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Effective shear stress of graded sediments
Author(s) -
Buscombe D.,
Conley D. C.
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2010wr010341
Subject(s) - stress (linguistics) , shear stress , critical resolved shear stress , flow (mathematics) , geotechnical engineering , function (biology) , inverse , sediment transport , granular material , mathematics , mechanics , statistical physics , sediment , geology , shear rate , geometry , physics , rheology , thermodynamics , paleontology , philosophy , linguistics , evolutionary biology , biology
A meta‐analysis of fractional mobilization data from 14 sets of experiments, totaling 103 different mixed sand and gravel beds and flow conditions, has been carried out in order to identify an expression for effective shear stress, here defined as the component of bed shear stress that is directly involved in transporting each grain size fraction in graded sediment. In doing so we test the assumption that excess stress should be defined solely in terms of a critical stress rather than effective stress, which exhibits sensitivity to the flow stage. In contrast to the approach which evaluates the size‐distribution effects on motion threshold by comparing inferred transport rates, an alternative approach is utilized which is based on the skill of reproducing the measured, mobilized particle size distribution. A simple equation is developed for mobilization of sediment mixtures, based on a well‐established transport law, and employing a classical “hiding function” approach to the problem of mitigating the bias toward mobilizing fine material in the mixture. We use inverse methods to find the optimal form of the hiding function which provides the best fit with the data. We find that the hiding function is indeed sensitive to the flow and bed composition. On this basis, a simple deterministic equation is proposed for fraction‐specific effective stress, which outperforms the next best existing formula based on critical stress by 34% on aggregate.