Rockfall hazard and risk assessment in the Yosemite Valley, California, USA

Abstract

Abstract. Rock slides and rock falls are the most frequent types of slope movements in Yosemite National Park, California. In historical time 392 rock falls and rock slides have been documented in the valley, and some of them have been mapped in detail. We present the results of an attempt to assess rock fall hazards in the Yosemite Valley. Spatial and temporal aspects of rock falls hazard are considered. A detailed inventory of slope movements covering the 145-year period from 1857 to 2002 is used to determine the frequency-volume statistics of rock falls and to estimate the annual frequency of rock falls, providing the temporal component of rock fall hazard. The extent of the areas potentially subject to rock fall hazards in the Yosemite Valley were obtained using STONE, a physically-based rock fall simulation computer program. The software computes 3-dimensional rock fall trajectories starting from a digital elevation model (DEM), the location of rock fall release points, and maps of the dynamic rolling friction coefficient and of the coefficients of normal and tangential energy restitution. For each DEM cell the software calculates the number of rock falls passing through the cell, the maximum rock fall velocity and the maximum flying height. For the Yosemite Valley, a DEM with a ground resolution of 10×10 m was prepared using topographic contour lines from the U.S. Geological Survey 1:24 000-scale maps. Rock fall release points were identified as DEM cells having a slope steeper than 60 • , an assumption based on the location of historical rock falls. Maps of the normal and tangential energy restitution coefficients and of the rolling friction coefficient were produced from a surficial geologic map. The availability of historical rock falls mapped in detail allowed us to check the computer program performance and to calibrate the model parameters. Visual and statistical comparison of the model results with the mapped rock falls confirmed the accuracy of the model. The model results are compared with a previous map of rockfall talus and with a geomorphic assessment of rock fall hazard based on potential energy referred to as a shadow angle approach, Correspondence to: F. Guzzetti (F.Guzzetti@irpi.cnr.it) recently completed for the Yosemite Valley. The model results are then used to identify the roads and trails more subject to rock fall hazard. Of the 166.5 km of roads and trails in the Yosemite Valley 31.2% were found to be potentially subject to rock fall hazard, of which 14% are subject to very high hazard.

Use of LIDAR in landslide investigations: a review

“…The first issue is to locate the source areas. This is often performed at Nat Hazards (2012) 61:5-28 15 regional scale using simple slope angle thresholds (Guzzetti et al 2003;Frattini et al 2008). But, as shown by Loye et al (2009), more information can be potentially extracted from the slope angle distribution.…”

Investigation and hazard assessment of the 2003 and 2007 Staircase Falls rock falls, Yosemite National Park, California, USA

“…Although the rock-fall shadow line provides useful firstorder hazard assessment, in some instances rock-fall hazards are not well represented by the shadow line concept because the use of the talus apex is somewhat arbitrary, because it does not represent well those areas that have not produced large talus deposits, and because complex topography above the talus can influence the distribution of rockfall debris (e.g., Guzzetti et al, 2003). To more accurately assess the extent of areas below Staircase Falls potentially subject to future rock falls, we employed STONE, a physically based computer program that simulates rock fall events in three dimensions (Guzzetti et al, 2002(Guzzetti et al, , 2003Agliardi and Crosta, 2003).…”

“…We conducted detailed investigations of these rock falls in order to 1) accurately map the distribution of rock debris at the base of the cliff, 2) assess the stability of the detachment area, 3) identify, if possible, the mechanism(s) that triggered the rock falls, 4) evaluate the geologic and hydrologic conditions that contributed to failure, and 5) assess future rock fall hazard from the Staircase Falls rock-fall detachment area using three-dimensional rock-fall simulations. This work furthers the ongoing study of geologic hazards in Yosemite Valley (e.g., Wieczorek et al, 1992;Wieczorek and Jäger, 1996;Wieczorek and Snyder, 1999;Wieczorek et al, 1998Wieczorek et al, 2000;Wieczorek and Snyder, 2004;Guzzetti et al, 2003;Bertolo and Wieczorek, 2005).…”

“…During the period 1857-2006, 55% of documented landslides had unreported or unrecognized triggers (Wieczorek and Snyder, 2004, and unpublished data). Through detailed study of individual landslide events (Wieczorek and Snyder, 1999;Wieczorek et al, 2000), compilation and analysis of an historic landslide database (Wieczorek et al 1992;Wieczorek and Jäger, 1996;Wieczorek and Snyder, 2004), geologic mapping (Wieczorek et al, 1998, and numerical modeling (Guzzetti et al, 2003), geologists have achieved a better scientific understanding of landslide triggering mechanisms and causative factors in Yosemite, and of the associated hazard and risk.…”

Modeling and managing uncertainties in rock-fall hazards

“…Several publications have presented generic risk assessment procedures for rock-fall hazards (Guzzetti et al 2003, Baillifard et al 2003, Budetta 2004. These methods provide integral procedures for estimating rock-fall risk that, to some extent, account for the uncertainties as described above.…”