Quantifying Historic Rock Fall on Coulterville Road, Yosemite National Park, using Field Observation and SfM Photogrammetry

Abstract:

Rock fall is common throughout Yosemite National Park and while the occurrence of larger, more hazardous events are well documented, the effects of smaller, more frequent rock falls on Park roads are less studied. This poses an ongoing challenge to the U.S. National Park Service (NPS), whose policies promote natural processes and view sheds rather than the use of engineered mitigation schemes on transportation corridors. The Old Coulterville Road is a historic route that formerly provided Park access through the steep Merced River Canyon west of Yosemite Valley, and permanently closed in 1982 when a large rockslide obstructed lower access. Rock fall debris has since accumulated on the road’s paved surface, which presents a unique opportunity to investigate the frequency and magnitude of smaller events and their cumulative impact over time; information that can be used to inform modern Park road conditions.

We identified the production of rock generated by the slopes adjacent to Old Coulterville Road by systematically characterizing slope metrics and quantifying rock fall debris that has amassed since the road’s closure. We binned rocks into four size categories based on their intermediate axes, and delineated observation sites where adjacent slope characteristics markedly changed. Our data show a total of 3,040 rock fragments accumulated on the 1.4 km of sampled roadway, ranging from 0.0056m³ to 5.40 m³, with a cumulative volume of 82.0 m³. Structure-from-Motion (SfM) photogrammetric models were produced with sub-millimeter error to verify the accuracy of our field assessment and inform volume calculations. To assess the volumetric change of accumulated rock, we compared the photogrammetric model of the existing road with a bare surface model, which provides digital estimates more inclusive than the field observations. Our results show the field binning method estimates 98.1% of the digitally modeled volume for segments with large boulders present (>70cm), and 87.6% for segments without. The total calculated volume of accumulated debris increased to 87.7 m³ when adjusting the field data accordingly. This study provides sound data on chronic small-scale rock fall that can be used to support road maintenance activity and planning, and emphasizes the use of SfM photogrammetry to confirm and refine field results.