Ninth International Conference on Permafrost ... - IARC Research

Liquid Water Destabilizes Frozen Debris Slope at the Melting Point:A Case Study of a Rock Glacier in the Swiss AlpsAtsushi Ikeda, Norikazu MatsuokaGraduate School of Life and Environmental Sciences, University of TsukubaIntroductionPositive correlation between temperatures and surfacevelocities of rock glaciers in various time scales indicatesthat ground warming accelerates rock glaciers (e.g., Kääbet al. 2007). In particular, rock glaciers lying close to thelower limit of local permafrost occurrence show highsurface velocities (>0.2 m a −1 ) and significant accelerationwhich appears to follow seasonal to decadal warming trends(Frauenfelder et al. 2003, Roer et al. 2005, Kääb et al. 2007,Delaloye et al. 2008). Such acceleration has been attributedto gradual heat conduction from the ground surface to thecreeping layer lying below several meters’ depth. In thisreport, we propose another model of acceleration by waterinfiltration, based on in situ monitoring of permafrost creepat the melting point (see also Ikeda et al. 2008).Study SiteThe studied rock glacier is the upper lobe of Büz Northrock glacier (BNU), located on the northeastern slope of apeak named Piz dal Büz (lat. 46°32′N, long. 9°49′E, 2955m a.s.l.) in the Upper Engadin, Switzerland (see also Ikeda& Matsuoka 2006 for detailed information). BNU originatesfrom the foot of a talus slope at 2840 m a.s.l. and terminatesat 2810 m a.s.l. The horizontal dimension (70 m long and120 m wide) represents almost the minimum size identifiedas a rock glacier. The steep frontal slope is 10 m high, slopingat 35°. The upper surface is smooth and the average slopeangle is 25°.A pit and borehole indicated that the major componentsof BNU are platy shale pebbles and cobbles, the intersticesof which are partly filled with sand and silt. Below the frosttable, the debris was entirely ice-cemented to the bottom ofthe hole at 5.4 m depth. The gravimetric ice content of boreholecores (c. 10 cm long) was 50% at 4 m depth and 28%at 5 m depth.MethodsInclinometers (BKJ-A-10-D, manufactured by KyowaElectronic Instruments, Japan) 35 cm long and 2.7 cm indiameter, installed at 4 m and 5 m depths in the borehole onAugust 9, 2000, measured the deformation of the perenniallyfrozen debris continuously. Each inclinometer sensedinclinations along two directions perpendicular to each otherwithin ±12.2° from the vertical with a resolution of 0.005°.The inclinations were recorded at 3 h or 6 h intervals by adatalogger until August 2, 2007. Ground temperatures (0.1°Cresolution) at depths of 0, 0.5, 1, 2, 3, 4, and 5 m were alsomonitored for the same period.Downslope inclination at a certain depth was calculatedfrom the sum of the horizontal vectors, which were definedas the tangents of the measured inclinations for the twoaxes. Strain rates (i.e., vertical velocity gradients) at the twodepths were also calculated. Note that inclination for one ofthe two axes at 5 m depth exceeded the measurement limit onFebruary 3, 2003. From the day to July 2005, the inclinationat 5 m depth was estimated from the measured inclination forthe other axis using the former linear relationship betweenthe two-axes’ inclinations (r 2 = 0.9995).Findings and a Presumed ModelBoth inclinometers showed fast continuous deformation(on average, 2.4° a −1 and 6.0° a −1 ) with large seasonal andinterannual variations, while the permafrost temperaturesremained almost at the melting point (Fig. 1). The movementof the inclinometers coincided with interannual changes inthe surface velocities (Ikeda et al. in press). The strain ratesat 5 m depth always surpassed those at 4 m depth, both ofwhich had parallel patterns of seasonal variations. The strainrates rapidly increased in the snowmelt periods, indicatedby the constant surface temperature at 0°C in early summer.In contrast, the strain rates gradually decreased below adry snow cover in winter, except for the 2000–01 winter.When the freezing index at the ground surface was small,the decrease in strain rate tended to be small (or even therate slightly increased in the 2000–01 winter), and the strainrate remained at a large value at the end of the dry snowperiod (Fig. 2). The highest strain rates at both depths wererecorded when the unusually thick snow cover in 2000–01was melting, whereas the net increases in the strain ratesduring a snowmelt period were smallest after the extremelysnowless 2001–02 winter. In addition, the magnitudes ofthe acceleration during the snowmelt periods appeared tocorrelate with the shearing strain (i.e., net deformation) inthe preceding dry snow periods.These phenomena suggest that the frozen debris ispermeable to water (mostly from snow melting), althoughice-saturated debris is generally regarded as impermeable.The large annual strain rates (>0.1 a −1 at 5 m depth) of thecoarse debris filled with ice probably result from thrustingup of debris particles over underlying particles within oneyear. The resulting dilatant deformation probably creates airvoids in the frozen debris, a network of which eventuallyallows water infiltration. The water infiltration acceleratesthe deformation by reducing effective stress. The refreezingof the pore water, which depends on the cooling intensityin winter, decelerates the deformation. The combination ofthese processes, possibly affected by different amounts ofannually developed air voids and available snowmelt water,controls the temporal variations in the deformation.109