGT) vary from -2 degrees Celsius along the coast of Vestpynten (cold permafrost zone) to close to 0 degrees Celsius near the shoreline of Varandey (warm permafrost zone). The field studies at the different sites highlight the importance of both the snow bank deposited on the lee side of the coastal bluff and the thermal regime of the bluff in the overall coastal recession process. Erosion occurs during snowmelt in June and is responsible for the coastal bluff crest recession. The snow bank on the beach and in front of the bluff may partly protect them from waves late in the season, but more important is that the snow bank isolates the ground from the cold winter air. Thawing of frozen bluff sediments and mass movements are exacerbated by excess water generated during snowmelt. The waves reaching the bottom of the bluff in summer will only remove the eroded and loose material. All the sites appeared to be heavily affected by these thermo-denudation processes. Based on field observations and measurements, a modelling approach is proposed to improve the predictions of Arctic coastal recession. The approach suggests three sequential modelling steps to assess recession rates of coastlines where thermodenudation is the dominant environmental force controlling bluff failure. First, a thermal model is built, representing the seasonal and annual thermal regime evolution of a coastal bluff. Then a simple soil model is applied to assess the stability of the thawing bluff depending on the previous thermal analysis. Finally, a hydrodynamic model is suggested to assess the amount of material removed by waves from the toe of the scree slope. In this thesis, detailed considerations are given to the first two steps of this modelling approach. The thermal and mechanical modelling approach has been tested on the three investigation sites and produce results closely resembling the data measured in the field. Being able to model/predict erosion in Arctic and permafrost-affected coasts with some accuracy may contribute in supplying both researchers and industrials partners with methods and guidelines to improve planning and design of infrastructures in Arctic coastal areas. However, numerical models and further field and laboratory work are recommended in order to calibrate, validate and improve the proposed model.