Quasi-horizontal circulation cells in 3D seawater intrusion

The seawater intrusion process is characterized by the difference in freshwater and seawater density that causes freshwater to float on seawater. Many confined aquifers have a large horizontal extension with respect to thickness. In these cases, while buoyancy acts in the vertical direction, flow is confined between the upper and bottom boundaries and the effect of gravity is controlled by variations of aquifer elevation. Therefore, the effective gravity is controlled by the slope and the shape of the aquifer boundaries. Variability in the topography of the aquifer boundaries is one case where 3D analysis is necessary. In this work, density-dependent flow processes caused by 3D aquifer geometry are studied numerically and specifically, considering a lateral slope of the aquifer boundaries. Sub-horizontal circulation cells are formed in the saltwater entering the aquifer. The penetration of the saltwater can be quantified by a dimensionless buoyancy number that measures the lateral slope of the aquifer relative to freshwater flux. The penetration of the seawater intrusion wedge is controlled more by this slope than by the aquifer thickness and dispersivity. Thus, the slope must be taken into account in order to accurately evaluate seawater intrusion

Elastic behaviour of nanophases in polyvinyl alcohol (PVA)/ bamboo charcoal (BC) nanocomposite films

Polyvinyl alcohol (PVA) nanocomposite films reinforced with bamboo charcoal (BC) nanoparticles were successfully fabricated via solution casting and their nanomechanical properties in terms of material phases were determined by a peak force quantitative nanomechanical tapping mode (PFQNM). Our experimental results revealed that the elastic modulus of PVA semicrystalline phase in stack-bundle form was 24± 4.2 GPa with the semicrystalline phase width being 20-76 nm, as opposed to 11.4± 3.1 GPa and 18-65 nm for corresponding PVA amorphous phase accordingly. The incorporation of BC nanoparticles enhanced the elastic moduli of both crystalline and amorphous phases of PVA by ~51% and ~100%, respectively. Moreover, the phase width was decreased to be in range of 5-53 nm for crystalline phase and 4-35 nm for amorphous phase

Airborne electromagnetic imaging of discontinuous permafrost

The evolution of permafrost in cold regions is inextricably connected to hydrogeologic processes, climate, and ecosystems. Permafrost thawing has been linked to changes in wetland and lake areas, alteration of the groundwater contribution to stream flow, carbon release, and increased fire frequency. But detailed knowledge about the dynamic state of permafrost in relation to surface and groundwater systems remains an enigma. Here, we present the results of a pioneering ~1,800 line-kilometer airborne electromagnetic survey that shows sediments deposited over the past ~4 million years and the configuration of permafrost to depths of ~100 meters in the Yukon Flats area near Fort Yukon, Alaska. The Yukon Flats is near the boundary between continuous permafrost to the north and discontinuous permafrost to the south, making it an important location for examining permafrost dynamics. Our results not only provide a detailed snapshot of the present-day configuration of permafrost, but they also expose previously unseen details about potential surface – groundwater connections and the thermal legacy of surface water features that has been recorded in the permafrost over the past 1,000 years. This work will be a critical baseline for future permafrost studies aimed at exploring the connections between hydrogeologic, climatic, and ecological processes, and has significant implications for the stewardship of Arctic environments

Social Movements and International Relations: A Relational Framework

Social movements are increasingly recognized as significant features of contemporary world politics, yet to date their treatment in international relations theory has tended to obfuscate the considerable diversity of these social formations, and the variegated interactions they may establish with state actors and different structures of world order. Highlighting the difficulties conventional liberal and critical approaches have in transcending conceptions of movements as moral entities, the article draws from two under-exploited literatures in the study of social movements in international relations, the English School and Social Systems Theory, to specify a wider range of analytical interactions between different categories of social movements and of world political structures. Moreover, by casting social movement phenomena as communications, the article opens international relations to consideration of the increasingly diverse trajectories and second-order effects produced by social movements as they interact with states, intergovernmental institutions, and transnational actors

Permafrost response to climate change: Linking field observation with numerical simulation

International audienceThe Scotty Creek basin (152 km 2) is located in the Northwest Territories, Canada, within the peat-covered discontinuous permafrost zone with a high density of wetlands. The extensive peat layer (up to 3-4 m thick) is underlain by generally clay-rich glacial sediments. The landcover consists of peat plateaus underlain by permafrost, permafrost-free channel fens, and connected and isolated permafrost-free ombrotrophic flat bogs, occurring as a complex mosaic of patches. The runoff from peat plateaus drains into isolated bogs and a network of connected bogs and fens. During the course of field studies since 1999, stark changes have been observed on the permafrost plateaus, including a deepening of active layer, soil settlement and depression formation, and changes in the lateral and vertical extent of the unsaturated zone. In general, the area of permafrost plateaus is decreasing, and the areas of fens and bog areas are increasing. These changes affect water flow and induce changes in heat transport, which in turn affect the aforementioned changes in permafrost plateaus (i.e. feedback processes). The goal of this study is to understand the feedbacks and their effects on permafrost degradation by used of the field observations and numerical simulations. We use a modified version of the three-dimensional SUTRA model that can simulate groundwater flow and heat transport, including freeze-thaw processes. Numerical simulation of heat transport accounts for the effects of latent heat associated with freezing and thawing, and variable heat capacity, thermal conductivity, and permeability as a function of ice content. The model is used to simulate the plateau-fen-bog complex, where intensive field studies have generated a large amount of data. The SUTRA model does not simulate complex surface processes such as radiative and turbulent heat exchange, snow accumulation and melt, and canopy effects. We use an energy and water transfer model, Northern Ecosystem Soil Temperature (NEST) to calculate the surface conditions and provide the surface boundary conditions for SUTRA. The SUTRA results are then compared to the long-term field and remote sensing data of permafrost degradation. Various hypotheses regarding permafrost degradation are tested: a) tree canopy density, water saturation, soil settlement, and snow cover impact the surface energy budget, b) warmer water flow in fens and connected bogs influences the relative roles of conductive and advective heat transfer, c) the type of mineral soil located below the peat (e.g. silty sand, clay) influences heat transport, water storage and possibly sub-permafrost flow and d) the isolated bogs become connected with each other to form the drainage network. In addition, model simulations are conducted for a small section (3 km 2) of the Scotty Creek basin to understand the current permafrost state and predict the degradation in the next twenty years using scenarios generated by a regional climate model