Determining the scales of collective entrainment in collision-driven bed load
<p>Fluvial bed-load transport is notoriously unpredictable, especially near the threshold of motion where stochastic fluctuations in sediment flux are large. Laboratory and field observations suggest that particles are entrained collectively, but this behavior is not well resolved. Collective...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-11-01
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| Series: | Earth Surface Dynamics |
| Online Access: | https://www.earth-surf-dynam.net/6/1089/2018/esurf-6-1089-2018.pdf |
| Summary: | <p>Fluvial bed-load
transport is notoriously unpredictable, especially near the threshold of
motion where stochastic fluctuations in sediment flux are large. Laboratory
and field observations suggest that particles are entrained collectively, but
this behavior is not well resolved. Collective entrainment introduces new
length scales and timescales of correlation into probabilistic formulations
of bed-load flux. We perform a series of experiments to directly quantify
spatially clustered movement of particles (i.e., collective motion), using a
steep-slope 2-D flume in which centimeter-scale marbles are fed at varying
rates into a shallow and turbulent water flow. We observe that entrainment
results exclusively from particle collisions and is generally collective,
while particles deposit independently of each other. The size distribution of
collective motion events is roughly exponential and constant across sediment
feed rates. The primary effect of changing feed rate is simply to change the
entrainment frequency, although the relation between these two diverges from
the expected linear form in the slowly driven limit. The total displacement
of all particles entrained in a collision event is proportional to the
kinetic energy deposited in the bed by the impactor. The first-order picture
that emerges is similar to generic avalanching dynamics in sandpiles:
“avalanches” (collective entrainment events) of a characteristic size relax
with a characteristic timescale regardless of feed rate, but the frequency of
avalanches increases in proportion to the feed rate. The transition from
intermittent to continuous bed-load transport then results from the
progressive merger of entrainment avalanches with increasing transport rate.
As most bed-load transport occurs in the intermittent regime, the length
scale of collective entrainment should be considered a fundamental addition
to any probabilistic bed-load framework.</p> |
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| ISSN: | 2196-6311 2196-632X |