Short-wave sand transport in the surf zone

Abstract

Morphodynamic models usually contain empirical parameterisations to estimate sand transport quantities from near-bed characteristics of the wave-orbital motion. While this results in reasonable predictions of the nearshore morphodynamics in deeper water, predictions in shallow water (≲ 3 m) are largely inaccurate. This precludes the use of these models to study, for example, the cross-shore sand transport toward the beach after the implementation of (shoreface) nourishments. Both acceleration skewness and surface-generated turbulence can induce additional sand suspension beneath breaking waves in the surf zone; however, their net effect on short-wave sand transport is not clear. The aim of this thesis is to improve our knowledge on turbulence, sand suspension and short-wave sand transport beneath irregular breaking waves. This aim is fulfilled through the analysis of turbulence and sand concentration measurements, collected beneath irregular waves in a field-scale laboratory flume and in natural surf zones. Detailed measurements of the turbulent kinetic energy, at three elevations above the bed, were collected during the BARDEXII experiment to analyse the importance of surface-generated turbulence as a function of the relative wave height. The vertical turbulence profile shows a transition from being dominated by bed-generated turbulence beneath nonbreaking waves (low relative wave height) above vortex ripples to being dominated by surface-generated turbulence beneath plunging breaking waves (high relative wave height) above subdued bed forms. To analyse the effect of breaking-induced turbulence on sand suspension, the sand concentration was measured at collocated sand concentration sensors. An event-based analyses shows that 50% of the turbulence events beneath plunging breakers can be related to concentration events. Beneath nonbreaking shoaling waves with vortex ripples, the sand concentration close to the bed peaks right after the maximum positive wave-orbital motion and shows a marked lag in the vertical. Beneath plunging waves with subdued bed forms, concentration peaks beneath the wavefront without any notable phase lags in the vertical. Further shoreward beneath bores, the sand concentration remains phase-coupled to the positive wave-orbital motion, but the concentration decreases toward the shoreline. The wave-driven suspended sand transport is thus onshore-directed and largest beneath plunging waves, while it is small and can also be offshore-directed beneath shoaling waves. To explore the generality of these findings and the resulting sand transport in the shallow surf zone, the magnitude, direction and relative importance of the short-wave sand flux in the BARDEXII dataset is compared with measurements collected on two natural sandy beaches. The relative importance of the short-wave sand flux in comparison with the mean flux, for all data combined, scales with (Auurms)/|u|, where Au is the velocity asymmetry, urms is the root-mean-square orbital motion and u is the mean cross-shore current. Contributions are high (~70% ) when this ratio is high (~10) and low and sometimes negative (~0% ) when this ratio is low (~ 1). These field and laboratory measurements under irregular waves thus support the hypothesis that the inclusion of velocity asymmetry in transport formulations would improve the performance of morphodynamic models in the shallow surf zone.