Relationship between Vertical Wave Asymmetry and the Fourth Velocity Moment in the Surf Zone: Implications for Sediment Transport

The commonly used Bailard sediment transport model relates sediment suspension to the fourth velocity moment. However field studies have shown that wave orbital acceleration, which is related to vertical wave asymmetry, is also important for sediment suspension. The general view is that asymmetry is unrelated to velocity moments. Here, incident wave cross-shore velocity data from dissipative beaches shows very similar cross-shore distributions for both vertical wave asymmetry and kurtosis. The implications for sediment transport of this possible relationship are investigated by obtaining the cross-spectra between sediment suspension and (i) a Bailard-type pick up function (u⁴)^3/4, and (ii) vertical asymmetry, as the Hilbert transform of the incident cross-shore velocity. For dissipative conditions, both the Bailard term and asymmetry represent similarly sediment suspension. For reflective conditions the Bailard term fails to describe suspension whilst vertical asymmetry is very strongly correlated to sediment suspension. Accurate representation of sediment suspension in models should include the effects of vertical asymmetry.