Experimental study of the effect of grain sizes in a bimodal mixture on bed slope, bed texture, and the transition to washload

Kimberly M. Hill, John Gaffney, Sarah Baumgardner, Peter Wilcock, Chris Paola

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


When fine sediment is added to a coarse-grained system, the mobility and composition of the bed can change dramatically. We conducted a series of flume experiments to determine how the size of fine particles introduced to an active gravel bed influences the mobility and composition of the bed. We initiated our experiments using a constant water discharge and feed rate of gravel. After the system reached steady state, we doubled the feed rate by supplying a second sediment of equal or lesser size, creating size ratios from 1:1 to 1:150. As we decreased the relative size of the fine particles, the system transitioned among three regimes: (1) For particle size ratios close to one, the bed slope increased to transport the additional load of similar-sized particles. The bed surface remained planar and unchanged. (2) For intermediate particle size ratios, the bed slope decreased with the additional fines. The bed surface became patchy with regions of fine and coarse grains. (3) For the largest particle size ratios (the smallest fines), the bed slope remained relatively unchanged. The subsurface became clogged with fine sediment, but fine particles were not present in the surface layer. This third regime constitutes washload, defined by those fractions that do not affect bed-material transport conditions. Our results indicate washload should be defined in terms of three conditions: small grain size relative to that of the bed material, full suspension based on the Rouse number, and a small rate of fine sediment supply relative to transport capacity.

Original languageEnglish (US)
Pages (from-to)923-941
Number of pages19
JournalWater Resources Research
Issue number1
StatePublished - Jan 1 2017

Bibliographical note

Funding Information:
We gratefully acknowledge funding for this research provided by the National Center for Earth Surface Dynamics (NCED) an NSF Science and Technology Center funded under agreement EAR-0120914, and by NSF grants CBET-0932735 and CBET-0756480. We thank Antoinette Abeyta for her assistance in performing some of the experiments, and Ben Erickson, Dick Christopher, and Jim Tucker for their technical assistance with flume modifications. The data generated from this work are provided in the tables and figures of this paper and, for the data in Figures, , and, in supporting information available electronically through the journal.


  • bank stability
  • bimodal mixtures
  • mobilization
  • sediment transport
  • washload


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