, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Contemporary periglacial processes in the Swiss Alps: seasonal, inter-annual and long-term variations

N. Matsuoka & A. Ikeda Institute of Geoscience, University of Tsukuba, Ibaraki, Japan K. Hirakawa & T. Watanabe Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan

ABSTRACT: Comprehensive monitoring of periglacial and mass wasting has been undertaken near the lower limit of the mountain permafrost belt. Seven years of monitoring highlight both seasonal and inter- annual variations. On the seasonal scale, three types of movements are identified: (A) small magnitude events associated with diurnal freeze-thaw cycles, (B) larger events during early seasonal freezing and (C) sporadic events originating from refreezing of meltwater during seasonal thawing. Type A produces pebbles or smaller fragments from rockwalls and shallow (10 cm) frost creep on debris slopes. Types B and C are responsible for larger debris production and deeper (50 cm) frost creep/. Some of these events contribute to perma- nent opening of rock joints and advance of lobes. Sporadic large boulder falls enhance inter-annual variation in rockwall retreat rates. On some debris slopes, prolonged snow melting occasionally triggers rapid flow, which causes inter-annual variation in rates of soil movement.

1 INTRODUCTION 9º40'E 10º00'E Piz Kesch Real-time monitoring of periglacial slope processes is 3418 Switzerland useful to predict ongoing slope instability problems in Inn alpine regions. Such a prediction, however, needs long- Piz d'Err Piz Ot term variations in slope processes caused by climate 3378 3246 Samedan Italia change to be distinguished from inter-annual scale vari- A ations. The latter may mask the long-term trends by St. Moritz Pontresina 46 º 30'E affecting the annual freeze-thaw depth. In addition, par- 1822 46 º 30'E tial melting of permafrost, which could trigger a large mass movement, may result either from an episodic B Piz Bernina warming event or from long-term warming. These situ- Piz Corvatsch 4049 ations call for long-term monitoring of slope processes. 3451 In this context, a monitoring project has been under- Italia taken since 1994 near the lower limit of the mountain 9º40'E 10º00'E permafrost belt in the Upper Engadin, Swiss Alps (Matsuoka et al. 1997, 1998). The monitoring involves Figure 1. Location map. The area A includes the Valletta, measurements of shattering, rock Padella, Büz, TFN and TFS sites and the area B includes the Murtèl site. creep (discussed elsewhere in this volume: Ikeda et al. 2003), soil movement and associated parameters. This paper presents data from the first seven years, focusing on seasonal, inter-annual and long-term variability of stripes. The Padella site (E-facing, ca. 2690 m ASL) is these processes. located on a debris-mantled slope, where numerous The monitoring sites described here include four stone-banked lobes are developed. rockwalls (Murtèl, Büz, TFN, TFS) and two debris slopes (Valletta, Padella) (Fig. 1). The Murtèl site (N-facing, 2890 m ASL) consists of greenschists while 2 ROCKWALL PROCESSES the Büz site (N-facing, 2880 m ASL) consists of shale. TFN and TFS (both 2850 m ASL) are located on the Milimetre-to-decimetre scale bedrock shattering has northern and southern faces of a small peak, respec- been investigated by rock joint opening and debris tively, which consists of massive limestone. The dislocation from painted rock faces. The volume of Valletta site (SW-facing, 2810 m ASL) is located near large boulder falls was also investigated in the thaw- the top of a small hill and displays miniature sorted ing period of 1997.

735 2.1 Rock joint opening 0.1–0.5 mm accompanied seasonal freezing in early winter (type B). Superimposed on type B was opening A crack extensometer connected to a data logger that occurred at the onset of seasonal thawing when recorded automatically the width of a rock joint at 1-h the rock surface was situated in the zero-curtain (type intervals and thermal probes measured rock tempera- C). Snow-melt water would fill the space of the rock tures in the joint (for details see Matsuoka et al. 1997). joint still at a subfreezing temperature and its refreez- Significant movement occurred at the Murtèl rock- ing may cause opening (Matsuoka 2001a). Although wall. Three types of movements (A–C) were identi- the type C events did not recur every year, individual fied (Fig. 2). The repetition of opening and closing of opening amounted to 0.5 mm or more. Furthermore, the order of 102 mm occurred frequently in autumn, this event usually induced permanent opening after accompanying diurnal freeze-thaw cycles (type A). complete thawing, while most of the type A and B Joint width also slightly fluctuated during summer, events were temporary. The mean opening rate over possibly related to wet–dry or warm–cool cycles, but 1994–2001 was ϳ0.1 mm a1 with a significant inter- the opening was much smaller. An expansion of annual variation (Table 1). All three events took place

