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

摘要

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.