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