Does Rock Mass Strength Control the Rate of Alpine Cliff Erosion?

Collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and plays a controlling role in mass removal from these systems. In this work we investigate the influence of rock mass strength on the retreat rate of alpine rock slopes. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, which combines numerous factors that affect the strength of a rock mass, such as intact rock strength, joint frequency, joint condition, and more. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back onto 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 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 erosion rates for each site. Our study area included a portion of the Sierra Nevada from Yosemite National Park in the south to Lake Tahoe in the north. Rockwall recession rates determined for 40 alpine cliffs in this region varied 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 that 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 correlation between the measured rockwall retreat rate and environmental attributes (such as site elevation, aspect, cliff slope length, and cliff slope angle), suggesting that rock mass strength is the dominant parameter controlling the rate of cliff erosion in our study area.