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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/esurf-2018-8
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
06 Feb 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).
Scales of collective entrainment and intermittent transport in collision-driven bed load
Dylan B. Lee and Doug Jerolmack Earth and Environmental Science, University of Pennsylvania, 251 Hayden Hall, 240 S 33rd St, Philadelphia, PA 19104, USA
Abstract. Fluvial bed-load transport is notoriously unpredictable, especially near the threshold of motion where stochastic fluctuations in sediment flux are large. A general statistical mechanics framework has been developed to formally average these fluctuations, and its application requires an intimate understanding of the probabilistic motion of individual particles. Laboratory and field observations suggest that particles are entrained collectively, but this behavior is not well resolved. Collective entrainment introduces new length and time scales 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 2D 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 into 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.
Citation: Lee, D. B. and Jerolmack, D.: Scales of collective entrainment and intermittent transport in collision-driven bed load, Earth Surf. Dynam. Discuss., https://doi.org/10.5194/esurf-2018-8, in review, 2018.
Dylan B. Lee and Doug Jerolmack
Dylan B. Lee and Doug Jerolmack
Dylan B. Lee and Doug Jerolmack

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Short summary
The transport of pebbles in a river is smooth and continuous under high flow conditions, but under typical flows becomes erratic and unpredictable. We perform experiments to learn more about the origins of this unpredictable, intermittent behavior. Our results show that this unpredictability is similar to how infrequent avalanches occur in a sandpile. Transport events are similar in size but become more infrequent and erratic as the river transports less and less sediment.
The transport of pebbles in a river is smooth and continuous under high flow conditions, but...
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