Permafrost

Summary

Permafrost () is soil or underwater sediment which continuously remains below for two or more years. Land-based permafrost can include the surface layer of the soil, but if the surface is too warm, it may still occur within a few centimeters of the surface down to hundreds of meters. It usually consists of ice holding in place a combination of various types of soil, sand, and rock, though in ice-free ground, perennially frozen non-porous bedrock can serve the same role. Around 15% of the Northern Hemisphere or 11% of the global surface is underlain by permafrost, with the total area of around . This includes substantial areas of Alaska, Greenland, Canada, and Siberia. It is also located in high mountain regions, with the Tibetan Plateau a prominent example. Only a minority of permafrost exists in the Southern Hemisphere, where it is consigned to mountain slopes like in the Andes or the Southern Alps of New Zealand, and beneath the massive ice sheets of the Antarctic. Permafrost con

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Statistical Analysis of Mountain Permafrost Temperatures

Evelyn Zenklusen Mutter

The consequences of climate change are clearly visible in the Swiss Alps. In the past decades, increasing air temperatures have induced pronounced glacier retreat and permafrost thawing. Thawing permafrost in steep terrain is a potential natural hazard, as it can become unstable and trigger mass movements such as settlement, debris flows or rock fall. The WSL Institute for Snow and Avalanche Research SLF has been measuring ground temperatures and displacements in permafrost boreholes for more than a decade. This thesis aimed to analyse these ground temperatures with statistical methods, assessing possible changes, trends and physical coherencies. In a first step, temporal changes in daily ground temperatures measured in two adjacent boreholes at Muot da Barba Peider in the Eastern Swiss Alps were analysed. Statistical models, which could for example describe changes in the amplitudes or in the mean, were used to estimate possible trends. The results for the period 1996 - 2008 revealed increasing summer temperature for the upper ground layers, whereas winter temperatures had decreased. For the frozen rock below 10 m however, a general temperature increase was found. Although increasing summer temperatures were consistent with the development of the air temperatures, decreasing winter temperatures could be attributed to a thin early winter snow depth. The general warming trend in the deeper layers was assigned to increased heat transfer through the mountain ridge of Muot da Barba Peider induced by warming air temperatures and lower snow depths. These results confirm that permafrost temperatures in the Alps are influenced by factors such as snow cover, surface properties, hydrology and topography. Increasing air temperatures do not necessarily induce permafrost thawing. Special attention was paid to the so-called "active layer", the topmost ground layer above permafrost, which seasonally thaws in summer. An increase in its thickness implies a potential increase of the material which might be released in a mass movement. Typical active layer characteristics have been analysed and compared for ten different permafrost sites. Whereas over the past decade, the active layer remained rather constant at the individual sites, significant differences due to local terrain properties were visible. The comparison of the daily development of the active layer thickness with that of the thawing degree days revealed that ice-rich ground layers delay the active layer development efficiently and thus isolate the permafrost from warm air temperatures. Two of the ten sites, however, revealed abrupt active layer deepening due to lateral air and/or water flows. Further investigation of the dependence between air and ground temperature was performed. Transfer function models were used to quantify the relation between air and ground temperatures measured at 0.5 m depth at seven different permafrost sites. The results showed that the delay between daily changes in the air and ground temperature ranges from one to six days, depending on the site. The most efficient relation was found for a very coarse-blocky rock glacier site, whereas scree slopes with smaller grain sizes showed a weaker and more prolonged relation. The main heat transfer mechanism in permafrost is conduction, but due to phase change, air or water flows, heat can also be transferred non-conductively. A statistical procedure including spectral analysis, order-restricted inference and a false discovery rate procedure has been developed to detect depths and frequencies at which significant non-conductive heat transfer processes occur. The application of the procedure to two-hourly borehole temperatures measured at Muot da Barba Peider revealed significant non-conductive heat transfer for the period 2005 - 2009 only in an ice-rich layer between 1 and 1.9 m depth. To gain deeper insight into the processes occurring shorter (three-week) periods have been analysed. The results showed non-conductive heat transfer processes that could be attributed to phase changes at the freezing front and at the base of the active layer in autumn, to convective air and vapour flows through the frozen scree in winter, and to phase changes and meltwater infiltration during the thawing period in spring. Analyses of borehole temperatures also revealed special phenomena. A case study for the famous permafrost site at Flüela Pass showed that the permafrost body inside the scree slope, close to the lake has degraded from 7 m to 3.5 m thickness within four years. This rapid permafrost degradation could be attributed to a seasonal ventilation system inside the scree, leading to condensation which melts the permafrost body from below. Increasing lake water temperatures might accelerate the phenomenon. The ground temperatures analysed in this work include about one decade of measurements and therefore are too short for climate-related statements. However, the statistical analyses performed give an interesting insight into the development of permafrost ground temperatures during the past ten years and their interaction with air temperature and local properties. The presented results furthermore emphasize the strong spatial variability of the ground thermal regime in mountain permafrost.