2.0 (A) Crack movement, 1994-2001 1.5 type C 1.0 type B type A 0.5 0.0 Expansion (mm) -0.5 20 15 (B) Crack-top temperature, 1994-2001 C) º 10 5 0 -5

Temperature ( -10 -15 1-Jan 1-Jan 1-Jan 1-Jan 1-Jan 1-Jan 1-Jan 1-Sep 1-Sep 1-Sep 1-Sep 1-Sep 1-Sep 1-Sep 1-May 1-May 1-May 1-May 1-May 1-May 1-May 1994 1995 1996 1997 1998 1999 2000 2001 Figure 2. Rock joint opening on the Murtèl rockwall, 1994–2001.

Table 1. Summary of periglacial monitoring, 1994–2001. Year 94/95 95/96 96/97 97/98 98/99 99/00 00/01 Murtèl (rockwall, 2890 m ASL) Mean annual rock temperature at 10 cm depth (°C) 1.8 2.5 1.7 1.6 1.9 1.3 0.2 Maximum joint opening (mm) 0.85 0.14 0.52 2.01 0.82 0.29 0.05 Annual joint opening (mm a1) 0.22 0.12 0.00 0.40 0.10 0.08 0.08 Valletta (sorted stripes, 2810m ASL) Mean annual soil temperature at 10 cm depth (°C) 0.0 0.2 0.7 0.8 0.6 0.4 0.9 Seasonal frost depth (cm) ϳ200 200 100 200 ϳ170 200 80 Seasonal frost heave (cm) 0.9 0.6 1.6 0.9 1.2 1.2 2.9 Annual surface movement shown by strain probe (cm a1) – 1.1 0.2 0.6 0.1 1.0 0.8 Annual surface movement shown by painted line (cm a1) –– –––2.4 1.9 Padella (solifluction lobe, 2690 m ASL) Mean annual soil temperature at 10 cm depth (°C) 0.7 0.5 NA NA NA 1.0 1.5 Seasonal frost depth (cm) 170 200 130 180 NA 190 50 Seasonal frost heave (cm) 4.8 5.3 5.1 4.8 NA 5.1 5.5 Annual surface movement shown by painted line (cm a1) –– ––3.7 3.2 1.1 Maximum snow depth (cm) NA NA 150 110 180 NA 320

NA Data not available.

736 mostly when the rock surface temperature reached or et al. 1999). It is most likely that the frequent produc- remained just below 0°C, indicating in-situ freezing tion of pebbles or smaller debris contributes to only as a primary cause of expansion. The type A and B a small part of the long-term rockwall retreat, while events occurred also on the Büz rockwall. occasional boulder falls govern the retreat. In fact, the volume of a large boulder fall from the Murtèl rock- wall during the thawing period of 1997, was equiva- 2.2 Rockfalls lent to the mean long-term retreat rate. The repetition of opening and closing of joints may eventually lead to rockfalls. Such rockfalls were eval- uated from rock fragments detached from 13 painted 3 DEBRIS SLOPE PROCESSES quadrangles (50 cm 50 cm). The volume of frag- ments was measured every summer and their total Automated monitoring of soil movement was con- then converted to annual rockwall retreat for each ducted at the Valletta and Padella sites. Vertical and quadrangle. downslope movements were measured with dilatome- Whereas a south-facing rockwall (TFS) produced ters and strain probes, respectively, and soil tempera- rock debris constantly over 7 years, north-facing rock- ture was monitored at various depths (for details see walls (TFN, Murtèl) showed irregular annual retreat Matsuoka et al. 1997). Data loggers recorded these rates with an extraordinary rate (1.5 mm a1) occur- quantities at 1–3 h intervals. ring once at TFN1 (Fig. 3). This contrast may reflect, at least partly, the type of joint opening. South-facing 3.1 Movement of sorted stripes rockwalls experience numerous short-term freeze- thaw cycles even in mid-winter because of the lack of Figure 4 displays five years of soil movement on sorted snow cover. This condition favours surficial, small stripes at the Valletta site. Mean annual near-surface but frequent joint opening (type A). In contrast, as soil temperatures were close to 0°C (Table 1), which indicated by data at Murtèl, the type B and C events implies the possible presence of permafrost below a probably prevail on north-facing rockwalls. Despite thick . However, permafrost is unlikely to their infrequent occurrence, the latter two events contribute to soil movement, because it lies, if present, produce deeper and more intensive opening, which far below the regolith-bedrock interface at ϳ60 cm may result in greater inter-annual variations in the depth. The ground temperatures showed significant retreat rate. inter-annual variability, such as frequent diurnal fluctu- However, most of the quadrangles showed very ations even in mid-winter (1995/96), the absence of small retreat rates (0.1 mm a 1), despite highly frac- both diurnal and seasonal fluctuations in winter tured nature (joint spacing 10 cm). The retreat rates (1996/97) and in-between conditions in the other are much smaller than the long-term rates estimated winters (Fig. 4C). from the volume of alpine talus cones or rock Three types of frost heave, equivalent to types A–C (e.g. 1–2mma 1 at the Murtèl rockwall: Haeberli of joint opening, were observed on a fine stripe (Fig. 4B). Diurnal frost heave cycles (type A) took place frequently in both autumn and spring. 1.6 Extremely high frequency variations were recorded 1.4 MU6 (Greenschist, N, 60º, when snow cover was lacking in winter (1995/96). J = 4.2 cm) Heaving ( growth) was confined to the top 1.2 TFN1 (Limestone, N, 70º, 5 cm of soil. Despite the small heave amounts J = 5.3 cm) 1 (1 cm), differential heave often took place between TFS2 (Limestone, SE, 75º, the coarse and fine stripes (Matsuoka et al. 2002). The 0.8 J = 2.5 cm) amount of seasonal heave (mostly type B) varied from 0.6 0.6 to 2.9 cm (Table 1). Relatively large heave was Erosion (mm/a) associated with significant winter snow cover. A small 0.4 heave event (type C) occurred during some zero- 0.2 curtain (seasonal thawing) periods. A comparison 0 between soil temperature profiles and seasonal heave 94/95 95/96 96/97 97/98 98/99 99/00 00/01 indicated that ice lenses grew largely within the top Period 20 cm during the type B and C events. Figure 3. Erosion rates of three painted quadrangles on Downslope soil movement mirrored frost heave rockwalls, 1994–2001. Legends involve lithology, aspect, activity (Fig. 4A). Whereas the surficial soil (0–12 cm gradient and mean joint interval (J). deep) responded mainly to the type A heave events,

