Flow competence: A criticism of a classic concept

The largest grains found in samples of transported sediment are commonly used to estimate flow competence. With samples from a range of flows, a relationship between the flow and the largest mobile grain can be derived and used to estimate the critical shear stress for incipient motion of the different grain sizes in the bed sediment or, inversely, to estimate the magnitude of the flow from the largest grain found in a transport sample. Because these estimates are based on an extreme value of the transport grain-size distribution, however, they are subject to large errors and are sensitive to the effect of sample size, which tends to vary widely in sediment transport samples from natural flows. Furthermore, estimates of the critical shear stress based on the largest sampled moving grain cannot be scaled in a manner that permits reasonable comparison between fractions. The degree to which sample size and scaling problems make largest-grain estimates of fractional critical shear stress deviate from a true relationship cannot be predicted exactly, although the direction of such a deviation can be demonstrated. The large errors and unknown bias suggest that the largest sampled mobile grain is not a reliable predictor of either critical shear stress or flow magnitude. It is possible to define a single flow competence for the entire mixture, based on a central value of the transport grain-size distribution. Such a measure is relatively stable, does not require between-fraction scaling, and appears to be well supported by observation.

[1]  F. Petit.  Evaluation of grain shear stresses required to initiate movement of particles in natural rivers , 1990 .

[2]  R. Ferguson,et al.  Influence of sand on hydraulics and gravel transport in a braided gravel bed river , 1989 .

[3]  R. Ferguson,et al.  Size‐selective entrainment of bed load in gravel bed streams , 1989 .

[4]  P. Wilcock,et al.  Experimental study of incipient motion in mixed‐size sediment , 1988 .

[5]  Peter R. Wilcock,et al.  Methods for Estimating the Critical Shear Stress of Individual Fractions in Mixed-Size Sediment , 1988 .

[6]  J. Smith,et al.  Calculations of the critical shear stress for motion of uniform and heterogeneous sediments , 1987 .

[7]  M. P. Mosley,et al.  Sediment variability and bed material sampling in gravel-bed rivers , 1985 .

[8]  M. A. Carson,et al.  Tractive stress and the onset of bed particle movement in gravel stream channels: Different equations for different purposes , 1985 .

[9]  R. J. Garde,et al.  Bed Load Transport of Coarse Nonuniform Sediment , 1984 .

[10]  D. N. Langhorne,et al.  A comparison between Shields' threshold criterion and the movement of loosely packed gravel in a tidal channel , 1984 .

[11]  E. Andrews.  Entrainment of gravel from naturally sorted riverbed material , 1983 .

[12]  G. Williams.  Paleohydrological methods and some examples from Swedish fluvial environments I. Cobble and boulder deposits. , 1983 .

[13]  J. Costa.  Paleohydraulic reconstruction of flash-flood peaks from boulder deposits in the Colorado Front Range , 1983 .

[14]  Michael Church,et al.  On the misuse of regression in earth science , 1977 .

[15]  V. Baker,et al.  Competence of Rivers To Transport Coarse Bedload Material , 1975 .

[16]  Robert T. Milhous,et al.  Sediment transport in a gravel-bottomed stream , 1973 .

[17]  P. Carling.  Threshold of coarse sediment transport in broad and narrow natural streams , 1983 .

[18]  H. Stefan,et al.  BEDLOAD TRANSPORT IN A MODEL GRAVEL STREAM. , 1980 .