EPFL2012

Snow cover variations and spring snowmelt runoff modelling in the source region of the Yellow River, China

Léo Zhou Ficheux

This study aims to assess the snow cover spatial and temporal variation in the Source Region of the Yellow River, to understand the effect of snowmelt on the hydrology and to evaluate the potential impact of increasing temperature on the spring snowmelt runoff. The Snowmelt Runoff Model (SRM) developed by Rango and Martinec was used and improved by adding diurnal temperature computation and snow storage computation to model the runoff for 2013, 2014, 2016 and 2017. The snow cover assessment was done using Tera-MODIS satellite images between 2001 and 2017. Results showed that spring snowmelt produces 38% of direct runoff before July and 15% of annual runoff in average. The snow cover is very sensitive to temperature and to winter precipitation. There is a significant decrease of snow cover in May since 2001 in parallel to a significant increase of temperature. The snow cover duration has decreased by more than 15 days (20%) in the permafrost area in 15 years. Simulation suggests that a temperature increase of 2°C or 4°C will reduce the spring snowmelt by at least 20% and 30% respectively, and spring peak runoff will occur up to 15 days and one month earlier. Future investigations should focus on the interaction between snow, frozen ground and ground water recharge in order to extend the knowledge on snowmelt impact to soil related processes which play a key role on the ecosystem degradation in the region.

2018

Curiously, the thawing of frozen soils containing ice lenses has only been the subject of rare fundamental studies even though it is the cause of the greatest damage to constructions. The reduction of bearing capacity caused by the thawing of the ice lenses is a destructive phenomenon, and thus costly, which concerns roadway and railway infrastructures, as well as the melting of mountain permafrost by global warming. The present research, especially experimental, has tried to explain how ice lenses melt and how the water produced by the melting acts on soil properties, in particular on their deformability. Numerous freezing and thawing tests have been carried out in a testing apparatus including : A mould containing a specimen 150 mm in diameter and 300 mm in height. It is slightly conical in order to reduce the friction against its walls due to swelling. Numerous gauges placed on the sides and in the specimen in order to measure temperature, unfrozen water content and suction. Three cryostats, which control temperature at the head of the specimen (positive and negative), at its base (always positive) and outside of the thermal insulation placed against the mould. A micro-camera (endoscope), which moves in a translucent tube placed along the axis of the specimen, which enables animations of the growth and melting of ice lenses to be made. A press to enable loading and unloading cycles to be applied to the specimen. An X ray device using lead shot placed in the specimen, which enables the measurement of deformation in the entire specimen during the freezing and thawing cycles. All of the tests were carried out on one very frost–susceptible silt. Their duration of approximately two months excluded the possibility of carrying out tests on several types of frost–susceptible soils. Some numerical simulations permitted the verification of the thermal behaviour of the test apparatus. Then, previous freezing and thawing tests carried out at full scale on road pavements, carried out in a large test pit, were reinterpreted in order to obtain improved information on the deformability (resilient moduli) of the frost–susceptible infrastructure which was made up of a silt similar to that tested in the laboratory tests described above. The results of the laboratory tests and the reinterpretation of the full–scale measurements on road pavements were used for two very different practical applications : the design of roadway and railway pavements by quantitative methods using, in particular, resilient moduli and the formation of mountain permafrost and its thawing due to global warming. In the general field of the physical phenomenon of freezing and thawing of frost–susceptible soils, the very elaborate experiments permitted the measurement, with precision and reliability, of numerous parameters which are involved in the phenomena of freezing and thawing of fine-grained soils. However, most of these phenomena were already known and their parameters have been determined rather well by numerous experiments. This research has, nevertheless, permitted the quantification of certain parameters a bit better. The greatest contribution to the understanding of these phenomena has been the visualisation of the growth and the thawing of ice lenses inside the specimen using an endoscope, which has never been carried out previously. The resulting animations show extremely well how the ice lenses form and melt in a virtually natural soil. In the field of freezing and thawing design for roads and railways, the research has shown that the use of a thawing resilient modulus combined with modern numerical design methods is completely possible. This research has provided several values for this modulus : for the silt used in the full–scale test pit, for various frost–susceptible soils, and, by a more elaborate method, for the silt used in this research. Also concerning this field, it has shown how the resilient modulus may be used in modern numerical methods and how to determine it in the laboratory. The results of this research will thus be very useful for the work of the Swiss committee which will very soon address the revision of Swiss standards for the design of pavements for roads and railways. In the field of the construction of roads and railways, this research has confirmed that the water, which comes from the melting of ice lenses, flows toward the soil zone from where it has been extracted then transported toward the freezing front. Any drainage of the formation level is thus of no practical use. Finally, in the field of melting of permafrost due to global warming and the initiation of debris flows in mountain permafrost, this research has shown, first of all, the importance of the capillary suction regime in the formation of mountain permafrost, which has never been well explained until now. It has also shown how the use of sophisticated numerical models in the prediction of the melting of mountain permafrost and of the initiation of debris flows is possible. Such use, however, is difficult and should not be considered as a practical tool for common use in the prevention of these dangerous phenomena. This research has also provided a practical method for evaluating the degree of supersaturation of permafrost.

EPFL2007