Ahmad, Nadeem (Norwegian University of Science and Technology (NTNU)) | Bihs, Hans (Norwegian University of Science and Technology (NTNU)) | Chella, Mayilvahanan Alagan (Norwegian University of Science and Technology (NTNU)) | Kamath, Arun (Norwegian University of Science and Technology (NTNU)) | Arntsen, Øivind A. (Norwegian University of Science and Technology (NTNU))
Computational fluid dynamics (CFD) modeling of breaking waves over a slope and the resulting erosion in the case of an Arctic coastline is presented in this study. The study is performed with the open-source numerical model REEF3D. First, the numerical model is validated for the simulation of incident waves, wave breaking on a slope, and the sediment transport process. The numerical results show good agreement with wave theory and experimental data. The validated numerical model for the hydrodynamics and the sediment transport process is then used to simulate the coastal erosion process under the breaking wave impact on a vertical bluff. An Arctic coastline at Bjørndalen region at Isfjorden, Svalbard, is chosen, where a significant coastal erosion was observed during a storm event in September 2015.
Most of the Arctic coastline is susceptible to climate change. Because of global warming and the transfer of additional heat fluxes, the frozen period of the upper active layers in the Arctic coastline is reduced. Consequently, coastline stability decreases during the extended warmer period. The average thickness of the active sediment layer in Svalbard, Norway, varies between 1.0 and 10.0 m and consists of coarse-grained sandy soil (Fromreide, 2014). Climate change can affect this Arctic coastline in two ways. First, the extended warmer period results in the formation of deeper and weaker active sediment layers (IPCC, 2007). Second, the melting of the Arctic ice sheets increases the sea level, resulting in higher tides. In combination, the higher tides approach the Arctic coastline (Thompson et al., 2016) and erode the weaker active sediment layer. A recent example of this change has been experienced in the Bjørndalen region in Isfjorden, Svalbard, where significant coastal erosion occurred during a storm event in September 2015. The waves reached the cabins built near Isfjorden and resulted in an almost 1.0-m-deep scour hole (Barstein, 2015). Therefore, in order to better understand the coastal erosion phenomenon in the Arctic regions, the processes of wave breaking and the resulting sediment transport have to be investigated in detail. The study is also important for the design of new coastal structures and suitable mitigation measures at the Arctic coastline.
Arctic regions have been determined to be particularly sensitive to a warming global climate both on the basis of climatic modeling and observation of dramatic changes in arctic landscapes and sea ice. As early as the 1990s, air temperatures in interior northwest Alaska were warming at a rate of 0.75 °C per decade. The resulting thawing of permafrost causes significant damage to buildings, roadways, and can lead to increased mass wasting (e.g., active layer detachments and thaw slumps) by melting the soil ice that "cements?? the grains together to resist soil movement, as well as ground subsidence. These climate-induced ground movements can threaten infrastructure, such as road, bridges, and pipelines, either by direct physical damage or indirectly, such as through changes in drainage patterns, increased risk of flooding and forest fires.
Because these geohazards often occur in remote locations with harsh weather conditions and limited access, and the precursor conditions for initiating them can occur gradually, methods for remotely monitoring changes in ground conditions and estimating ground failure risks have significant engineering and economic value. The research described in this paper addresses this need by developing techniques for detecting changes in permafrost and seasonally frozen soil terrains using satellite and airborne remote sensing data, and combining these data with mathematical models to estimate the risk of ground failures due to soil thawing. The methodology consists of combining multiple sources of satellite-acquired synthetic aperture radar (SAR) data with high resolution optical-band data and aerial photography to map frozen ground and associated changes in soil moisture, and to detect vertical and lateral ground movements.
The remote sensing data interpretations along with traditional soil and vegetation mapping are used to inform mathematical models of permafrost and frozen soil stability. These models are used to develop maps of the probability of ground movements associated with permafrost degradation and seasonally frozen soil under current and future climate conditions. The models and slope stability risk algorithm were applied to a portion of Kobuk Valley National Park, Alaska for which soil and vegetation land cover maps were available. The cryosphere model results suggest that the same relative change in active-layer thickness occurs across the landscape, but warmer locations experience a larger absolute change in active-layer thickness and may experience permafrost loss as a consequence. The slope failure risk algorithm indicates that the upland areas are most susceptible to slope failure, particularly south-facing slopes, but low-cohesion low-land soils and steep river banks are also susceptible to failure.