Laboratory investigations of beach morphology change under wave action are undertaken to gain insight into coastal processes, design coastal structures and validate the predictions of numerical models. For the results of such experiments to be reliable, it is necessary that they are repeatable. The equilibrium beach concept, that beach morphology will evolve to a quasi-static equilibrium shape for a given forcing suggests that experiments should be repeatable to some degree. However, sediment transport in turbulent breaking and broken waves is complex and highly variable and the level of repeatability at different temporal and spatial scales is challenging to measure, as such, previous work has restricted comparisons to small numbers of waves. Here we use the results of two identical, 20-h large-scale wave flume experiments to investigate the repeatability of sediment transport and beach morphology change under waves at timescales down to individual swash events. It is shown that while flow characteristics from identical swash events are very repeatable, the sediment transported can be very different in both magnitude and direction due to differences in turbulence, sediment advection and morphological feedback. Over longer periods containing multiple matching swash events however, the beach responds in a very similar manner, with the level of morphological repeatability increasing with time. The results also demonstrate that gross swash zone sediment transport remains high even as a beach profile approaches quasi-equilibrium, but the proportion of individual swash events that cause large sediment fluxes (>±7.5 kg/event/m) reduces with time. The results of this laboratory study indicate that beach morphology change has a level of determinism over timescales of several minutes and longer, giving confidence in the results from physical modelling studies. However, the large differences in sediment transport from apparently identical swash events questions the value in pursuing numerical predictions of sediment transport at the wave-by-wave timescale unless the reversals in sediment transport between apparently near identical swash events can also be predicted.

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Monitoring sandy shoreline evolution from years to decades is critical to understand the past and predict the future of our coasts. Optical satellite imagery can now infer such datasets globally, but sometimes with large uncertainties, poor spatial resolution, and thus debatable outcomes. Here we validate and analyse satellite-derived-shoreline positions (1984–2021) along the Atlantic coast of Europe using a moving-averaged approach based on coastline characteristics, indicating conservative uncertainties of long-term trends around 0.4 m/year and a potential bias towards accretion. We show that west-facing open coasts are more prone to long-term erosion, whereas relatively closed coasts favor accretion, although most of computed trends fall within the range of uncertainty. Interannual shoreline variability is influenced by regionally dominant atmospheric climate indices. Quasi-straight open coastlines typically show the strongest and more alongshore-uniform links, while embayed coastlines, especially those not exposed to the dominant wave climate, show weaker and more variable correlation with the indices. Our results provide a spatial continuum between previous local-scale studies, while emphasizing the necessity to further reduce satellite-derived shoreline trend uncertainties. They also call for applications based on a relevant averaging approach and the inclusion of coastal setting parameters to unravel the forcing-response spectrum of sandy shorelines globally.

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High quality laboratory measurements of nearshore waves and morphology change at, or near prototype-scale are essential to support new understanding of coastal processes and enable the development and validation of predictive models. The DynaRev experiment was completed at the GWK large wave flume over 8 weeks during 2017 to investigate the response of a sandy beach to water level rise and varying wave conditions with and without a dynamic cobble berm revetment, as well as the resilience of the revetment itself. A large array of instrumentation was used throughout the experiment to capture: (1) wave transformation from intermediate water depths to the runup limit at high spatio-temporal resolution, (2) beach profile change including wave-by-wave changes in the swash zone, (3) detailed hydro and morphodynamic measurements around a developing and a translating sandbar.

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Overwash hydrodynamics datasets are mixed in quality and scope, being hard to obtain due to fieldwork experimental difficulties. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring it to model overwash on a low-lying barrier island. Fieldwork was performed on Barreta Island (Portugal), in December 2013, during neap to spring-tides, when significant wave height reached 2.64 m. During approximately 4 hours, more than 120 shallow overwash events were measured with a video-camera (at 10 Hz), a pressure transducer (at 4 Hz) and a current-meter (at 4 Hz). This high-frequency fieldwork dataset includes runup, overwash number, depth and velocity. Fieldwork data along with information from literature were used to setup XBeach model in non-hydrostatic mode. The baseline model had variable skills over the duration of the overwash each 30 minutes. The baseline model was forced to simulate overwash with different nearshore morphology, grain-size and lagoon water level. An average decrease of about 30% overwash was obtained due to changes in the nearshore episode, performing better during the rising tide than during the falling tide. Model average number of events RMSE (root-mean-square-error) was 7 events morphology, mostly a small vertical growth of the submerged bar. The coarser and finer grain-sizes tests produced an 11% change in overwash, with less overwash on the coarser barrier. Changing lagoon water levels had a reduced effect on overwash hydraulics.@en

A quasi-3D process-based and time dependent groundwater model is developed and coupled to a hydrodynamic storm impact model to simulate the effect of infiltration on overwash on a gravel barrier. The coupled model is shown to accurately reproduce groundwater variations, runup properties and overwash time series measured in the gravel barrier during a large-scale physical model experiment. The coupled model is applied to study the influence of hydraulic conductivity on overwash volumes. It is shown that modeled overwash volumes are significantly affected by infiltration for hydraulic conductivity values greater than 0.01ms^-1.

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