Debris flow initiation in proglacial gullies on Mount Rainier, Washington

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/geomorphology.Effects of climate change, retreating glaciers, and changing storm patterns on debris flow hazards concern\ud managers in the Cascade Range (USA) and mountainous areas worldwide. During an intense rainstorm in\ud November 2006, seven debris flows initiated from proglacial gullies of separate basins on the flanks of Mount\ud Rainier. Gully heads at glacier termini and widespread failure of gully walls imply that overland flow was transformed\ud into debris flow along gullies. We characterized gully change and morphology, and assessed spatial\ud distributions of debris flows to infer the processes and conditions for debris flow initiation. Slopes at gully\ud heads were greater than ~0.35 m m⁻¹ (19°) and exhibited a significant negative relationship with drainage\ud area. A break in slope–drainage area trends among debris flow gullies also occurs at ~0.35 m m⁻¹, representing\ud a possible transition to fluvial sediment transport and erosion. An interpreted hybrid model of debris flow initiation\ud involves bed failure near gully heads followed by sediment recruitment from gully walls along gully\ud lengths. Estimates of sediment volume loss from gully walls demonstrate the importance of sediment inputs\ud along gullies for increasing debris flow volumes. Basin comparisons revealed significantly steeper drainage\ud networks and higher elevations in debris flow-producing than non-debris flow-producing proglacial areas. The\ud high slopes and elevations of debris flow-producing proglacial areas reflect positive slope–elevation trends for\ud the Mount Rainier volcano. Glacier extent therefore controls the slope distribution in proglacial areas, and thus\ud potential for debris flow generation. As a result, debris flow activity may increase as glacier termini retreat\ud onto slopes inclined at angles above debris flow initiation thresholds