1. Rock wall thermal regime characterization in high mountain areas
European Geosciences Union General Assembly 2007 Vienna, Austria, 15 – 20 April 2007
Rock wall thermal regime characterization in high mountain areas
and related permafrost degradation: preliminary data from the Western Alps .
E. Cremonese (1), U. Morra di Cella (1), P. Pogliotti (2), M. Giardino (2), S. Gruber (3)
(1) ARPA Valle d’Aosta – A. O. Cambiamenti Climatici, Italia; (2) Dipartimento di Scienze della Terra, Università degli studi di Torino, Italia; (3) Laboratoire Edytem, Université de Savoie, Le Bourget du Lac, France CONTACT:
4. Instrumentation
Introduction
3. Methodology
Measured variables at all sites are: rock temperature at different depths (3, 30 and 55 cm), air temperature and relative humidity close to rock-atmosphere interface. Two sites (CC and AdM) have been dedicated to intensive measurements that also include solar radiation (incoming and outgoing short / long wave), wind speed and wind direction. Project’s first year has been mainly dedicated to the development and the improvement of measurement methodology leading to reliable and accurate data: results of this methodological approach are shown in the boxes below.
Permafrost degradation of ice-filled discontinuities in high mountain areas has been hypothesized to be one of the main causes of rockfalls and rock wall instabilities occurred in the last years in these ambients. In the context of global warming, alpine regions are extremely sensitive to increasing temperatures and permafrost has revealed to be a privileged geoindicator of climate change effects. Action PERMA_TEMP is one of the action of PERMAdataROC project (co-funded by Interreg III ALCOTRA Program) whose main objective is the development of rock wall instability monitoring strategy.
Radiometric
parameters
Rock surface
energy balance
Spatial modelling
at regional scale
Air temperature &
relative humidity
Rock wall temperature
Air temperature and relative humidity
Solar radiation, wind speed and wind direction
First measurement have been made without using a radiation shield (RS); after assessing the bigb differences shown in the graphs below all air temeperature sensors have been equipped with standard radiation shield.
At Carrel Hut south site (CCS), we installed a combination of a net radiometer (NrLite) and an albedometer (CM7B). Both were installed in vertical position parallel to the rock wall. Measurements at this sites last for more than one year.
At Aiguille du Midì south site (AdMS) we installed an authomatic weather station with a net radiometer (CNR1) which measures all radiometric components (SWin, SWout, LWin, LWout) and a sonic anemometer. All instruments are parallel to rock surface.
60 cm
2 cm
30 cm
Environmental variables
Very weak!!
Action PERMA_TEMP activity
Wind speed
and direction
Transfer
function
Δtime (3-55cm)
Δenergy (3-55cm)
Theoretical
Thermal conductivity
Configuration A
CM7B intercomparison
Air temperature
noRS RS
1. Objective
Rock
Temperature
CNR1
Anemometer
Air
Rock wall
temperature
Signal
processing
The aim of PERMA_TEMP action is the direct measurement of rock wall thermal regimes, in order to asses the effect of global warming, and consequent permafrost degradation, on rock wall stability. Rock temperature data, in association with meteo-climatic and radiative parameters, will be used to calibrate and validate models of rock wall temperature regimes and their regional distribution in high-mountain areas, aiming to obtain a better understanding of the mechanisms triggering rockfall phenomena.
In configuration A sensors cables were exposed to the frost and wind action and many have been damaged. Therefore configuration B has been developed and some comparative tests have been conducted for appraising the differences caused by the new configuration.
CM7B+NrLite
Thermal conductivity variability
evaluation
Relative humidity RS
noRS
In order to asses the uncertainities coming from vertical installation, a one-day intercomparison experiment has been carried out with a CM7B horizontally installed (picture above).
Function of...
Configurations comparison
3 cm
30 cm
55 cm
The anemometer has been oriented so that north corresponds to an ascensional wind.
To install and to maintain this kind of instruments in high mountain environment we have to face big logistical constraints.
Measurements can be affected by ice formation and its permanence during ugly weather days and datalogging has to be guaranteed by properly working batteries.
The effect of vertical installation
Configuration B
CNR1
Anemometer
Topography
&
Morphology
Rock sampling
Thermal conductivity laboratory measure
2. The monitoring sites
more resistant
and reliable!! h
Geological variables
noRS
Future developments RS
Lithology
Geotechnical test
Compressive strength module v
Aiming to cover wide climatic, geological, geomorphological, topographic and altitudinal gradients, five monitoring areas have been equipped in the Mont Blanc massif and the Matterhorn (western Alps). Four of them are located in the Mont Blanc massif (Aiguille du Midì; Les Drus; Col d’Entrèves; Col du Peuterey) and one is the south-west face of Matterhorn. In these five areas a total amount of nine measurement sites are instrumented; in each site, sensors have been placed in different exposition, fracturation density and steepness (snow cover) context, with the purpose to investigate the effect of such variables on rock wall thermal regimes.