737 5 Displacement (cm) 4 -1 0134 2 Profile by A. Deformation of strain probe 0 cm 12 cm 0 excavation on 24 cm 36 cm 3 10 24 July 2001 20 2 30 1 Depth (cm) 40

Displacement (cm) 0 -1

2 B. Frost heave 1 0

Heave (cm) -1 -2

40 C. Soil temperature 0 cm 30 30 cm 20 150 cm 10 0 Temperature ( º C) -10 -20 -00 g 1-Oct-95 1-Apr-96 1-Oct-96 1-Apr-97 1-Oct-97 1-Apr-98 1-Oct-98 1-Apr-99 1-Oct-99 1-Apr-00 1-Jun-96 1-Jun-97 1-Jun-98 1-Jun-99 1-Jun-00 1-Feb-96 1-Feb-97 1-Feb-98 1-Feb-99 1-Feb-00 1-Dec-95 1-Dec-96 1-Dec-97 1-Dec-98 1-Dec-99 1-Aug-95 1-Aug-96 1-Aug-97 1-Aug-98 1-Aug-99 1-Au

Figure 4. Soil movement (downslope and vertical) and temperatures on sorted stripes, Valletta site, 1995–2000.

the subsoil (12–24 cm deep) moved with annual frost 1 cm. The seasonal heave (mostly type B) was much heave cycles (types B, C). As a result, the velocity larger than the type A events, amounting to ϳ5cm profile was concave downslope (see the inset in regardless of the seasonal frost depth (Table 1). A Fig. 4A), which suggests the primary role of diurnal large part of the type B heave occurred before the frost creep (cf. Matsuoka 2001b). Such a shallow frost table reached a depth of 50 cm, suggesting that movement mainly reflects the thin fine debris layer. ice lenses formed largely within the top 50 cm. A type The mean surface velocity was ϳ0.5 cm a1 over 6 C heave event was sometimes superimposed on the years, with the highest value of 1.1 cm a1 in the first type B (e.g. in 1998), but the magnitude was much year (Table 1). smaller than the latter (Fig. 5B). Although monitoring with a strain probe was suc- cessful only in the 1998–2000 period, the available 3.2 Movement of solifluction lobes data indicated a nearly constant surface velocity (2–3cma1), a maximum depth of movement of Results of monitoring on a solifluction lobe at the 40 cm, and a straight-to-concave downslope velocity Padella site are summarized in Table 1 and Figure 5. profile (Fig. 5A). These characteristics were consistent This site recorded 0.6–0.7°C higher mean annual soil with manual measurements with painted lines and temperatures than the Valletta site. Permafrost is prob- flexible tubes. The velocity profile mainly reflected ably absent, but seasonal frost penetrates deeply, with seasonal , suggesting the primary role of a large inter-annual variation in depth (50–200 cm) annual frost creep and gelifluction (cf. Matsuoka according to snow conditions. 2001b). Such a deeper movement in comparison with Short-term frost heave cycles (type A) occurred in that at the Valletta site reflects the thick fine sediment autumn, but they were rare in spring because of the (2 m) at the foot slope location. late-lying snow cover (Fig. 5B). A large heave event A different type of movement took place on another (5.6 cm) occurred in early October 1999, which lobe located 50 m upslope of the Padella site, where accompanied short-term freezing to a depth of 20 cm. prolonged water supply from a late-lying snow patch With this exception, the type A heave rarely exceeded can raise soil water content above a critical level for a

