Near‐Bed Sediment Transport During Offshore Bar Migration in Large‐Scale Experiments

Abstract

This study presents novel insights into hydrodynamics and sediment fluxes in large‐scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). An Acoustic Concentration and Velocity Profiler provided detailed information of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport. The morphological evolution was characterized by offshore migration of the outer breaker bar. Decomposition of the total net transport revealed a balance of onshore‐directed, short wave‐related and offshore‐directed, current‐related net transport. The short wave‐related transport mainly occurred as bedload over small vertical extents. It was linked to characteristic intrawave sheet flow layer expansions during short wave crests. The current‐related transport rate featured lower maximum flux magnitudes but occurred over larger vertical extents. As a result, it was larger than the short wave‐related transport rate in all but one cross‐shore position, driving the bar’s offshore migration. Net flux magnitudes of the infragravity component were comparatively low but played a nonnegligible role for total net transport rate in certain cross‐shore positions. Net infragravity flux profiles sometimes featured opposing directions over the vertical. The fluxes were linked to a standing infragravity wave pattern and to the correlation of the short wave envelope, controlling suspension, with the infragravity wave velocity.

Near‐Bed Sediment Transport Processes During Onshore Bar Migration in Large‐Scale Experiments: Comparison With Offshore Bar Migration

“…Close to the bar crest and under E2, intrawave time series (Figure 6d/7d in Grossmann et al. (2022) and Figure 3d) and time‐averaged concentration profiles (Figure 5) indicate larger amounts of sediment in suspension. Here, visual observations showed a planar bed so that diffusive mixing processes should dominate.…”

“…Like the changes in wave height, this is closely related to and caused by wave breaking. The measured maximum H rms , sw and H rms , ig are 40%–50% larger under E2 (Grossmann et al., 2022). Set‐down and set‐up, however, are of similar magnitudes and cross‐shore evolutions.…”

“…In combination this leads to onshore migration of the bar, maintaining its general shape. In contrast, E2 featured offshore transport rate increasing on the offshore side of the bar crests followed by decreases on their onshore side, causing offshore bar migration (Grossmann et al., 2022).…”

“…To explain how different time scale‐related processes contributed to the observed total transports, sediment fluxes were Reynolds‐decomposed (Figures 7g–7r; see Grossmann et al. (2022) for details on the decomposition, which neglects turbulent fluxes, and note that infragravity contributions were negligibly small). In all positions, the short wave‐related flux profiles (panels g–l) are characterized by onshore contributions near the bed ( ζ < 0.045 m).…”

“…Figure 3 presents ensemble‐averaged measurements resolving T r in a single A1 test 0.4 m offshore of the bar crest (see Grossmann et al. (2022) for similar plots under E2). During T r three wave groups occur (delimited by vertical dashed lines in the figure).…”