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Onshore bar migration is a characteristic bar behavior during post‐storm beach recovery. The present large‐scale experiments, feature bichromatic wave groups over an initially steep (1:15), fully‐evolving beach. The same accretive wave condition is applied on two different post‐storm beach profiles featuring outer and inner bars. They are characterized by a larger (smaller) shoreline erosion and a larger (smaller) outer breaker bar located farther away from (closer to) the shoreline depending on the larger (smaller) energy of the storm condition. After a considerable post‐storm recovery time, similar equilibrium profiles are obtained, stressing the link between wave condition and equilibrium beach configuration. However, the evolution toward the equilibrium is different and depends on the initial morphological condition (post‐storm beach profile). After the larger storm, the morphological evolution is termed accretive merging (AM) and characterized by merging of the two bars (outer bar dissipation). After the smaller storm, the morphological evolution denoted as accretive non‐merging (AN) is characterized by onshore migration of the two bars with constant distance between them (bar maintenance). This study focuses on processes around the outer bar. During AN it features wave breaking, causing large suspended net offshore transport. AM, in contrast, mainly features bedload related to short wave asymmetries and low decomposed net transport rate magnitudes. High suspended net offshore transport occurs solely onshore of the outer bar trough. This causes filling of the bar trough and bar dissipation during migration. Additionally, processes around the outer bars are linked to accretion onshore of the bars and at the shoreline. Plain Language Summary: Nearshore sandbars are seabed features that protect coastal infrastructure behind many sandy beaches around the world. In response to waves they change in shape and distance to the beach, playing a key role in beach recovery after storms. To improve understanding of their onshore movement (migration), experiments were conducted which represented natural conditions in a controlled laboratory setting. In this context, the underwater transport of sand was measured. Application of the same recovery wave condition to two different post‐storm beach profiles resulted in two different types of bar onshore migration. After long experimental duration, both types evolved toward relatively similar beach profiles but the different ways of reaching them might have important practical consequences. In general, bar migration was governed by the interaction of onshore‐ and offshore‐transporting processes. They, in turn, were influenced by wave breaking. Bars, furthermore, cause wave breaking—closing the feedback loop. Ultimately, the different wave breaking locations, resulting from the shape of the post‐storm profiles, caused the different types of bar onshore migration during recovery. The present results provide crucial information for the development of mathematical models to forecast beach evolution and safeguard human interests in the coastal zone. Key Points: Accretive wave condition determines quasi‐equilibrium recovery beach profile irrespective of initial profilesConfiguration of post‐storm profiles determines type of accretive morphological evolution toward the quasi‐equilibrium profileOuter bar maintenance associated with wave breaking over the bar while breaking farther onshore leads to outer bar dissipation [ABSTRACT FROM AUTHOR]

Detailed information on nearshore sediment transport processes during onshore bar migration were obtained from large‐scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). Detailed measurements of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport are presented. The analysis focuses on onshore migration under an accretive wave condition but comparison to an erosive condition highlights important differences. Decomposition shows that total transport mainly results from a balance of short wave‐related, bedload onshore transport and current‐related, suspended offshore transport. When comparing the accretive to the more energetic erosive condition, the balance shifts toward onshore transport, and onshore migration, because the short wave‐related transport does not decrease as much as the current‐related transport. This is related to the effects of skewness and asymmetry combined with less sediment suspension in the water column and undertow magnitude under the accretive condition. Transports from streaming in the wave boundary layer and from infragravity waves become visible but only play a subordinate role. Identified priorities for numerical model development include parametrization of wave nonlinearity effects and better description of wave breaking and its influences on sediment suspension. Plain Language Summary: Nearshore sandbars are seabed features that protect coastal infrastructure behind many sandy beaches around the world. In response to waves they change in shape and distance to the beach. To improve understanding of their onshore and offshore movement (migration), experiments representing natural conditions in a controlled laboratory setting were done. In this context, the underwater transport of sand was measured on the basis of flow velocities and sediment concentration in the water. The most important factors for migration direction were: shape of the waves (asymmetries about horizontal and vertical axes), wave breaking and resulting flows, and processes to lift sediments into the water column and keep them suspended. Under storm waves, the breaking‐induced flows dominated the sediment transport so that bars moved offshore. In calmer conditions, the shape of the waves dominated the transport so that bars moved onshore. The inherent complexities of these processes make their mathematical replication difficult, as shown by comparison to literature. Consequently, priorities for advancing the forecasting of sandbar movement, improving coastal protection, were identified. Key Points: Migration results from balance of onshore‐directed, short‐wave related bedload and offshore‐directed, current‐related suspended transportsFrom storm to recovery waves there is change to onshore migration because stronger reduction in offshore‐ than onshore‐directed transportsStreaming and infragravity wave influences are visible but negligibly low; wave breaking and nonlinearity influences are more important [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Nearshore sandbars are seabed features that protect coastal infrastructure behind many sandy beaches around the world. In response to waves, they change in shape and distance to the beach. To improve understanding of their offshore movement, experiments representing natural conditions in a controlled laboratory setting were done. In this context, the underwater transport of sand was measured. The experiments featured groups of single short waves (less than 2.5 m long), which transported sand in different ways. Single short waves largely transported sand toward the beach, and in a very thin layer close to the seabed. Currents, caused by the short waves and their breaking, largely transported sand away from the beach, and over a much wider layer. Interactions between the single short waves in a group created longer waves. They transported sand partly toward and partly away from the beach. The currents transported sand more effectively than the single short waves, which explains the sandbar's offshore movement. The long waves transported less sand and only became important when transports from currents and short waves canceled each other out. This study provides useful information for better forecasting of sandbar movement, improving coastal protection. Key Points: Bar offshore migration resulted from balance of offshore‐directed, current‐related and onshore‐directed, short wave‐related transportsShort wave‐ (current‐) related transport dominant in sheet flow (suspended) layer. As they oppose, infragravity transport non‐negligibleThe presence/absence of time‐lags between concentration and hydrodynamics in the suspended/sheet flow layers affects transport direction [ABSTRACT FROM AUTHOR]

