Abstract
A heavy rainstorm on the evening of July 25, 1991 spawned numerous debris flows, rockfalls, and rockslides along New Mexico Highway 68 in the Rio Grande gorge, closing the highway for nearly a day and temporarily damming the river. The most spectacular event was the fall, slide, and tumble of a 2.7 × 10 5 kg (300 ton) boulder, which left a large crater in the roadway before coming to rest across the Rio Grande. We describe the geologic factors that combine to promote slope instability along the gorge, and analyze the dynamics of a tumbling boulder. We estimate that the July 1991 boulder was traveling at approximately 21 m/sec (47 mi/hr) when it struck the highway with a total kinetic energy of about 8.5 × 10 7 N-m (6 × 10 7 ft-lb), which is several hundred times greater than the capacity of commercially available rockfall protection nets.
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