DIFFERENTIAL BEDLOAD TRANSPORT RATES IN A GRAVEL-BED STREAM - A GRAIN-SIZE DISTRIBUTION APPROACH

The grain‐size distributions of bedload gravels in Oak Creek, Oregon, follow the ideal Rosin distribution at flow stages which exceed that necessary to initiate breakup of the pavement in the bed material. The distributions systematically vary with flow discharge and bed stress, such that at higher flow stages the grain sizes are coarser while the spread of the distribution decreases. A differential bedload transport function for individual grain‐size fractions is formulated utilizing the dependence of the 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. Successful reproduction of the measured fractional transport rates and bedload grain‐size distributions in Oak Creek by this approach demonstrates its potential for evaluations of transport rates of size fractions in gravel‐bed streams. The approach will be useful in investigations of downstream changes in bed material grain‐size distributions. Copyright © 1990 John Wiley & Sons, Ltd