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A comparison between Shields' threshold criterion and the movement of loosely packed gravel in a tidal channel
Author(s) -
HAMMOND F. D. C.,
HEATHERSHAW A. D.,
LANGHORNE D. N.
Publication year - 1984
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1984.tb00722.x
Subject(s) - shields , flume , geology , entrainment (biomusicology) , channel (broadcasting) , sediment , range (aeronautics) , sediment transport , seabed , geotechnical engineering , mechanics , geomorphology , flow (mathematics) , materials science , petrology , shield , physics , oceanography , acoustics , composite material , engineering , rhythm , electrical engineering
Observations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲ D ≲ 5.0cm the critical friction velocity u * c , corresponding to the initiation of sediment transport, is given by u * c =7.0 D 0.2 . At large values of D within the quoted range, the value u * c is significantly lower than would be obtained by a Shields experiment ( u * c ∞ D 0.5 ). By comparing our values of u * c with those obtained under well‐controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0 c is valid.