Bed slope effects on turbulent wave boundary layers: 2. Comparison with skewness, asymmetry, and other effects

A numerical model solving incompressible Reynolds-averaged Navier-Stokes equations, combined with a two-equation k-omega model for turbulence closure, is used to systematically compare the relative strength of bed shear stress quantities and boundary layer streaming under wave motions from four contributions believed to play a prominent role in cross-shore boundary layer and sediment transport processes: (1) converging-diverging effects from bed slope, (2) wave skewness, (3) wave asymmetry, and (4) waves combined with superposed negative currents (intended to loosely represent, for example, return currents or undertow).

The effects from each of the four components are isolated and quantified using a standard set of bed shear stress quantities, allowing their easy comparison. For conditions representing large shallow-water waves on steep slopes, the results suggest that converging-diverging effects from beach slope may make a significant onshore bed load contribution.

Generally, however, the results suggest wave skewness (in addition to conventional steady streaming) as the most important onshore contribution outside the surf zone. Streaming induced within the wave boundary layer is also investigated for each component, and skewness and asymmetry are demonstrated to promote largely negative streaming velocities, consistent with earlier work.

For hydraulically smooth cases, however, a thin region of positive streaming is revealed in the viscous sublayer which is effectively absent in hydraulically rough conditions. Cross-shore boundary layer dynamics are generally acknowledged to be a complicated combination of numerous competing physical processes, and this work is among few to systematically compare a number of these.

It is hoped that the results will be useful in assessing which of the contributions considered will be most prominent for a given set of local wave and beach slope conditions.