Surface kinematics of the Ferguson rock slide revealed by repeat lidar and GPS measurements, Highway 140, California

摘要

High-resolution topographic data, such as that collected using lidar ("light detection and ranging"), allow examination of the complex morphology of landslide masses. When these data are collected repeatedly over temporally significant time intervals (i.e., days to years), the kinematics of slide motion can be extracted. This information can guide assessments of expected future deformation, and in turn assist hazard and risk assessments as well as steer the design of potential mitigation options. Here, we examine the motion of a large (approximately 800,000 m~3) rock block slide reactivation located in northern California. The Ferguson rock slide moved during the particularly wet spring of 2006 in an area of prehistoric instability as evidenced by multiple headscarps in the upper portion of the slope. The landslide is located on one side of the narrow Merced River canyon where both the river, nationally designated as Wild and Scenic, and California State Highway 140 share the canyon bottom. The 2006 reactivation caused a 3-month closure to this section of the highway, which receives about 875,000 vehicle trips per year and serves as the main all-weather entrance to the iconic and heavily visited Yosemite National Park. As of summer 2013, talus from the landslide still blocked the original roadway and traffic used a one-lane temporary road to detour around the closure. We present surface and cross-section deformation analyses of the landslide surface using a total of four high-resolution terrestrial lidar data sets collected at approximately two-year intervals following the landslide reactivation. We couple these data sets with differential GPS data collected semi-continuously at three locations on the landslide surface during approximately this same time interval (late-2006 to late 2012) to examine patterns of motion within the slide. Our results provide a more complete understanding of the complex interactions between the upper, driving part of the landslide and the conveyor belt pathway that creates and deposits talus on the original roadway and into the river. Overall, we find that rock slide motion is mostly translational, and it moves at higher velocity in its middle and lower areas compared to the upper blocks. However, we also find that overall velocities have decreased over the 6-year period of investigation. This case study illustrates the use of repeat high-resolution topography for guiding hazard assessments related to ongoing motion of large landslides.