LIDAR AND 3D MODEL ANALYSIS OF A POTENTIAL ROCKFALL SOURCE IN ZION NATIONAL PARK, UTAH

Rockfall is an important mechanism of erosion in high-relief areas. Rockfall events can damage infrastructure, including roads, homes, and other buildings, and cause loss of life. Multiple modes of failure exist, including detachment of exfoliation sheets and toppling of pre-existing blocks and pillars. Rockfall events have been linked to the freeze-thaw cycle, as well as daily and seasonal temperature variation, but the processes by which rockfalls are triggered remain incompletely understood.

In this study, we used terrestrial lidar data to better understand the geometry and conditions for failure of a potential rockfall source area at Zion National Park. At Zion, rockfall is a significant hazard, having previously closed roads and trails, as well as harmed visitors. The study site contains a precariously balanced pillar of cliff-forming sandstone that overlies a slope-forming package of mudstones; the pillar is situated above the historic Pine Creek housing area in the Park. Our analysis revealed the exact geometry of the precariously balanced pillar, which is about twenty meters tall and eight meters wide.

We collected terrestrial LiDAR data to generate a detailed 3D model of the pillar to assess its stability and conditions for toppling. The 3D model was used to calculate center of mass and displacement the pillar could exhibit before failure. The pillar is about twenty meters tall and eight meters wide. The thickest part of the pillar is the base and it thins at the top.

We used a free software package, Blender, to conduct physical simulations of the pillar failing to determine the vertical tilt required from its current orientation for failure to occur. Results show that it could rotate 8.4°–10.4° before failure. The top of the pillar would have to be displaced about three meters for the pillar to topple gravitationally. This displacement is much higher from the displacement that crack meter data show gathered from Zion National Park. Failure simulations and crack meter data suggest that under current trends of motion, gravitational failure of the pillar is on the order of thousands of years before failure. Potential factors that could accelerate failure include seismic shaking and failure of underlying material.