File:Mount Rainier Lahars- Hazards for the Puyallup and Nisqually River Drainages.webm

English: Large lahars (volcanic mudflows) pose substantial threats to people and property downstream from Mount Rainier, Washington. Lahars can occur during an eruption, as happened at Mount St. Helens on May 18, 1980. Lahars can also begin as large landslides that occur without warning, traveling down river channels to densely populated areas downstream.

To inform potentially affected communities, assist school districts with implementing state mandatory lahar drills, and help emergency management officials prepare for the possibility of a large lahar, the USGS prepared state-of-the-art numerical simulations (D-Claw) of hypothetical lahars that originate as landslides high on Mount Rainier’s western flank and descend the Puyallup and Nisqually River valleys.

The simulations focused on the Sunset Amphitheater and Tahoma Glacier Headwall, areas identified as the most landslide-prone part of the volcano. Two different volumes were modeled: 260 million cubic meters, the approximate volume of the Electron Mudflow that occurred at Mount Rainier about 500 years ago, and 52 million cubic meters, the volume of a large landslide that occurred at Mount Meager volcano, British Columbia, in 2010. The computer model predicted how flow would move across the terrain if it consisted of a fairly dense (hard) rock mass (the low mobility scenario) and how the flow would behave if it consisted of weakened, clay-rich rock (the high mobility scenario). The simulations predict inundation patterns, depths, and speeds of lahars for landslides of different volumes and mobilities, information not available in a prior hazards assessment report.

The report confirms our general understanding of lahar threats and analyses of lahar velocity, depths, and areas of inundation in the present USGS hazard assessment and related hazard documents. The results provide needed information for the continued development of evacuation routes, and general emergency response plans.

The report does not predict when the next large lahar will occur. It provides examples of how lahars of differing sizes and mobilities could behave. The simulations rely on decades of research on the geology of Mount Rainier, the physics of landslide initiation and lahar dynamics, and computational methods for computer simulations.

Open-file Report 2021-1118, "Modeling the Dynamics of Lahars that Originate as Landslides on the West Side of Mount Rainier, Washington", is authored by USGS scientists David George, Richard Iverson, and Charles Cannon of the USGS Cascades Volcano Observatory and USGS Oregon Water Science Center.

Video: https://www.usgs.gov/media/videos/mount-rainier-lahars-hazards-puyallup-and-nisqually-river-drainages