MODELING THE DYNAMICS OF DEBRIS-AVALANCHE LAHARS THAT ORIGINATE ON CASCADE VOLCANOES

Lahars that begin as debris avalanches pose significant threats to populations downstream from Cascade volcanoes. Field evidence at Mount Rainier indicates that at least one Holocene lahar involving more than 100 million m³ of debris probably occurred without any precursory volcanic activity that foreshadowed the event. Computational models that illustrate the probable scope and dynamics of similar future events can help guide development of real-time warning systems and evacuation plans.

Our computational model D-Claw can seamlessly simulate a debris avalanche’s onset, its transformation into a liquefied lahar that travels far downstream, and its possible entrainment of water and sediment en route. We have tested D-Claw predictions using experimental data and also data from a 50 million m³ debris-avalanche lahar that occurred in 2010 at Mount Meager, a volcano located about 160 km north of Vancouver, British Columbia, near the northern end of the Cascade Range (Guthrie et al., 2012, Nat. Haz. Earth System Sci., 12, 1277–1294). The Mount Meager event began with gravitational failure of a steeply sloping mass of hydrothermally altered rock, similar to the onset inferred for some large lahars at Mount Rainier. Our simulations of the Mount Meager event demonstrate that D-Claw can reproduce key features of debris avalanche and lahar behavior with a minimum of model tuning. These features include a 270-m vertical runup and flow bifurcation at a major stream confluence as well as a distal runout that was limited by spreading and deposition on a broad river floodplain. Our most realistic simulations also account for grain-size segregation and its effect on development of multiple surges within the flow.

Recently we have performed D-Claw simulations that illustrate the potential effects of hypothetical debris-avalanche lahars that originate from slope failures of hydrothermally altered rock in the Sunset Amphitheater high on the western flank of Mount Rainier. The simulations employ material property values identical to those we used to simulate the 2010 event at Mount Meager, and they use avalanche source-area volumes of 100 million m³. An important finding is that modeled lahar runout dynamics and distances vary considerably in response to small differences in the precise location and shape of the source area.