Investigation of the logarithmic model applied to bed shear stresses in the swash zone

Abstract:

Accurate understanding of beach face sediment transport in the swash zone is essential to improve existing models for predicting beach morphological changes. In the swash zone, bed shear stresses are the dominant driving mechanism of both bed-load and suspended-load sediment transport.

A detailed comparison is presented of swash zone bed shear stresses obtained from direct measurements and velocimetry derived estimates, as measured in the large-scale GWK wave flume facility in Hannover, Germany. Bed shear stresses were measured directly by flush mounted shear plates and estimated using the logarithmic model for velocity profiles obtained from Acoustic Doppler Velocity Profilers (ADVP). The swashes measured were generated by large-scale (H > 0.9m, T > 8s) monochromatic and solitary waves on a planar fixed-bed beach with a rough surface (d₅₀ = 4.6mm). The logarithmic model and its application to swash flows are investigated in detail for the ensemble and individual swash events.

The results confirm the concerns of others about log-law suitability in the swash zone and extend the prior works to fully prototype scale. The logarithmic model proves reasonably valid in uprush but increasing invalid through backwash where there is clear evidence of a systematic departure from log-law theory. The cause of the disparity is investigated and considered to be the result of unsteady hydrodynamics, free-surface pressure gradients and complex boundary layer evolution. In the latter stages of backwash the boundary layer becomes emergent further disrupting the flow, re-aerating and tending towards more complex turbulent sheet-flow behaviour. Adjustment to the depth-averaged void fraction cannot account for the magnitude of the discrepancy, indicating that the formulation of the logarithmic model itself is decreasingly valid as the flow thins and decelerates throughout backwash. Though it is conceptually appealing and relatively simple to apply, the results further confirm the limited application of the logarithmic model to sediment transport through the full uprush – backwash cycle.