ROCK-FALL INVESTIGATION AND HAZARD ASSESSMENT IN YOSEMITE VALLEY, CALIFORNIA, USING AIRBORNE AND TERRESTRIAL LASER SCANNING DATA

Rock falls are common from the glacially steepened walls of Yosemite Valley, shaping the valley’s iconic rock formations but also posing potentially serious hazards. Laser scanning data allows for new analyses to investigate rock falls and assess hazards with increased accuracy and precision. We utilize both airborne and ground-based terrestrial LiDAR data to (1) investigate prehistoric rock fall frequency and magnitude, (2) document and analyze modern rock falls, (3) monitor cliff faces, and (4) assess future rock fall susceptibility and runout. Postglacial talus volumes yield annually averaged rock-fall rates that can be compared across different slope angles, aspects, and lithology, providing estimates of rock fall rates over geologic timescales. Repeat scanning of three recent large (850 to 45,200 m³) rock falls provide accurate and precise rock-fall volumes at far range (volumetric precision of ~1% even at scan distances >1.2 km). At close range, terrestrial laser scanning can potentially resolve small (~1 cm) deformation of rock flakes, which we demonstrate occurs on the cliffs of Yosemite Valley. Airborne and terrestrial laser scanning also provide data for assessing hazards associated with potential future rock falls. Structural data, obtained by plane-fitting of discontinuity-controlled bedrock surfaces and verified by field measurements, clarify potential rock fall modes. Raw point cloud data prove more valuable for structural analyses than gridded DEMs, as point cloud data better resolve overhanging surfaces and fine-scale features such as sheeting joints that are essential components of stability assessment. Automated structural analyses, computed with Coltop3D software, and simple kinematic tests indicate relative rock-fall susceptibility for Yosemite Valley’s cliffs by mapping the dominant discontinuity sets and identifying cliff areas potentially subject to planar or wedge failures. Numerical model (STONE) simulations, calibrated to recent rock falls, delineate runout areas for rock falls originating from areas identified as having high rock-fall susceptibility. Each of these LiDAR-based analyses provides critical information for park managers charged with balancing public use with public safety in one of the world’s most frequently visited National Parks.