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Iverson, Denlinger and Major investigated slurries predominantly made up of
a water-saturated mixture of sand and gravel, with a fine fraction of only a few
percent [IVE 97, IVE 03a, IVE 05, MAJ 96]. Experiments were run on the USGS
flume and consisted of releasing a volume of slurry (approximately 10 m 3 )downa
31 , 95-m-long flume. At the base of the flume, the material spread out on a planar,
nearly horizontal, unconfined runout zone. Flow-depth, base normal stress, and base
interstitial flow pressure were measured at different places along the flume. Iverson
and his colleagues observed that at early times, an abrupt front formed at the head
of the flow, followed by a gradually tapering body, then a thin, more watery tail. The
front remained relatively dry (with pore pressure dropping to zero) and of constant
thickness, while the body elongated gradually in the course of the flow. Over the
longest part of the flume, the basal pore pressure nearly matched the total normal
stress, which means that shear strength was close to zero and the material was liquefied
within the body [IVE 97].
Figure 1.8 shows a sequence of aerial photographs taken when the material spread
out on the runout surface. Self-organization of the slurry flow into a coarse-grained
boundary and a muddy core became quite visible as the flow traveled the runout
Figure 1.8. Snapshots showing slurry flow discharging from the U.S. Geological Survey
Debris-flow Flume and crossing the unconfined, nearly horizontal runout zone. The dark-toned
material around the perimeter of the flow was predominantly gravel, while the light-toned
material in the center of the flow was liquified mud. Figure reproduced from [IVE 03a]
(courtesy of R.M. Iverson)
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