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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. 

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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.