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The beach slope β is a key component characterizing the coastal response to wave forcing. Here we investigate the rapid adaptation of the upper beach slope to a given wave forcing, for the case of a lower flat platform. Such types of morphology are found on coral and rocky reef beaches and low tide terrace environments. The influence of the lower platform on this rapid equilibrium beach state is shown to be significant depending on the breaking wave regime. In particular, the width of the platform and its water level can affect the wave dissipation along the inner surf and thus the wave structure entering the swash. This paper provides a classification of the beach slope equilibrium values as a function of the Dean number on a short time scale (individual wave action), based on both offshore and swash wave conditions. A decreasing trend of the beach slope with increasing offshore Dean number (Ω 0) is found for Ω 0 ≲ 2. 7. For Ω 0 ≳ 2. 7 it is observed that the beach slope gradient is strongly controlled by the surf zone dissipation and it becomes necessary to define the swash Dean number (Ω s w) to classify the slope. Finally, a semi-empirical formula for the beach slope evolution in the case of a low tide platform is introduced and tested on two natural low-tide terrace beaches. • Definition of rapid equilibrium beach face slope associated to individual wave action. • Distinction between swash and surf dominated beaches on LTT beaches using Dean number based on offshore or local swash flow characteristics (Ω 0 or Ω s w). • Swash-dominated beaches respond directly to offshore wave forcing. • In surf-dominated beaches the inner-surf zone cannot be disregarded. • A simplified model is proposed to predict the rapid evolution of the beach slope as a function of the swash Dean number Ω s w. [ABSTRACT FROM AUTHOR]

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. • Large-scale flume experiments are used to assess morphological repeatability. • The results of laboratory investigations of morphology change are repeatable. • Sediment fluxes from almost identical waves vary in magnitude and direction. • Over longer timescales, sediment fluxes from identical wave groups converge. • Gross sediment transport remains high even as beaches approach quasi-equilibrium. [ABSTRACT FROM AUTHOR]

Short-crested breaking waves that result from directionally spread wave conditions dissipate energy and generate turbulence within the surf zone, altering sediment transport processes, wave runup, and forces on structures. Additionally, vertical vorticity generated near crest ends during breaking, which depends on the gradient in wave height along a crest, may enhance nearshore dispersion of pollutants, nutrients, and larvae. Although directionally spread irregular wave fields are ubiquitous on ocean and large lake coastlines, the dependence of short-crested breaking wave characteristics (including the along-crest length and number of crest ends) on offshore wave conditions is not well established. To assess this relationship, laboratory experiments with alongshore-uniform barred bathymetry were performed in a large-scale directional wave basin. A three-dimensional scanning lidar, trinocular camera stereo processing methods, and in situ measurements were used to study short-crested wave field breaking characteristics in the laboratory, yielding a dataset with dense spatio-temporal coverage relative to prior laboratory or field measurements. Wave height estimates are similar for remotely sensed and in situ observations, except in the outer surf zone where plunging breaking occurred. Directional wave properties estimated with an array of in situ or remotely sensed sea-surface elevation estimates are similar and yield smaller directional spreads than single-point colocated pressure and velocity based in situ estimates when waves are less directionally spread. Using a breaking crest identification procedure combining visible imagery and stereo sea-surface elevation, we find that the average along-crest length of breaking waves decreases and the average number of crest ends increases with increasing directional spread. