Can a Single Representative Grain Size Describe Bed Load Transport in the Surf Zone?

Bed load transport models for nearshore environments typically represent the distribution of grain sizes with a single grain size, often D50. (Dxx is the diameter for which xx percent of the grains by weight are finer.) The assumption that a single grain size appropriately represents transport processes is challenged by three-dimensional discrete particle computer simulations of bed load transport in the surf zone. One series of numerical experiments uses a mixed size distribution of coarse grains ranging from 0.7 mm to 1.7 mm in diameter and having D50 = 1.1 mm; and a second series of experiments uses six very sharply peaked grain size distributions having nominal diameters of 0.7 mm, 0.9 mm, 1.0 mm, 1.2 mm, 1.3 mm, and 1.7 mm, which correspond to D00, D20, D40, D60, D80, and D100, respectively, from the mixed distribution. Comparison of simulated transport rates for several waveforms each having a constant maximum wave orbital velocity of 1 m/s reveals that the single representative grain size whose bed load transport rate is equivalent to the mixed size distribution increases from D75 under Stokes-like waves, to D85 under near-breaking waves, to D95 under a bore. Mechanics of size-sorting processes during transport may explain these results: for the mixed size distribution the portion of the total transport rate attributed to the coarsest fraction of grains is large, because larger grains migrate to the top of the bed load layer. The velocity gradient within the bed load layer increases roughly linearly from the immobile grains up to the poorly defined top of the layer. The thickness of the layer and the velocity gradient across it may account for the increase of the representative grain size as waves progress across the surf zone. Research supported by the National Ocean Partnership Program.