Elsevier

Geomorphology

Volume 371, 15 December 2020, 107456
Geomorphology

Interpreting rockfall activity on an outcrop–talus slope system in the southern Japanese Alps using an integrated survey approach

https://doi.org/10.1016/j.geomorph.2020.107456Get rights and content

Highlights

  • Seasonal rockfall activity was interpreted with a suite of survey approaches.

  • The timing of rockfall was different between talus slopes and outcrops.

  • Rockfall from outcrops was active during winter due to freeze-thaw cycles.

  • Rockfall also originated from the redistribution of sediment on talus slopes.

Abstract

Understanding relationships between meteorological conditions and rockfall activity is important for the management of rockfall risks as well as for the estimation of future rockfall activity under climate change. Seasonal activity of rockfall on talus slopes depends on the actual triggers of rockfall, which in turn may vary as well between outcrops because of different rockfall sources and the influence of topography. In this study, we interpret spatial and temporal variations in rockfall activity and related sediment transfer processes in an outcrop-talus slope system in central Japan through monitoring using rock temperature and moisture sensors, sediment traps, and time lapse cameras. In addition, tree-ring analysis has been added to understand possible changes of rockfall activity over longer time periods. Results suggest that the frequency of grouped gravel transport (i.e., soil creep) was higher than that of individual transport (i.e., rockfall and dry ravel) on the talus slope, whereas rockfall likely is the predominant process from the outcrops. Observations from sediment traps showed that coarse gravel is characterized by higher bounce heights and that it travelled longer distances than fine gravel. Rockfall from the outcrops was most active during dormancy of trees (which locally lasts from autumn to early spring) and can be related to freeze-thaw cycles. In contrast, rockfall and other sediment transfer processes were monitored on the talus slope all year round. Secondary movement of gravels on the talus slope, such as soil creep during rainfall events, likely triggers rockfall during times when the outcrops are not producing rockfall (i.e. from late spring to summer). Consequently, our results suggest that the spatio-temporal rockfall activity varies substantially between outcrops and talus slopes and that differences in process behavior should be taken into account in future disaster mitigation measures.

Introduction

Rockfall is considered one of the most frequent, but also most damaging natural hazard process in mountain regions (Allen and Huggel, 2013; Mourey et al., 2019). Rockfall is triggered mainly by climatic factors, such as freeze-thaw cycles, rainfall, and/or wind (Gruber et al., 2004a; Stoffel and Huggel, 2012; Delonca et al., 2014; Collins and Stock, 2016; D'Amato et al., 2016; Pratt et al., 2019); but also by non-climatic drivers such as earthquakes (Heron et al., 2014; Valagussa et al., 2014; Stoffel et al., 2019). Due to the seasonal changes in meteorological conditions, including temperature and precipitation, rockfall activity is often constrained to a short time period of the year (Matsuoka and Sakai, 1999; Sass, 2005a; Stoffel et al., 2005; Schneuwly and Stoffel, 2008; Thapa et al., 2017; Pratt et al., 2019). However, in mountain regions, rockfall activity in general but also its seasonality could change as a result of the current and future climate warming (Pepin et al., 2015). Therefore, understanding rockfall-climate linkages is of paramount importance for the implementation of strategies that will allow a proper rockfall risk management under current and future climate conditions (Paranunzio et al., 2016; Moos et al., 2019).

The primary sources of rockfall are often formed by outcrops on steep hillslopes (Matsuoka and Sakai, 1999; Gruber et al., 2004a; Strunden et al., 2015; Collins and Stock, 2016). Outcrops are frequently exposed to physical and biological weathering processes (e.g., freeze-thaw cycles, temperature changes, expansion of cracks by tree roots) which in turn promote the production of debris and the release of rockfall (Matsuoka and Sakai, 1999; Gruber et al., 2004b; Gunzburger et al., 2005; Dorren et al., 2007). Boulders released from outcrops immediately gain high kinematic energy on the usually steep terrain (Dorren, 2003; Dunham et al., 2017), resulting in long travel distances and destructive power that can provoke severe damage (Michoud et al., 2012; Wei et al., 2014). Talus slopes, defined as terrains formed by the accumulation of rockfall debris from farther upslope, are often exposed to high rockfall activity. Although rockfalls originating from the outcrop is one of the most important sediment transfer process on talus slopes (Trappmann et al., 2013; Corona et al., 2017; Thapa et al., 2017), the redistribution of deposits, which previously originated from outcrops, can occur on talus slopes as well (Krautblatter and Moser, 2009; Veilleux et al., 2020). Thus, the timing of rockfall from outcrops and that originating from the redistribution of sediments on talus slopes could be different, because of the dissimilar triggering factors involved in the two processes in terms of topography and/or composing material. So far, most rockfall studies have focused on rockfall from outcrops, without taking into consideration the sediment redistribution on talus slopes (Sass and Krautblatter, 2007; Krautblatter and Moser, 2009; Moya et al., 2010).

