Investigating 163 Years of Rock Falls in Yosemite National Park using a New Spatially Enabled Inventory Database

Yosemite National Park experiences dozens of rock falls, rock slides, and debris flows each year. These events are documented in a park-wide inventory database that records 1,489 events between 1857 and 2020. All recorded events are associated with a range of nonspatial attributes including, date, time, event type, rock type, volume, probable triggers, infrastructure impacts, injuries, and fatalities. The majority of events are rock falls (69%), followed by rock slides (23%) and debris flows (5%), most of which occurred from the cliffs of Yosemite Valley. The updated inventory is spatially enabled with each event assigned NAD83 UTM coordinates, NAVD88 elevation, and inferred uncertainty. This new digital geodatabase offers increased capability for convenient storage of rock fall information, conducting rapid analyses within GIS software and developing interactive maps using web-mapping applications for widespread visualization of rock fall data. All events and associated attribute data can be queried and analyzed for event distribution by time, location, correlation to environmental conditions, or dependence on spatial variables such as slope, aspect, elevation, and rock type. For example, nearly two-thirds of rock falls originate from above the Last Glacial Maximum trimline, likely because glaciation scoured loose, decomposed rock from below the trimline. Event volumes span six orders of magnitude (0.02 to 200,000 m3), with magnitude-frequency relationships indicating that large-volume rock falls accomplish the majority of cliff erosion. Rock falls caused damage to roads, trails, vehicles, and structures and accounted for 16 documented fatalities and 90 reported injuries. Just under half (45%) of all reported events have an unknown or unrecognized trigger, indicating either incomplete information on the environmental conditions present, or that obvious triggering conditions were lacking, suggesting subtle and previously unrecognized factors such as thermal stresses. For those events with recognized triggers, precipitation in various forms is the primary trigger (80%). Further analysis of this inventory database will be critical for efficient quantitative characterization of rock fall hazard and risk, identifying areas of rock fall susceptibility, and assessment of rock fall frequency.