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April 29, 2022

U.S. Geological Survey scientists studying Mount Rainier and its hazards for the past 70 years have long recognized that the greatest danger may not be a volcanic eruption, but instead large mudflows called lahars.

 A distant view of Mount Rainier volcano over Puyallup Valley, near Orting, Washington.
A distant view of Mount Rainier volcano over Puyallup Valley, near Orting, Washington.

As a result, researchers have developed state-of-the-art computer simulations that are part of a new USGS report, Modeling the Dynamics of Lahars that Originate as Landslides on the West Side of Mount Rainier, Washington. Animations of possible lahar arrival times and flow depths can help emergency managers, public officials and communities in lahar hazard zones continue to prepare by developing alert systems and evacuation routes that lead away from valley floors. 

Lahars are rapidly moving flows of water, mud, rock, and woody debris that originate on volcanoes. They can flow almost as fluidly as water, yet be as dense as wet concrete. Large lahars can bulldoze or bury nearly everything it their paths, leaving behind deposits of rocky sediment. They commonly accompany volcanic eruptions, but can also develop from landslides that occur on steep volcano slopes in the absence of any eruptive activity. In such cases, lahars may occur without warning, accentuating the importance of anticipating their downstream behavior and hazards.    

The study focuses on the west flank of Mount Rainier, which previous USGS research identified as the most landslide-prone part of the volcano. The new report examines unheralded lahars- lahars not caused by volcanic activity. The simulations do not predict when the next large lahar will occur. Rather, they provide examples of how lahars of differing sizes move and flow, with detailed information about anticipated depth, speed and range of movement in the densely developed valleys of the Puyallup and Nisqually rivers that drain the western side of Mount Rainier. 

Mount Rainier volcano hazards and lahar arrival times in Puyallup and Nisqually River valleys
Mount Rainier (Washington) volcano hazard zones and estimated lahar (volcanic mudflow) arrival times for the Puyallup and Nisqually River valleys. Lahar arrival times derived from computer simulations described in Modeling the Dynamics of Lahars that Originate as Landslides on the West Side of Mount Rainier, Washington, USGS Open-File Report 2021-1118, Lahars may be larger or small than depicted and arrival times will vary during an actual lahar.

The best way to mitigate volcano hazards is to ensure local communities are aware of the threats and take steps to minimize risk. A 2015 USGS study determined that over 90,000 people live in Mount Rainier lahar hazard zones, along with over 50,000 employees working in about 3,800 business. USGS scientists are working closely with emergency managers and public officials at Mount Rainier National Park and in the communities and counties that contain the Puyallup and Nisqually river valleys downstream from Mount Rainier to communicate the key findings of the new lahar simulations and upgrade and expand an automated lahar detection system installed in 1998.

The report’s findings supplement the existing information within the USGS volcano hazards assessment for Mount Rainier and its surroundings. The new lahar simulations were performed using D-Claw, a computational model developed by USGS scientists in collaboration with university researchers. The physics represented by the model equations were gradually revealed during decades of large-scale experiments performed at the USGS debris-flow flume in Oregon. D-CLAW can be applied in several ways. It can be used to model what might happen in the future, as has been done for this report on the downstream impacts of hypothetical lahars at Mount Rainier. It can also be used to provide insight on events of the past. Previous applications of the D-Claw model have included simulation of the disastrous Oso, Washington, landslide in 2014.



Video Transcript
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.
D-Claw computer simulation of landslide that begins on Mount Rainier's west flank (Tahoma Glacier Headwall).
Close-up oblique views of Mount Rainier’s west side showing simulated lahar flow depths from a landslide originating in the area of the Tahoma Glacier Headwall (T-260-HM simulation). Imagery appears blurry where lahar material is absent because D-Claw’s adaptive mesh refinement (AMR) employs very coarse resolution in those areas. As modeled, the landslide transforms into highly mobile flows, which enter both the Puyallup River valley (heading from the South Mowich, Puyallup, and Tahoma Glaciers) and the Nisqually River valley (heading from the Tahoma and South Tahoma Glaciers). Color shading indicates landslide and lahar flow depths in meters (m). Time (t) is indicated in hours:minutes:seconds. Additional simulations are available in Modeling the Dynamics of Lahars that Originate as Landslides on the West Side of Mount Rainier, Washington, USGS Open-File Report 2021-1118,