Because slope gradients of talus slopes often are close to the angle of repose, sediments can sometimes travel in the form of rockfall without water supply (Carson, 1977; Imaizumi et al., 2017a). Sediment on talus slopes is also transported by freezing-thawing processes and water supply in the form of solifluction and debris flows, respectively (Sass and Krautblatter, 2007; Owkzarek, 2010). Therefore, sediment transfer processes other than rockfall need to be interpreted as well if one aims at understanding sediment transfer activity on the talus slope comprehensively.

The timing of rockfall activity has been addressed repeatedly in past research and was interpreted using various methods. Tree-ring analysis can identify long histories (>100 years) of rockfall activity (Stoffel et al., 2005, Stoffel et al., 2011; Perret et al., 2006; Šilhán et al., 2012) as it relies on the dating of growth disturbances in tree-ring series that were caused to trees through the impact of rock fragments (Trappmann et al., 2013). The quality and length of records obtained by this method depend on species, age, and location of affected trees (Trappmann et al., 2014). Sediment traps can measure rockfall amounts directly (Krautblatter and Moser, 2009; Imaizumi et al., 2015, Imaizumi et al., 2017b). The temporal resolution of rockfall activity can be improved further by increasing the sampling frequency, for instance, using time-lapse photography at daily or sub-daily scales (e.g., Matsuoka, 2019). In this case, however, large efforts are needed to keep monitoring active over longer periods. Periodical geomorphometry (e.g., terrain laser scanning, structure from motion assessments) can help in the interpretation of the timing and location of rockfall activity as well (Abellán et al., 2009; Gigli et al., 2014). Although the spatial resolution of geomorphometry has been improved substantially (Strunden et al., 2015), these approaches cannot be applied over a wide area in cases where the terrain is covered by dense vegetation. In addition, simulations with rockfall models have been developed, and have contributed to successfully explain real rockfall events (Dorren, 2003; Haas et al., 2012; Corona et al., 2017). However, simulated rockfall frequencies will not likely agree with real rockfall records unless the model is properly calibrated (Trappmann et al., 2014). Hence, each interpretation of rockfall activity has clear advantages but also some limitations. A combination of multiple methods is therefore needed to overcome the limitations inherent to each method (Trappmann et al., 2014; Matsuoka, 2019).

The purpose of this study therefore is to provide new insights on the seasonal timing of rockfall and related sediment transfer processes within an outcrop-talus slope system. Here, we observed rockfall activities with an integrated field monitoring approach on a mountain slope located in the southern Japanese Alps, where climatic conditions are characterized by high humidity in summer and intense frost in winter. As a result, high rockfall activity has been observed from fractured bedrock in steep terrain (Imaizumi et al., 2017b; Matsuoka, 2019). We gathered a unique dataset that combines long-term seasonal records of rockfall and related sediment transfer activity from tree-ring series for over twenty years with shorter, yet highly-resolved data derived from the monitoring of rock temperature and moisture, time-lapse photography, and monitoring with sediment traps. On the basis of this broad dataset, we then interpret seasonal differences in rockfall and other sediment transfer processes between the outcrops and the talus slopes. We also discuss advantages and disadvantages of methods by comparing data obtained with different monitoring approaches.

