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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]

Abstract Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (T g = 15.0 s and 27.7 s). For the shortest wave group, due to bore focusing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of wave–swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave–swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave–backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra–swash cross–shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration is influenced by horizontal advection, leading to large sheet-flow layer thickness. However, for the longer wave groups, the local vertical exchange of sediment in the sheet–flow layer is dominant, with the presence of a pick–up and mirroring upper layer similar to oscillatory sheet–flow measurements. These results reaffirm the important effects of the wave group structure and the wave–swash interactions on the swash zone sediment dynamics and beach face evolution. Highlights • The swash zone of short groups oscillates at the group period while for long groups it oscillates with individual waves. • Short groups induced sediment transport is dominated by horizontal advection with important wave-swash interactions. • The sheet flow layer (SFL) of short groups is thickest and it develops at the group period time scale. • The long groups SFL is thinnest and dominated by vertical sediment exchange occurring at the time scale of individual waves. [ABSTRACT FROM AUTHOR]

In this paper, large-scale experimental data are presented showing the beach profile morphological evolution induced by four different bi-chromatic wave conditions characterized by very similar energy content between them but varying the modulation period. Important differences were observed in the resultant beach profiles as a function of the wave group periods. Larger variability in the profile evolution is generally observed for larger wave group periods and, more importantly, as the wave group period increases the distance between the generated breaker bar and the shoreline increases. The measured primary wave height to depth ratio ( γ ) increases with the wave group period, which is consistent with the observed larger wave height at the breaking location. The primary wave breaking location is also observed at increasing distances with respect to the initial shoreline as the wave group period increases. The variation in γ with wave group period is related to the selective energy dissipation of the higher primary frequency component ( f 1 ) during the wave group shoaling. Broad bandwith conditions (reduced wave group period) lead to larger dissipation of wave heights at the f 1 component relative to f 2 resulting in a reduction in the wave modulation and primary wave height at the breaking location. Suspended sediment fluxes obtained from collocated velocity and sediment concentration measurements in the surf zone showed a consistently larger contribution of the mean return flow to the suspended sediment fluxes compared with the wave group and primary wave components. The distinct beach profile evolution in terms of bar location is interpreted from an increasing distance of the mean breakpoint location and the location of maximum return flow with respect to the shoreline as the wave group period increases. [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: This work presents a detailed analysis of suspended sediment concentration measured in the inner surf and swash zone under large-scale wave flume conditions. Controlled hydrodynamic conditions tested in the laboratory corresponded to highly energetic (erosive) conditions. Hydrodynamic, bottom evolution and suspended sediment concentration (SSC) data were acquired at several locations within the inner surf and the swash zone. A limited number of hydrodynamic type of events have been identified to induce important suspended sediment concentrations. These hydrodynamic conditions include incident broken wave, wave capture, weak and strong wave backwash interactions or pure backwash events. The acquired data showed the variability of the suspended sediment concentration pattern depending on the cross-shore location from which measurements were taken. This suspension mechanism also varied depending on the time series characteristic. There was a clear predominance of incident wave/bore as a forcing term in the surf zone, whilst the SSC measured in the swash zone showed that wave-swash interaction controls the most significant events. Finally, the importance of a wave-by-wave analysis is highlighted in this study. It was observed that despite the fact that most of the time series induce profile accretion in the mid and upper swash area, only one or two swash events are able to erode extensive parts of the beach and change the final output of the entire time record from accretion to erosion. [Copyright &y& Elsevier]

The evolution of different initial beach profiles towards the same final beach configuration is investigated based on large-scale experimental data. The same wave condition was performed three times, each time starting from a different initial profile morphology. The three different initial profiles are an intermediate energy profile with an offshore bar and a small swash berm, a plane profile and a low energy profile with a large berm. The three cases evolve towards the same final (equilibrium) profile determined by the same wave condition. This implies that the same wave condition generates different sediment transport patterns. Largest beach changes and differences in hydrodynamics occur in the beginning of the experimental cases, highlighting the coupling between morphology and hydrodynamics for beach evolution towards the same profile. The coupling between morphology and hydrodynamics that leads to the same final beach profile is associated with differences in sediment transport in the surf and swash zone, and is explained by the presence of bar and berm features. A large breaker bar and concave profile promote wave energy dissipation and reduce the magnitudes of the mean near-bed flow velocity close to the shoreline limiting shoreline erosion. In contrast, a beach profile with reflective features, such as a large berm and a small or no bar, increases negative velocity magnitudes at the berm toe promoting shoreline retreat. The findings are summarised in a conceptual model that describes how the beach changes towards equilibrium from two different initial morphologies. [ABSTRACT FROM AUTHOR]