Detailed measurements of bed level motions and sheet flow processes in the lower swash are presented. The measurements are obtained during a large-scale wave flume experiment focusing on swash zone sediment transport induced by bichromatic waves. A new instrument (CCM + ) provides detailed phase-averaged measurements of sheet flow concentrations, particle velocities, and bed level evolution during a complete swash cycle. The bed at the lower swash location shows a clear pattern of rapid erosion during the early uprush and progressive accretion during the middle backwash phase. Sheet flow occurs during the early uprush and mid and late backwash phases. Sheet flow sediment fluxes during these instances are highest in the pick-up layer. Sediment entrainment from the pick-up layer occurs not only during instances of high horizontal shear velocities but also in occurrence of wave–backwash interactions. As opposed to oscillatory sheet flow, the pivot point elevation of the sheet flow layer is time-varying during a swash event. Moreover, the upper sheet flow layer concentrations do not mirror the concentrations in the pick-up layer. Both differences suggest that in the lower swash zone the dynamics of the upper sheet flow layer are not only controlled by vertical sediment exchange (such as in oscillatory sheet flows) but are strongly affected by horizontal advection processes induced by the non-uniformity of the flow. [ABSTRACT FROM AUTHOR]

Abstract: The present study presents a database of hydrodynamic properties and suspended sediment concentration collected within the inner surf and swash zones aiming to improve the current understanding of the sediment dynamics occurring within the beach area closest to the shoreline. Experimental measurements were conducted in a large-scale wave flume under high-energy wave conditions at three cross-shore locations, representing inner surf and swash zone conditions. 47 tests, each one comprising 500 wave trains with identical wave conditions were measured. Obtained hydrodynamic properties and suspended sediment concentrations were observed to be highly repeatable between successive tests despite the sediment suspension event-like pattern, the beach evolution between tests, and the apparent randomness of the sediment suspension phenomenon. The hydrodynamics close to the shoreline (inner surf and swash zone) is dominated by short incident broken waves and long-wave water level oscillations induced by wave grouping. The analyzed time series of measured water surface elevation, horizontal velocity, computed Turbulent Kinetic Energy (TKE), and sediment concentration revealed that the suspended sediment concentration in this coastal zone does not correlate strongly with either the incident bore height, or the short-wave horizontal velocity or the TKE; in other words high/low values of these variables do not always promote high/low values of sediment suspension. In contrast, the highest suspended sediment concentrations were observed to occur by the combined action of incident bores and the trough of long-period water level oscillations. This pattern was more apparent in the inner surf than in the swash zone. High suspended sediment concentrations were also observed to coincide with negative peaks in long-wave horizontal velocity modulation resulting in enhanced negative sediment transport rates and beach erosion close to the shoreline. [Copyright &y& Elsevier]

Bar and berm morphology characterize the seasonal beach evolution, and determine the protection against storm erosion as well as the touristic use of beaches. Thus, they are of particular interest for coastal management and engineering in the nearshore zone. This study used large-scale wave flume experiments to observe the transition from fully dissipative to fully reflective beach profile at a high level of detail. Starting from a post-storm profile generated under energetic waves, a very low energy wave condition caused dissipation of the outer and the inner bar, shoreline recovery, and berm accretion. Measurements revealed feedback between hydrodynamics and beach profile evolution with an onshore shift of the wave breaking location. As a result, the magnitude and cross-shore evolution of wave asymmetry-related bedload net onshore and suspended net offshore transport changed. The relative magnitudes of the two transport components and the way they shifted relative to each other caused the observed beach recovery. Additionally, a link between bar and berm morphology (surf-swash sand exchange) was observed. The shifting breakpoint enabled sustained, wave asymmetry-related onshore transport in the inner surf zone, feeding the berm accretion which occurred through advective swash zone processes including berm overwash. • Large-scale experiments of beach recovery and berm accretion. • Four morphological phases during transition from dissipative to reflective profile. • Shifting breakpoint controls transport processes and morphology. • Important roles of bedload, undertow, wave asymmetry and berm overwash. • Useful insights into surf and swash zone dynamics (sand exchange). [ABSTRACT FROM AUTHOR]