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Partial transport of a sand/gravel sediment
Author(s) -
Wilcock Peter R.,
McArdell Brian W.
Publication year - 1997
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 - 10.1029/96wr02672
Subject(s) - flume , sediment transport , grain size , geology , bed load , range (aeronautics) , sediment , geotechnical engineering , hydrology (agriculture) , mineralogy , soil science , geomorphology , materials science , mechanics , flow (mathematics) , composite material , physics
Grains of a single size within a mixed‐size bed are entrained over a range of flows. Within this range some grains exposed on the bed surface are active (entrained at least once over the duration of a transport event), while the remaining surface grains are immobile, a condition we define as partial transport. We demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand/gravel mixture. The active proportion of the bed surface increases with bed shear stress τ 0 . At a given τ 0 , 90% of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed. Mobilization of grains in a size fraction increases from 10% to 90% over a range of τ 0 of a factor of 2. The bounds of this range increase with grain size D i so that at a given τ 0 , sizes over a range of a factor of 4 are in a state of partial transport. Fractional transport rates are independent of D i for fully mobilized fractions and decrease rapidly with D i for partially mobile fractions. Partial transport is associated with substantial transport rates of finer, fully mobile sizes, limits both the rate and size distribution of grain exchange with the bed subsurface, and may be the dominant transport condition in many gravel‐bed rivers.