Section snippets

Study site

We conducted field observations in the Ikawa University Forest of the University of Tsukuba, located at the southern end of the southern Japanese Alps (Fig. 1). Average annual precipitation at the site is 2800 mm for the period 1993–2002 and under the influence of the East Asian Monsoon (Imaizumi et al., 2010). Heavy rainfalls (total amount > 100 mm) occur during the “Baiu” rainfall season (i.e. June and July) and during typhoon activity (from August to early October). Winter snowfall occurs

Monitoring of climate and ground conditions

Bedrock thermal and hydrological conditions were investigated at sandstone outcrops in the upper part of site S (just above SU: Fig. 1b). The top 10 mm of a thermistor probe (2.2 mm in diameter) was inserted in an open crack (5–7 mm wide at the surface) in the bedrock and fixed with silicone rubber. The measured data represent rock surface (crack-top) temperature recorded at hourly intervals in a miniature data logger (TR52i logger, T & D corporation) with a precision of 0.3 °C. Continuous data

Rock temperature and moisture at outcrops

Rock surface temperature in the outcrop area showed similar temporal variation to the air temperature (Fig. 5). Freeze-thaw activity at the rock surface was high in the period from late December to early April, because rock surface temperature rose above and fell below 0 °C several tens of times during winter (Fig. 5b). The freeze-thaw cycles at the surface mostly occurred diurnally, but sometimes subzero temperatures continued for several days in January and February (Fig. 6a), indicating

Sediment transport type

Multiple types of sediment transfer processes, including rockfall, dry ravel, and soil creep, have been documented by on-site monitoring with sediment traps and time-lapse cameras (TLCs) at Ikawa forest, southern end of the southern Japanese Alps. In addition to the transportation of individual, isolated sediment, talus slopes also experienced grouped transportation (Sass and Krautblatter, 2007; Owkzarek, 2010), for instance through soil creep monitored with TLCs. In the present case, the

Conclusions

Activity of rockfall and other sediment transfer processes on an outcrop-talus slope system was interpreted for a site in the southern Japanese Alps by combining a broad suite of methods, including tree-ring analysis, the monitoring of rockfall by sediment traps, and time-lapse photography. Observations using sediment traps indicate that coarse gravels attain higher jump heights and show longer travel distance than fine gravels. Our monitoring also revealed that the seasons with active rockfall

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This study was supported by JSPS Grant Numbers 17H02029, 18H02235, and 19K01156. The monitoring site was provided by Kato-shoji. Rainfall data used in this study was provided by the Ikawa University Forest, University of Tsukuba. We thank the staff of the Ikawa University Forest, Yosuke Yamakawa, Yoshikazu Endo, and Yusuke Ueji, who supported our field work. Field surveys were supported by Sota Nakano, Masahiro Masumoto, and Okabe Shinya, who were students in Faculty of Agriculture, Shizuoka

References (72)

  • J. Moya et al.

    Tree-ring based assessment of rockfall frequency on talus slopes at Solà d’Andorra, Eastern Pyrenees

    Geomorphology

    (2010)
  • Y. Okura et al.

    The effects of rockfall volume on runout distance

    Eng. Geol.

    (2000)
  • O. Sass et al.

    Debris flow-dominated and rockfall-dominated talus slopes: genetic models derived from GPR measurements

    Geomorphology

    (2007)
  • K. Šilhán et al.

    Tree-ring analysis in the reconstruction of slope instabilities associated with earthquakes and precipitation (the Crimean Mountains, Ukraine)

    Geomorphology

    (2012)
  • M. Stoffel et al.

    Reconstructing past rockfall activity with tree rings: some methodological considerations

    Dendrochronologia

    (2006)
  • P. Thapa et al.

    Quantification of controls on regional rockfall activity and talus deposition, Kananaskis, Canadian Rockies

    Geomorphology

    (2017)
  • A. Valagussa et al.

    Earthquake-induced rockfall hazard zoning

    Eng. Geol.

    (2014)
  • S. Veilleux et al.

    Talus slope characterization in Tasiapik Valley (subarctic Québec): evidence of past and present slope processes

    Geomorphology

    (2020)
  • G.T. Vieira et al.

    Ground temperature regimes and geomorphological implications in a Mediterranean mountain (Serra da Estrela, Portugal)

    Geomorphology

    (2003)
  • L.W. Wei et al.

    The mechanism of rockfall disaster: a case study from Badouzih, Keelung, in northern Taiwan

    Eng. Geol.

    (2014)
  • A. Abellán et al.

    Detection of millimetric deformation using a terrestrial laser scanner: experiment and application to a rockfall event

    Nat. Hazards Earth Syst. Sci.

    (2009)
  • R.S. Anderson

    Near-surface thermal profiles in alpine bedrock: implications for the frost weathering of rock

    Arct. Alp. Res.

