Influence of rock mass strength on the erosion rate of alpine cliffs

Abstract

Collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first-order control on the morphologic development of these landscapes. In this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, a metric that combines numerous factors contributing to the strength of a rock mass. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face, while accounting for the loss of production area as talus buries the base of the wall. Selecting sites within basins swept clean by advancing Last Glacial Maximum (LGM) glaciers allowed us to estimate the time period over which talus accumulation occurred (i.e. the production time). Dividing the magnitude of normal cliff retreat by the production time, we calculated recession rates for each site. Our study area included a portion of the Sierra Nevada between Yosemite National Park and Lake Tahoe. Rockwall recession rates determined for 40 alpine cliffs in this region range from 0·02 to 1·22 mm/year, with an average value of 0·28 mm/ year. We found good correlation between rockwall recession rate and SMR which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength. Analysis of the individual components of the SMR reveals that joint orientation (with respect to the cliff face) is the most important parameter affecting the rockwall erosion rate. The complete SMR score, however, best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes. Our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation, aspect, or slope angle.Intact rock strength, the presence and geometry of discontinuities (frequency, orientation, persistence, aperture, roughness, infilling), and the rockwall environment (degree of weathering, amount of water or vegetation) combine to control the mechanical behavior of a rock slope

Quantifying the effect of forests on frequency and intensity of rockfalls

“…The constant α of the cumulative power-law distribution was defined as 12 in our study, corresponding to a rockfall retreat rate of approximately 0.2 mm yr −1 for the considered volume range (0.05 and 2.0 m 3 ). This is in the typical range of rockfall retreat rates in alpine regions (Sass and Wollny, 2001;Hoffmann and Schrott, 2002;Moore et al, 2009). …”

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

“…In contrast to hill slope processes, glacial valley widening and fluvial erosional processes (e.g., Selby, 1982;Augustinus, 1992;Sklar and Dietrich, 2001;Brook et al, 2004;Moore et al, 2009), the relevant rock properties for subglacial erosion are not well known (e.g., Haynes, 1968;Gordon, 1981).…”

Controls on Rockfall-Talus Process-Response Systems, Kananaskis, Canadian Rockies

“…Sauchyn & Gardner, 1983;Brocklehurst & Whipple, 2002;Moore et al, 2009;Hoffman et al, 2013). A question of interest remains is whether rockfall occurrence following deglaciation is paraglacially or periglacially dominated.…”

“…The source area for talus are steep rock slopes, usually above 45-50˚ (Martin, 2000). Therefore, talus source areas in our study area were analyzed and it was determined that a threshold of approximately 45˚ is required for a slope to be source area for talus deposits (Moore et al, 2009;Frattini et al, 2008). For each basin, the area of the basin having slope gradients > 45˚ (herein referred to as steepland) was determined and used in the calculation of erosion rates.…”

“…Rockfall is one of the major erosional processes in the alpine environment (Ballantyne, 2002;Moore et al, 2009) and has a significant role in the total denudation of mountain slopes (Matsuoka & Sakai, 1999). The accumulation of rock debris falling from a cliff and depositing at its base is called a rockfall talus slope (Figure 1.1) (Luckman, 2007).…”