ENTRAINMENT OF BED SEDIMENT BY DEBRIS FLOWS: RESULTS FROM LARGE -SCALE EXPERIMENTS — IJEGE
 
 
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ENTRAINMENT OF BED SEDIMENT BY DEBRIS FLOWS: RESULTS FROM LARGE -SCALE EXPERIMENTS



Abstract:
When debris flows grow by entraining sediment, they can become especially hazardous owing to increased volume, speed, and runout. To investigate the entrainment process, we conducted eight largescale experiments in the USGS debris-flow flume. In each experiment, we released a 6 m3 water-saturated debris flow across a 47-m long, ~12-cm thick bed of partially saturated sediment lining the 31ยบ flume. Prior to release, we used low-intensity overhead sprinkling and real-time monitoring to control the bed-sediment wetness. As each debris flow descended the flume, we measured the evolution of flow thickness, basal total normal stress, basal pore-fluid pressure, and sediment scour depth. When debris flows traveled over relatively dry sediment, net scour was minimal, but when debris flows traveled over wetter sediment (volumetric water content > 0.22), debris-flow volume grew rapidly and flow speed and runout were enhanced. Data from scour sensors showed that entrainment occurred by rapid (5-10 cm/s), progressive scour rather than by mass failure at depth. Overriding debris flows rapidly generated high basal pore-fluid pressures when they loaded and deformed bed sediment, and in wetter beds these pressures approached lithostatic levels. Reduction of intergranular friction within the bed sediment thereby enhanced scour efficiency, entrainment, and runout.

Authors:
Mark E. Reid - U.S. Geological Survey, 345 Middlefield Road, MS 910 - Menlo Park - CA 94025 USA
Richard M. Iverson - Cascades Volcano Observatory, U.S. Geological Survey, 1300 SE Cardinal Ct. #100 - Vancouver - WA 98683 USA
Matthew Logan - Cascades Volcano Observatory, U.S. Geological Survey, 1300 SE Cardinal Ct. #100 - Vancouver - WA 98683 USA
Richard G. Lahusen - Cascades Volcano Observatory, U.S. Geological Survey, 1300 SE Cardinal Ct. #100 - Vancouver - WA 98683 USA
Jonathan W. Godt - U.S. Geological Survey, Denver Federal Center, Box 25046, MS 966 - Denver - CO 80225 USA
Julie P. Griswold - Cascades Volcano Observatory, U.S. Geological Survey, 1300 SE Cardinal Ct. #100 - Vancouver - WA 98683 USA
Keywords
Debris flow, sediment entrainment, scour, largescale experiment, pore-fluid pressure, soil moisture.
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