Grain-size distributions of gravels transported as bedload in Oak Creek, Oregon,
show systematic variations with changing flow discharges. At low discharges the gravel
distributions are nearly symmetrical and Gaussian. As discharges increase, the
distributions become more skewed and follow the ideal Rosin distribution. The patterns of
variations are established by goodness-of-fit comparisons between the measured and
theoretical distributions, and by Q-mode factor analysis. Two end members are obtained
in the factor analysis, respectively having almost perfect Gaussian and Rosin
distributions, and the percentages of the two end members within individual samples
vary systematically with discharge.
Transformation from the Gaussian to a Rosin distribution with increasing discharge
may be explained by processes of selective entrainment of grains from a bed of mixed
sizes. Samples of bed material in Oak Creek follow the Rosin distribution. At high
discharges, the transported bedload approaches the grain sizes of that bed-material
source and mimics its Rosin distribution. Random-selection processes must be more
important to grain entrainment at lower discharges, so that the resulting Gaussian
distributions of transported bedload reflect similar distributions of bed stresses exerted
by the stream flow.
The results from Oak Creek demonstrate that the competence of the flow is
reflected in the entire distribution of transported gravel sizes. A sequence of layers of
fluvial gravels, modern or ancient, might show systematic variations between coarse
Rosin and finer-grained Gaussian distributions, and these could be used to infer
frequencies of various discharges and to establish a relationship to the source sediment.
A differential bedload transport function is formulated utilizing the dependence of
two parameters in the Rosin distribution on the flow stress. The total transport rate,
which is also a function of the flow stress, is apportioned within the Rosin grain-size
distribution to yield the fractional transport rates. The derived bedload function has the
advantage of yielding smooth, continuous frequency distributions of transport rates for
the grain-size fractions, in contrast to the discrete transport functions which predict
rates for specified sieve fractions. A group of differential transport frequency curves
can be constructed that reflects a particular stream's bedload transport characteristics.
Successful reproduction of the measured fractional transport rates and bedload grainsize
distributions by this approach demonstrates its potential in flow-competence
estimates, evaluations of differential transport rates of size fractions, and in
investigations of downstream changes in bed material grain-size distributions.
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