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Contemporary Periglacial Processes in the Swiss Alps: Seasonal, Inter-Annual and Long-Term Variations

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Contemporary Periglacial Processes in the Swiss Alps: Seasonal, Inter-Annual and Long-Term Variations Permafrost, 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 weathering 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 (Ͻ10 cm) frost creep on debris slopes. Types B and C are responsible for larger debris production and deeper (Ͻ50 cm) frost creep/gelifluction. Some of these events contribute to perma- nent opening of rock joints and advance of solifluction lobes. Sporadic large boulder falls enhance inter-annual variation in rockwall retreat rates. On some debris slopes, prolonged snow melting occasionally triggers rapid soil 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 30'E 30'E º St. Moritz ations. The latter may mask the long-term trends by Pontresina º 46 affecting the annual freeze-thaw depth. In addition, par- 1822 46 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 bedrock shattering, rock glacier 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 10Ϫ2 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 aϪ1 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) Ϫ1.8 Ϫ2.5 Ϫ1.7 Ϫ1.6 Ϫ1.9 Ϫ1.3 0.2 Maximum joint opening (mm) 0.85 0.14 0.52 2.01 0.82 0.29 0.05 Annual joint opening (mm aϪ1) 0.22 Ϫ0.12 0.00 0.40 0.10 0.08 Ϫ0.08 Valletta (sorted stripes, 2810m ASL) Mean annual soil temperature at 10 cm depth (°C) 0.0 Ϫ0.2 0.7 0.8 0.6 Ϫ0.4 0.9 Seasonal frost depth (cm) ϳ200 Ͼ200 100 Ͼ200 ϳ170 Ͼ200 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 aϪ1) – 1.1 0.2 0.6 Ϫ0.1 1.0 Ϫ0.8 Annual surface movement shown by painted line (cm aϪ1) –– –––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 Ͻ5.5 Annual surface movement shown by painted line (cm aϪ1) –– ––3.7 3.2 1.1 Maximum snow depth (cm) NA NA Ͼ150 110 Ͼ180 NA Ͼ320 NA ϭ 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 ϫ 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 aϪ1) 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 active layer. 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.
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