Differences between breaker bar evolution under high energy (erosive) and subsequent mild energy (accretive) wave conditions are studied based on three data sets from large-scale morphodynamic experiments. Under erosive wave conditions, a clear linear relationship between the height and the cross-shore location of the breaker bar is observed. The similar cross-shore shape of the sediment transport magnitudes underlines the similarity of the bar evolution under erosive wave conditions. Under the subsequent accretive wave conditions, two major evolution patterns are observed during onshore migration (decaying or non-decaying breaker bar). In the presented data, the feedback between the wave conditions and the beach morphology determines if a bar decays during onshore migration. This feedback involves that the wave breaking location depends on the morphology and on the wave conditions resulting in differences in sediment transport patterns between the two types of onshore bar migration. A comparison against numerous other experimental beach profile data sets strongly supports the linear relationship between bar height and bar location under erosive wave conditions. For accretive wave conditions, the number of comparable data sets is very limited. The comparison underlines that breaker bar evolution under accretive wave conditions cannot be condensed to a single pattern as under erosive wave conditions. [ABSTRACT FROM AUTHOR]

New large-scale experiments on the influence of storm sequencing on beach profile evolution are presented. The experiments comprised three sequences that commenced from the same initial beach profile. Each sequence consisted of two storms of different energy, with each storm followed by a recovery phase. A specific focus of the experiments was the influence of storm chronology as well as the influence of the recovery wave energy and duration on beach evolution. The breaker bar and the shoreline are studied as indicators for the beach response. Both evolve towards an equilibrium location for each wave condition where the breaker bar reaches its equilibrium much faster than the shoreline. Overall, no enhanced beach erosion due to storm sequencing is observed. Despite a similar cumulative wave power of the three sequences, the final beach configuration of each sequence seems to be determined by the last wave condition instead of previous storms. However, storm sequencing is important when the beach profile has not yet fully recovered before being disrupted by the subsequent storm. In this case, the second storm does not necessarily cause further erosion but can result in onshore sediment transport and hence, form part of the recovery. The bulk onshore sediment transport as well as the shoreline recovery rate vary depending on the wave condition and the profile disequilibrium and show a maximum value for a recovery condition with an intermediate wave energy. The very low energy condition that was performed for a long duration (24 hours) generated a near-stationary beach profile with typical features of a reflective beach. • New large-scale experiments on storm sequencing and beach response. • The beach evolves towards an equilibrium state under the performed storm sequences. • The rate of beach change varies depending on the antecedent beach morphology. • Largest bulk onshore sediment transport occurs for intermediate low energy conditions. • A very low energy condition of long duration (24 h) results in a reflective beach profile. [ABSTRACT FROM AUTHOR]

Sand transport processes and net transport rates are studied in a large-scale laboratory swash zone. Bichromatic waves with a phase modulation were generated, producing two continuously alternating swash events that have similar offshore wave statistics but which differ in terms of wave-swash interactions. Measured sand suspension and sheet flow dynamics show strong temporal and spatial variability, related to variations in flow velocity and locations of wave capture and wave-backwash interactions. Suspended and sheet flow layer transport rates in the lower swash zone are generally of same magnitude, but sheet flow exceeds the suspended load transport by up to a factor four during the early uprush. The bed level near the inner surf zone is relatively steady during a swash cycle, but changes of O (cm/s) are measured near the mid swash zone where wave-swash interactions lead to strongly non-uniform flows. The two alternating swash events produce a dynamic equilibrium, with bed level changes up to a few mm induced by single swash events, but with net morphodynamic change over multiple events that is two orders of magnitude lower. Most of the intra-swash and the single-event-averaged bed level changes in the swash zone are caused by a redistribution of sediment within the swash. The transport of sediment across the surf-swash boundary is minor at intra-swash time scale, but becomes increasingly significant at swash-averaged time scales or longer (i.e., averaged over multiple swash events). •Large-scale wave flume experiments involving two alternating bichromatic wave induced swash events. •Sediment mobilization as sheet load and suspended load increases substantially from inner surf to lower swash zone. •Sheet flow transport dominates the total transport during the early uprush and during instants of strong wave-backwash interactions. •Single swash events can produce much greater net transport rates and bed level changes than the overall trend over multiple events. •The sediment exchange across the surf-swash boundary becomes increasingly significant when integrated over larger time scales. [ABSTRACT FROM AUTHOR]