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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]

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]

Beach profile evolution under storm sequence forcing presents an emerging research topic that has only been investigated at a limited number of sites. The occurrence and effects of storm sequencing on beach profile evolution are studied at Hasaki Beach, Japan, using weekly beach profile and two-hourly offshore wave measurements. During the 25-year study period, the supratidal beach at Hasaki is subjected to long-term accretion and steepening while the shoreline shows a long-term oscillation. In addition, oscillations of the supratidal beach volume and the shoreline at semi-annual and annual intervals are identified, which are largely controlled by the variability of the wave height. Hasaki Beach is subjected to frequent storms that often cluster in sequences, especially during the extra-tropical cyclone season (January to March). The majority of storms and sequences generate erosion of the beach above the low water level but some also lead to recovery. Despite a tendency for storms and storm sequences with larger power to cause more erosion, the present data does not demonstrate increased beach erosion by storm sequences. Following these findings, the tendency of the beach to evolve towards equilibrium and the importance of the antecedent beach morphology are demonstrated. • Accretion and steepening of the supratidal beach at Hasaki during the 25-year study period. • Annual and semi-annual oscillations of the shoreline, supratidal and intertidal beach volume are controlled by the wave energy. • Storms and storm sequences are quantitatively defined for Hasaki Beach and the definitions are discussed. • No indication for increased erosive effect due to storm sequences compared to individual storms of equivalent power • The antecedent beach morphology and the evolution of the beach towards equilibrium are found to be important. [ABSTRACT FROM AUTHOR]

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]

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]

Large-scale experiments on beach profile evolution, performed within the HYDRALAB+ experiments 'RESIST', will be presented providing new insights into the influence of storm sequencing on beach response. Three storm sequences with the same cumulative wave power were generated; each sequence consisted of two high energy (storm) wave conditions each of which was followed by a recovery stage. Beach profile measurements, observations of the wave breaking location and sediment bed samples are used to study the influence of storm sequences on beach profile evolution, including the effect of storm chronology and beach recovery of different energy and duration.The beach profile is found to evolve towards an equilibrium state that is specific for each wave condition indicating no cumulative effect of storm sequencing on beach erosion in the present experiments. This means that, despite the same cumulative power of each sequence, the final beach configuration of each sequence is different and it depends on the wave condition that was performed at the end of the sequence. Storm sequencing is, however, important for the rate of beach change: As the beach evolves towards equilibrium for each wave condition, the rate of beach change varies for the same wave conditions but different initial beach profiles (different sequencing). This means that the same wave condition can result in either beach erosion or recovery depending on if the beach is in recovered or eroded condition compared to the equilibrium beach configuration for these wave conditions.This contribution will present one of the first large-scale data sets comprising more than one high energy and subsequent recovery stage in one sequence. We will discuss the influence of the storm chronology, beach equilibrium configuration and recovery conditions on the beach profile evolution. [ABSTRACT FROM AUTHOR]