738 Profile by excavation A. Deformation of strain probe Displacement (cm) 6 on 1 Aug. 2000 -2 0 2 4 6 8 0 4 10 0 cm 20 2 12 cm Logging start 30 24 cm Depth (cm) 40 Displacement (cm) 0 36 cm 50

-2

6 B. Frost heave 4 2 0 Data not available Frame bent by in 1998-1999 snow pressure Heave (cm) -2 -4

30 C. Soil temperature 0 cm 20 30 cm Data not available 225 cm 10 in 1998-1999

0

Temperature ( º C) -10

-20 1-Apr-01 1-Apr-00 1-Oct-00 1-Apr-99 1-Oct-99 1-Apr-98 1-Oct-98 1-Oct-97 1-Jun-01 1-Jun-00 1-Jun-99 1-Jun-98 1-Feb-01 1-Feb-00 1-Feb-99 1-Feb-98 1-Dec-00 1-Dec-99 1-Dec-98 1-Dec-97 1-Aug-01 1-Aug-00 1-Aug-99 1-Aug-98 1-Aug-97 Figure 5. Soil movement (downslope and vertical) and temperatures on a solifluction lobe, Padella site, 1997–2001.

Distance (cm) 0 50 100 150 200 250 300 350 400 4 SEASONAL, INTER-ANNUAL AND -20 LONG-TERM VARIATIONS 0 The seven years of monitoring have highlighted tem- 20 poral changes in alpine periglacial processes, both on 40 seasonal and inter-annual time scales. Firstly, on the seasonal time scale, three types of 60 frost action operate during different periods. The 80 95/96 type A action originates from short-term freeze-thaw 96/97 cycles which take place mainly before seasonal freez- 100 Displacement (cm) 97/98 ing and after seasonal thawing. Despite low magni- 120 98/99 tude, its high frequency, regularity and ubiquity allow 99/00 140 type A to dominate over a wide area and to promote 00/01 surficial but rapid movement. Such a movement is 160 probably responsible for pebble falls and small-scale Figure 6. Annual surface movement on a solifluction . Seasonal freezing involves the type lobe, upslope of the Padella site, 1995–2001. B (accompanying frost penetration in early winter) and C (generated by refreezing of melt water) actions. rapid flow. In fact, painted lines demonstrated the On thawing, these two types combine to induce occurrence of a rapid soil flow, with downslope move- deeper and more voluminous movement and, in ment of about 100 cm, during the thawing periods of terms of the erosion rate, much more important than 1996 and 2000 (Fig. 6). type A.