Fracturation
density
Structural analysis and statistical elaboration of geologic data
5. Preliminary data series
Rock wall temperature
Radiation
Rock wall temperature
Radiation
Snow / Hoarfrost formation on instruments (see picture) can lead to different kind of errors: some instruments show an interruption of data measurements (i.e. sonic anemometer) while others give unreliable data (i.e. radiometer). In order to invalid data from all the sensors during period of hoarfrost formation and permanence, we used anemometer status report.
The exposition effect
Location of Matterhorn sites
Cheminée
Carrel hut (m3830)
CHEM 2
Cheminée site 1
1 - Carrel hut site (CC)
2 - Cheminée (CHEM)
3 - Oriondè (O)
CCS
The snow cover effect
Matterhorn south face landscape
Carrel Hut
Oriondè
Carrel hut South site
Morpho-structural analysis 3
Around Carrel Hut, 5 morpho-structural measure lines to identify main discontinuity systems and their characteristic of density and persistence have been performed.
The morphology of Matterhorn’s SW ridge is guided by 4 principal discontinuity systems (underlined in picture). The statistic elaboration of structural data underlines that on southern slope the fracturation density and persistence are greater than on the northern one; moreover the compression strenght module, measured by Schmidt hammer test on both expositions, underlines a worse quality of rock on southern slopes. Probably the greater thermal excursion occurring on south exposition and the thicker rock depth interested by this excursion, makes cryogenic alteration processes (frost shattering, frost wedging, ecc… ) on this face more efficient than on northern slope. 1
Aiguille du Midì
Matterhorn
Rock wall thermal gradients at CCW, CCS and CHEM stations.
CCW
ONFR
OFR
July 2006 has been separate in three meteorologically homogeneous periods.
1) 03/ /07: middly warm period.
2) 10/ /08: very warm period.
3) 02/ /08: middly cold period.
The differences among profiles are imputable to elevation, exposure and micro-morphologies variations.
On southern slope thermal excursions at 30 cm depth are 3-4 times greater than northern slope.
Carrel hut North-West site
Oriondé: high and low fractured
Location of Mont Blanc massif sites
Air temperature close to rock-atmosphere interface
LEGEND
Absolute maximum temperature
Maximum mean temperature
Absolute minimum temperature
Minimum mean temperature
Mean temperature
Therefore hypothesizing a downward linear gradient, the 0°C isotherm is attested at 240 cm depth on southern slope and around 75 cm depth on northern slope during very warm period.
On 25 july a middly rockfall is occurred on NW face few dozen meters under CCW logger; it’s interesting to observe that during 10/ /08 period the minimum mean temperature was constantly above 0°C (see the arrow). 4 5
6. Thermographic approach
7. Expected results and developments
(1)
(2)
(3)
(4)
The PERMAdataRoc project will officially end on march 2008; data acquisition will continue for the following years aiming to acquire longer and more meaningful temporal series of such parameters.
Acquired data will be used for validation and calibration of mathematical models for distribution, degradation and temperature evolution of permafrost in global warming context.
Rock temperature data will be treated by signal processing method for appraising rock wall thermal conductivity variability and its relation with some geological variables like schistosity, lithology and fracturation density.
Future application of thermography techniques will provide fundamental information regarding the spatial distribution of rock surface temperature data. 9
4 - Pilier d’angle (CPS)
5 - Aiguille Blanche (CPN)
6 - Les Drus (DRN + DRS)
7 - Aiguille d’Entrèves (CEN)
8 - Tour Ronde (CES)
9 - Aiguille du Midì (AdM)
Beside we show some preliminary results obtained from thermographic analysis (FLIR ThermaCAM P640) at Aiguille du Midi site (single day measurements 05/04/07). The main purpose of this approach is obtain surface temperature distribution and to analyze its spatial variability.
The picture shows the data collected on northern (upper raw) and southern (lower raw) slopes. Images 1 and 2, displayed with the same temperature range give evidence of the temperature difference between the two expositions: northern slope appears entirely below 0°C, while the southern one side is consistently warmer. The enhanced temperature contrast images (3 and 4) underline the surface temperature spatial variability of both expositions.
A first attempt to compare temperature measured by rock temperature sensors and derived by thermographic data is shown in graphic above: preliminary elaborations give encouraging results for future applications of this integrated approach. 8 7 6