Abstract
A growing body of research indicates that rock slope failures, particularly from exfoliating cliffs, are promoted by rock deformations induced by daily temperature cycles. Although previous research has described how these deformations occur, full three-dimensional monitoring of both the deformations and the associated temperature changes has not yet been performed. Here we use integrated terrestrial laser scanning (TLS) and infrared thermography (IRT) techniques to monitor daily deformations of two granitic exfoliating cliffs in Yosemite National Park (CA, USA). At one cliff, we employed TLS and IRT in conjunction with in situ instrumentation to confirm previously documented behavior of an exfoliated rock sheet, which experiences daily closing and opening of the exfoliation fracture during rock cooling and heating, respectively, with a few hours delay from the minimum and maximum temperatures. The most deformed portion of the sheet coincides with the area where both the fracture aperture and the temperature variations are greatest. With the general deformation and temperature relations established, we then employed IRT at a second cliff, where we remotely detected and identified 11 exfoliation sheets that displayed those general thermal relations. TLS measurements then subsequently confirmed the deformation patterns of these sheets showing that sheets with larger apertures are more likely to display larger thermal-related deformations. Our high-frequency monitoring shows how coupled TLS and IRT allows for remote detection of thermally induced deformations and, importantly, how IRT could potentially be used on its own to identify partially detached exfoliation sheets capable of large-scale deformation. These results offer a new and efficient approach for investigating potential rockfall sources on exfoliating cliffs.
Highlights
Rock slope failure leading to rockfall is a dynamic erosional process that controls the evolution of many landscapes and, in particular, steep bedrock formations (Varnes 1978; Hutchinson 1988; Evans and Hungr 1993; Hungr et al 1999; Rosser et al 2005; Rabatel et al 2008; Stock and Uhrhammer 2010; Krautblatter et al 2012; Janeras et al 2017)
We present results coupling terrestrial laser scanning (TLS) surveying with infrared thermography (IRT) techniques to further characterize the links between daily thermal changes and rock face deformation
The comparison of the thermograms acquired on this rock wall identified 11 exfoliation sheets that behave thermally like the Rhombus Wall flake; their edges are colder on all the thermograms, and their central part undergoes the most significant nocturnal cooling (e.g., ΔIRT of −13.4 ± 1.0 °C for flake “F3” between 17:30 and 01:30 PDT) (Figs. 8B and 9C)
Summary
Rock slope failure leading to rockfall is a dynamic erosional process that controls the evolution of many landscapes and, in particular, steep bedrock formations (Varnes 1978; Hutchinson 1988; Evans and Hungr 1993; Hungr et al 1999; Rosser et al 2005; Rabatel et al 2008; Stock and Uhrhammer 2010; Krautblatter et al 2012; Janeras et al 2017). We present results coupling TLS surveying with infrared thermography (IRT) techniques to further characterize the links between daily thermal changes and rock face deformation To undertake these studies, we conducted two intraday monitoring campaigns on two granitic cliffs in Yosemite Valley, CA, USA: the Rhombus Wall and El Capitan. Most rockfalls have been documented to occur due to precipitation-related seepage into rock fractures (Stock et al 2013), some rockfalls occur due to thermally generated stresses (Collins and Stock 2016) wherein parts of rock cliffs (i.e., especially partially detached exfoliation sheets) expand and contract in response to diurnal variations of temperature These types of cliffs, including the Rhombus Wall and El Capitan, are the subject of our study as they are well suited for characterization by thermal detection methods. These animations provide useful tools for visualizing the coupled thermo-deformation behavior of the cliffs
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