    (1998)
  • M.A. Carson

    Angle of repose, angle of shearing resistance and angle of talus slopes

    Earth Surf. Process.

    (1977)
  • B.D. Collins et al.

    Rockfall triggering by cyclic thermal stressing of exfoliation fractures

    Nat. Geosci.

    (2016)
  • J. D'Amato et al.

    Influence of meteorological factors on rockfall occurrence in a middle mountain limestone cliff

    Nat. Hazards Earth Syst. Sci.

    (2016)
  • T. Dambara

    Synthetic vertical movements in Japan during the recent 70 years

    J. Geod. Soc. Jpn.

    (1971)
  • F.V. De Blasio et al.

    Properties of talus slopes composed of flat blocks

    Norsk Geografisk Tidsskrift–Norwegian J. Geogr.

    (2010)
  • A. Delonca et al.

    Statistical correlation between meteorological and rockfall databases

    Nat. Hazards Earth Syst. Sci.

    (2014)
  • L.K.A. Dorren

    A review of rockfall mechanics and modelling approaches

    Prog. Phys. Geogr.

    (2003)
  • L. Dorren et al.

    State of the art in rockfall–forest interactions

    Schweizerische Zeitschrift fur Forstwesen

    (2007)
  • M.C. Eppes et al.

    Deciphering the role of solar-induced thermal stresses in rock weathering

    Geol. Soc. Amer. Bull.

    (2016)
  • E.J. Gabet

    Sediment transport by dry ravel

    J. Geophys. Res.

    (2003)
  • G. Gigli et al.

    Terrestrial laser scanner and geomechanical surveys for the rapid evaluation of rock fall susceptibility scenarios

    Landslides

    (2014)
  • S. Gruber et al.

    Permafrost thaw and destabilization of Alpine rock walls in the hot summer of 2003

    Geophys. Res. Lett.

    (2004)
  • S. Gruber et al.

    Rock-wall temperatures in the Alps: modelling their topographic distribution and regional differences

    Permafr. Periglac. Process.

    (2004)
  • F. Haas et al.

    Runout analysis of a large rockfall in the Dolomites/Italian Alps using LIDAR derived particle sizes and shapes

    Earth Surf. Process. Landf.

    (2012)
  • Cited by (10)

    • Power law models for rockfall frequency-magnitude distributions: review and identification of factors that influence the scaling exponent

      2022, Geomorphology
      Citation Excerpt :

      Second, rockfall frequency and volume data can be collected by recording new rockfalls as they occur by monitoring rock slopes. One option is to use rockfall collectors to catch and measure fallen material directly (Imaizumi et al., 2020; Krautblatter et al., 2012; Heckmann et al., 2016). In this case, rockfalls are recorded by making repeat site visits for frequent measurements of size and frequency.

    • Contribution of dendrogeomorphology to the dating of secondary processes on dormant rockslides

      2022, Quaternary Geochronology
      Citation Excerpt :

      Intense summer rainfall was also identified as a rockfall trigger by Mainieri et al. (2020) in the French Alps. Interestingly, we did not observe any relationship to freeze-thaw cycles, as observed in various mountain environments worldwide (Šilhán et al., 2011; Stoffel et al., 2019; Imaizumi et al., 2020). This fact could have, however, been caused by the temperature measurements (ca. 30 km from the studied rockslides), which did not exactly represent the specific microclimate of the study site.

    • Improved tree-ring sampling strategy enhances the detection of key meteorological drivers of rockfall activity

      2021, Catena
      Citation Excerpt :

      To overcome this lack of historical archives, rockfall frequency and failure have been estimated in the past using sediment traps (Sass, 2005), terrestrial laser scans (Rabatel et al., 2008; D’Amato et al., 2016), or time-lapse cameras (Kellerer-Pirklbauer and Rieckh, 2016). Even though these methods allowed investigation of specific events (Matsuoka, 2019; Imaizumi et al., 2020), detailed rockfall monitoring efforts rarely exceed a few years at a specific site (Weber et al., 2019) and thus fail to encompass the typically wide range of rockfall processes on a slope (Sass, 2005). On forested slopes, tree damage and its dating with dendrogeomorphic approaches have been used widely to reconstruct past process activity (Stoffel and Corona, 2014).

    View all citing articles on Scopus
    View full text