Delaloye Reynald
Effets de la circulation d'air par effet de cheminée dans l'évolution du régime thermique des éboulis froids de basse et moyenne altitude
Project Number: CH-3606
| Project Type: |
Dissertation |
| Project Duration: |
01/01/2007 - 11/14/2011 project completed |
| Funding Source: |
other , |
| Project Leader: |
Prof. Reynald Delaloye
Géographie physique - Géomorphologie
Dépt. des Géosciences - Géographie
Université de Fribourg
Chemin du Musée 4
1700 Fribourg
Phone: +41 (0) 26 300 90 21 ; +41 (0) 26 300 90 10
FAX: +41 (0) 26 300 97 46
e-Mail: reynald.delaloye(at)unifr.ch
http://www.unifr.ch/geoscience/geographie |
Research Areas:
Disciplines:
| |
|---|
| climatology, atmospheric physics, aeronomy |
Keywords:
Overcooled talus slopes, relict rock glaciers, dynamic ice cave, chimney effect air circulation, low elevation permafrost, electrical resistivity tomography monitoring
Abstract:
In the Swiss Alps, mountain permafrost – subsurface materials remaining at negative ground temperatures throughout the year – is encountered discontinuous above about 2300 m.a.s.l. Nevertheless, abnormally cold ground conditions indicating possible occurrence of isolated permafrost patches have been reported in many locations at much lower elevation where the mean annual air temperature is definitely positive. All these cold environments are inside caves or are located in the lower parts of debris accumulations, mostly talus slopes and relict rock glaciers. The main objective of this thesis is to identify and try to understand the processes governing the presence and determining the evolution of these overcooled environments located below the timberline. Twelve sites located in the Jura Mountains, the Prealps and the Alps in western Switzerland were investigated through various types of measurements, mainly thermal records and geophysics.
The results support the hypothesis that a process of reversible internal air circulation (called chimney effect or wind tube) allows the occurrence of cold environments in the lower parts of debris accumulations located a middle and low elevation in mid-latitude areas. The origin of the airflow, as its velocity and direction, are essentially driven by thermal difference between the air inside the ventilated system (the talus slope) and the external air (the atmosphere). Thus the chimney effect air circulation is characterized by a reverse of the airflow direction when a thermal threshold is reached. This value could be seen as the average temperature of the whole porous system and is closed to the mean annual external air temperature. Two major seasonal phases can be distinguished: during winter, an ascent of relatively warm light air tends to occur in the upper part of the debris accumulation. This leads to a dynamic low in the lower part, causing a forced aspiration of cold external air deep inside the ground, even trough a thick – but porous – snow cover. Airflow velocities indicating aspiration of external air were recorded in wind hole during winter under the snowpack. Conversely, the summer phase is characterized by a gravity discharge of dense cold air in the lower parts of the system, causing an inflow of warmer external air in the upper parts of the talus slope. Within these two major seasonal phases, ventilation can still reverse during short periods of mild weather in winter or during colder meteorological conditions in summertime. As a consequence, this process leads to a negative annual ground surface thermal anomaly reaching 3 to 7.5°C depending on the site in the lower parts of talus slopes, compared to the mean annual air temperature. Conversely in their upper parts, a positive annual ground surface thermal anomaly from 2 to 4°C is observed.
The thermal monitoring conducted at the ground surface in several porous debris accumulations, since 1997 for the longest series of measurements, has shown that the thermal regime is similar between the different investigated sites, and also that the key factor of evolution is the temperature of the external air during the winter. Periods of prolonged and very intense cold weather in winter promote a deep freezing of the ground. As a consequence of this forced aspiration of air during wintertime, a cold reservoir formed deep inside the talus slope. A part of the heat advected by the airflow is transmitted by conduction to the rock debris and the finer sediments located below the porous debris accumulation. A well-defined relationship is also recorded between the winter cooling of the talus slope and the ground thermal conditions prevailing in summertime. It may be noted that a particularly mild winter with limited freezing of the ground as in 2006/2007 causes a significant increase in ground surface temperatures during summer and fall. The growth of temperate permafrost below the porous talus slope of Dreveneuse d’en Bas has been reported in two boreholes in 2004-2006. Following the mild winter 2006-2007, this frozen ground disappeared and finally reformed in 2010 consecutively to the very cold winter 2009-2010.
On the other hand the external air temperature in summer and the importance of snow cover contribute very modestly to changes in the thermal regime in the lower parts of the debris accumulations. This particular thermal behaviour significantly differs from the thermal regime observed in conventional permafrost situations (eg. rock glacier) at high altitude.
Repeated time-lapse geophysical measurements (electrical resistivity and seismic refraction tomography) illustrate the two-dimensional rapid cooling of the whole talus slope by air ventilation. Electrical resistivities in the lower half of the talus slope increase exponentially as the ground temperatures decrease below the freezing point. A cold reservoir formed during the winter, whose size is directly dependent on external air temperatures. The talus slope also dries during winter, while a significant amount of interstitial ice is formed during the snowmelt periods in spring.
The validity of geophysics and temperature measurements at the ground surface was confirmed by direct measurements inside the small dynamic Diablotins ice cave, a permeable system ventilated by chimney effect. The thermal measurements clearly show that the effects of air circulation are similar between the inside of the ice cave and the porous debris accumulations. During winter, the ice cave is strongly cooled and dried, and a significant decrease in the ice mass by sublimation is observed. The rock walls surrounding the gallery act as a reservoir of heat. In contrast, the ice mass increases and remains stable during the spring and summer as a result of refreezing of snowmelt percolating water. Continuous measurements of the cave climate initiated in June 2009 also showed the predominant role of winter meteorological conditions in seasonal changes of the ice mass. Thus the ice-free state of the cave in the early 1990 occurred during mild winters with little snowfall, while the strong increase of ice mass in 1994 and 1995 follows cold and very snowy winters.
A part of this thesis was also devoted to methodological issues. A multi-methodological approach has been particularly effective for understanding the complex process of air circulation by chimney effect into porous environments. The importance of repeated thermal and geophysical measurements (monitoring), the systematic application of quality testing and the comparison of results obtained through different methods are needed to avoid over-interpretation or misinterpretation of results. Concerning the geoelectrical methods, the analysis of vertical electrical sounding curves is a good quality control of electrical resistivity tomography.
Leading questions:
Is the particular thermal regime from scree of Creux-du-Van generalizable to other cold scree area (Jura, Pre-Alps, Alps) from different altitude, orientation and geological conditions? Is it possible to theorize and explain the behaviour of air circulation using mathematical equations? What are the key factors control (meteorological variables) determining the evolution of the thermal regime of cold scree? What is the part of the evaporation / sublimation, melting and condensation in different parts of the landslide process? Can we define the extent and the way in which the tank cold formed inside a cold scree ventilated is created (and empty itself)??
Publications:
Morard, Sébastian. 2011. Effets de la circulation d'air par effet de cheminée dans l'évolution du régime thermique des éboulis froids de basse et moyenne altitude. Geo-Focus, No. 29. Université de Fribourg, Suisse.
pdf Thesis
Last update: 7/18/17
Source of data: ProClim- Research InfoSystem (1993-2024)
Update the data of project: CH-3606
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