000170281 001__ 170281
000170281 005__ 20190717172522.0
000170281 0247_ $$2doi$$a10.5075/epfl-thesis-5270
000170281 02470 $$2urn$$aurn:nbn:ch:bel-epfl-thesis5270-5
000170281 02471 $$2nebis$$a6695013
000170281 037__ $$aTHESIS
000170281 041__ $$aeng
000170281 088__ $$a5270
000170281 245__ $$aStatistical Analysis of Mountain Permafrost Temperatures
000170281 269__ $$a2012
000170281 260__ $$bEPFL$$c2012$$aLausanne
000170281 300__ $$a163
000170281 336__ $$aTheses
000170281 520__ $$aThe 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.
000170281 6531_ $$aActive layer
000170281 6531_ $$aAlpine permafrost
000170281 6531_ $$aborehole temperatures
000170281 6531_ $$aspectral analysis
000170281 6531_ $$atransfer function model
000170281 6531_ $$atrend analysis
000170281 6531_ $$aheat transfer
000170281 700__ $$aZenklusen Mutter, Evelyn
000170281 720_2 $$aDavison, Anthony Christopher$$edir.$$g111184$$0240476
000170281 720_2 $$aPhillips, Marcia$$edir.
000170281 8564_ $$zTexte intégral / Full text$$yTexte intégral / Full text$$uhttps://infoscience.epfl.ch/record/170281/files/EPFL_TH5270.pdf$$s12021388
000170281 909C0 $$xU10124$$pSTAT$$0252136
000170281 909CO $$pthesis-bn2018$$pDOI$$pSB$$ooai:infoscience.tind.io:170281$$qDOI2$$qGLOBAL_SET$$pthesis
000170281 918__ $$dEDMA$$cMATHAA$$aSB
000170281 919__ $$aSTAT
000170281 920__ $$b2012
000170281 970__ $$a5270/THESES
000170281 973__ $$sPUBLISHED$$aEPFL
000170281 980__ $$aTHESIS