739 Secondly, three types of movements are subject to Dramis, F., Govi, M., Gugliemin, M. & Mortara, G. 1995. inter-annual variation. The annual frequency of type A Mountain permafrost and slope instability in the Italian events varies mainly with the duration of the snow- Alps: the Val Pola landslide. Permafrost Periglac. cover period, and this variation may affect the annual Process. 6: 73–82. movement. The inter-annual variability of type B and Haeberli, W. 1996. On the morphodynamics of ice/debris- transport systems in cold mountain areas. Norsk C events depends on the seasonal frost (or thaw) depth Geogr. Tidsskr. 50: 3–9. and snow conditions. The amounts of seasonal frost Haeberli, W., Wegmann, M. & Vonder Mühll, D. 1997. heave and solifluction may significantly change in Slope stability problems related to glacier shrinkage regions with cold permafrost, where upward freezing and permafrost degradation in the Alps. Eclogae Geol. from the permafrost table causes ice segregation near Helvetiae 90: 407–414. the base of the active layer (e.g. Lewkowicz & Clarke Haeberli, W., Kääb, A., Wagner, S., Vonder Mühll, D., 1998). In contrast, inter-annual variation in the Geissler, P., Haas, J.N., Glatzel-Mattheier, H. & 14 solifluction rate is likely to be small in regions with Wagenbach, D. 1999. Pollen analysis and C age of deep seasonal frost or warm permafrost like the study moss remains in a permafrost core recovered from the active Murtèl-Corvatsch, Swiss Alps: geo- sites, because frost heave and solifluction occur largely morphological and glaciological implications. Jour. within the top 50 cm of soil regardless of the seasonal Glaciol. 45: 1–8. frost (or thaw) depth. In such an environment, inter- Harris, S.A. 2001. Twenty years of data on climate- annual variation in soil movement depends mainly on permafrost-active layer variations at the lower limit of prolonged snow melting that triggers a rapid soil flow. alpine permafrost, Marmot Basin, Jasper National The annual rockwall retreat would change consider- Park, Canada. Geogr. Ann. 83A: 1–13. ably with the occurrence of a large boulder fall, which Ikeda, A., Matsuoka, N. & Kääb, A. 2003. A rapidly mov- requires a number of annual freeze-thaw cycles and ing small rock glacier at the lower limit of the moun- removal of the underlying smaller clasts (Matsuoka & tain permafrost belt in Swiss Alps. In this volume. Sakai, 1999). Lewkowicz, A.G. & Clarke, S. 1998. Late-summer solifluc- tion and active layer depths, Fosheim Peninsula, The final remark is on the effect of long-term Ellesmere Island, Canada. In A.G. Lewkowicz & climate change on periglacial processes. Indeed, a M. Allard (eds.), Proc., 6th Intern. Conf. Permafrost, number of monitoring/modelling projects have been Yellowknife, Canada: 641–666. Centre d’études concerned with the influence of global warming on nordiques, Univ. Laval: Sainte-Foy. alpine periglacial processes (e.g. Haeberli 1996, Matsuoka, N. 2001a. Direct observation of frost wedging in Wegmann & Gudmundsson 1999, Davies et al. 2001). alpine bedrock. Earth Surf. Process. Landforms 26: Long-term warming results in thinning of the sea- 601–614. sonal frost depth in non-permafrost sites. This poten- Matsuoka, N. 2001b. Solifluction rates, processes and land- tially impedes frost-induced processes, where ice forms: a global review. Earth-Sci. Rev. 55: 107–134. segregation occurs in the lower part of the seasonal Matsuoka, N. & Sakai, H. 1999. Rockfall activity from an alpine cliff during thawing periods. Geomorphology frost. Warming would affect more significantly the 28: 309–328. active layer processes in permafrost sites, particularly Matsuoka, N., Abe, M. & Ijiri, M. 2002. Differential frost where melting of ice lenses accompanies the active heave and sorted patterned ground: field measure- layer deepening. On a time scale of decades or shorter, ments and a laboratory experiment. Geomorphology: however, inter-annual fluctuation or an episodic warm in press. year may often hinder such a long-term trend, because Matsuoka, N., Hirakawa, K., Watanabe, T. & Moriwaki, K. it may involve temporary melting of the uppermost 1997. Monitoring of periglacial slope processes in the few decimetres of permafrost. In contrast, except Swiss Alps: the first two years of frost shattering, where permafrost is very thin, inter-annual fluctuation heave and creep. Permafrost Periglac. Process. 8: 155–177. or an episodic event may not result in basal melting of Matsuoka, N., Hirakawa, K., Watanabe, T., Haeberli, W. & permafrost (cf. Harris 2001), which potentially desta- Keller, F. 1998. The role of diurnal, annual and millen- bilizes a decametre-thick material (e.g. Dramis et al. nial freeze-thaw cycles in controlling alpine slope 1995, Haeberli et al. 1997). instability. In A.G. Lewkowicz & M. Allard (eds.), Proc., 7th Intern. Conf. Permafrost, Yellowknife, Canada: 711–717. Centre d’études nordiques, Univ. REFERENCES Laval: Sainte-Foy. Wegmann, M. & Gudmundsson, G.H. 1999. Thermally Davies, M., Hamza, O. & Harris, C. 2001. The effect of rise induced temporal strain variations in rock walls in mean annual temperature on the stability of rock observed at subzero temperatures. Proc. 6th Intern. slopes containing ice-filled discontinuities. Permafrost Symp. Thermal Engineering and Sciences for Cold Periglac. Process. 12: 137–144. Regions, 511–518. Springer-Verlag: Heidelberg.

740