Grain-size segregation and levee formation in geophysical mass flows
Data from large‐scale debris‐flow experiments are combined with modeling of particle‐size segregation to explain the formation of lateral levees enriched in coarse grains. The experimental flows consisted of 10 m3 of water‐saturated sand and gravel, which traveled ∼80 m down a steeply inclined flume...
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| Published in | Journal of Geophysical Research: Earth Surface Vol. 117; no. F1 |
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| Main Authors | , , , , , |
| Format | Journal Article |
| Language | English |
| Published |
Washington, DC
Blackwell Publishing Ltd
01.03.2012
American Geophysical Union |
| Subjects | |
| Online Access | Get full text |
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| Summary: | Data from large‐scale debris‐flow experiments are combined with modeling of particle‐size segregation to explain the formation of lateral levees enriched in coarse grains. The experimental flows consisted of 10 m3 of water‐saturated sand and gravel, which traveled ∼80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface. We measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage. Levees formed by progressive downslope accretion approximately 3.5 m behind the flow front, which advanced steadily at ∼2 m s−1 during most of the runout. Segregation was measured by placing ∼600 coarse tracer pebbles on the bed, which, when entrained into the flow, segregated upwards at ∼6–7.5 cm s−1. When excavated from the deposit these were distributed in a horseshoe‐shaped pattern that became increasingly elevated closer to the deposit termination. Although there was clear evidence for inverse grading during the flow, transect sampling revealed that the resulting leveed deposit was strongly graded laterally, with only weak vertical grading. We construct an empirical, three‐dimensional velocity field resembling the experimental observations, and use this with a particle‐size segregation model to predict the segregation and transport of material through the flow. We infer that coarse material segregates to the flow surface and is transported to the flow front by shear. Within the flow head, coarse material is overridden, then recirculates in spiral trajectories due to size‐segregation, before being advected to the flow edges and deposited to form coarse‐particle‐enriched levees.
Key Points
Coarse particle levees form in debris flows by progressive streamwise accretion
Coarse grains are transported to the flow front, then laterally into the levees
Grain size segregation profoundly affects flow dynamics and deposit structure |
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| Bibliography: | ark:/67375/WNG-M6STTM1C-3 ArticleID:2011JF002185 istex:0CD27B6D95F229114669496CBD3F8F1743721FB3 NERC - No. NE/E003206/1; No. NE/G523747/1 EPSRC - No. GR/S50052/01; No. GR/S50069/01 Animation 1. The 25th August 2009 experiment at the USGS debris-flow flume. The animation shows the debris flow resulting release of ∼ 10 m3 of water-saturated sand and gravel from a hopper at the top of the 95 m long flume. Colored tracer cubes are seeded onto the flow surface when it first reaches the flume mouth. As the flow deposits on the runout pad, a board is brought down across the flow to divert the watery "tail" to one side.Animation 2. An overhead view of runout of the 25th August experiment. The rows of colored tracer cubes that are seeded onto the flow surface are initially shown on the far left. Coarse-enriched levees form spontaneously during the runout, which channelize the flow and cause the deposit to maintain a constant width. The grid shows 1 m squares.Animation 3. An animation of , showing the path of a sequence of particles that initially start at the same transverse locations. Particles marked 1, which are initially the farthest downstream, are the first to reach the flow head and be advected towards the lateral margins. These particles deposit first, followed by particles 2, 3, 4, and 5 in turn. When deposited in the levees, the spatial order of the particles is reversed, with particles marked 5 farthest downstream.Animation 4. Surface flow speeds of the 25th August experiment, calculated from particle tracking velocimetry. A detailed velocity field is shown for the frame at t = 3.50 s in . Animation 4 shows that this snapshot is representative of the flow throughout the regime of steady front propagation and levee formation. The streamwise growth of the levees (shown in black) and the lack of downstream-variation in the velocity field of the levee-channelized section are also evident.Animation 5. An overhead view of the 25th August experiment, in a frame moving with the flow front. In this frame, the near-steady nature of flow can be seen. Surface material in the central channel moves to the right, towards the flow front, while surface material at the lateral margins of the flow moves to the left, away from it. Animation 5 shows the runout between t ∼ 2 s, when the levees first begin to form on the runout pad, to t ∼ 4.5 s, when the flow diverter is brought down. The animation is slowed down by a factor of six.Animation 6. Particle paths through the debris-flow head with a plug-flow vertical velocity profile (shown in ). Particles at the surface and base of the flow travel at the same velocity. They are transported into the flow head near the center, and leave the head at the lateral margins. All particles on the surface remain there, with none passing over the flow front.Animation 7. Particle paths through the debris-flow head with a simple shear vertical velocity profile (shown in ). Particles are transported into the head at the upper half of the flow, and leave it in the lower half. Nearly all particles pass over the flow front.Animation 8. Particle paths through the debris-flow head with a combination of basal slip and shear (shown in ). Surface particles near the flow certerline pass over the flow front and are transferred to the base of the flow, but those closer the edges of the flow remain on the surface as they are advected out of the head. ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 14 ObjectType-Feature-1 |
| ISSN: | 0148-0227 2169-9003 2156-2202 2169-9011 |
| DOI: | 10.1029/2011JF002185 |