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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. 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. 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 Accretive wave condition determines quasi‐equilibrium recovery beach profile irrespective of initial profiles Configuration of post‐storm profiles determines type of accretive morphological evolution toward the quasi‐equilibrium profile Outer bar maintenance associated with wave breaking over the bar while breaking farther onshore leads to outer bar dissipation

The propagation of bichromatic wave groups over a constant 1:100 beach slope and the influence of the group modulation is presented. The modulation is controlled by varying the group frequency, fg, which is shown to remarkably affect the energy transfer to high and low frequency components. The growth of the high frequency (hf) wave skewness increases when fgdecreases. This is explained by nonlinear coupling between the primary frequencies, which results in a larger growth of hf components as fgdecreases, causing the hf waves to break earlier. Due to high spatial resolution, wave tracking has provided an accurate measurement of the varying breakpoint. These breaking locations are very well described ( R2>0.91) by the wave‐height to effective‐depth ratio (γ). However, for any given Iribarren number, this γis shown to increase with fg. Therefore, a modified Iribarren number is proposed to include the grouping structure, leading to a considerable improvement in reproducing the measured γ‐values. Within the surf zone, the behavior of the Incident Long Wave also depends on the group modulation. For low fgconditions, the lf wave decays only slightly by transferring energy back to the hf wave components. However, for high fgwave conditions, strong dissipation of low frequency (lf) components occurs close to the shoreline associated with lf wave breaking. This mechanism is explained by the growth of the lf wave height, induced partly by the self‐self interaction of fg, and partly by the nonlinear coupling between the primary frequencies and fg. Nonlinear energy transfers explain the group modulation influence on high and low frequency wave dynamicsThe growth of the high frequency wave skewness increases when the group frequency decreases promoting wave breaking at larger water depthsLong wave dissipation close to the shoreline is low for long wave groups but high for the shortest wave groups suggesting long wave breaking

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. 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. 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 Migration results from balance of onshore‐directed, short‐wave related bedload and offshore‐directed, current‐related suspended transports From storm to recovery waves there is change to onshore migration because stronger reduction in offshore‐ than onshore‐directed transports Streaming and infragravity wave influences are visible but negligibly low; wave breaking and nonlinearity influences are more important

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. 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. 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 Bar offshore migration resulted from balance of offshore‐directed, current‐related and onshore‐directed, short wave‐related transports Short wave‐ (current‐) related transport dominant in sheet flow (suspended) layer. As they oppose, infragravity transport non‐negligible The presence/absence of time‐lags between concentration and hydrodynamics in the suspended/sheet flow layers affects transport direction

The transport of plastic particles from inland sources to the oceans garbage patches occurs trough coastal regions where the transport processes depend highly on wave‐induced motions. In this study, experimental measurements of the plastic particles wave‐induced Lagrangian drift in intermediate water depth are presented investigating the influence of the wave conditions, particle size and density on the motion of relatively large plastic particles. A large influence of the particle density is observed causing particles to float or sink for relative densities lower and larger than water respectively. The measured net drift of the floating particles correlates well with theoretical solutions for particle Stokes drift, where the net drift is proportional to the square of the wave steepness. Floating particles remain at the free water surface because of buoyancy and no evidence of any other influence of particle inertia on the net drift is observed. Nonfloating particles move close to the bed with lower velocity magnitudes than the floating particles’ motion at the free surface. The drift of nonfloating particles reduces with decreasing wave number, and therefore wave steepness. Marine plastic pollution attracts significant attention from scientists and the general public. Most focus is on the floating plastic that accumulates in the centers of the ocean basin. However, for plastic originating from land to end up in these open‐ocean regions, it first needs to be transported through the coastal zone. Little is known how coastal waves transport plastic. Here, we use a 16 m‐long wave‐flume to measure how waves transport both floating and nonfloating plastic particles in the laboratory. The floating particles move with the waves as predicted by the so‐called Stokes drift, while nonfloating particles feel the effect of the bottom shear. These results improve our understanding of how plastics move from the coast to the open ocean and vice versa, thereby supporting improvements in our modeling capacity of the transport of marine plastic litter. A 16 m wave flume is used to measure wave transport of floating and non‐floating plasticFloating plastic transport correlates well with Stokes drift and buoyancy is the only non‐inertial effectNon‐floating particles experience shear forces. Transport varies with particle size and density A 16 m wave flume is used to measure wave transport of floating and non‐floating plastic Floating plastic transport correlates well with Stokes drift and buoyancy is the only non‐inertial effect Non‐floating particles experience shear forces. Transport varies with particle size and density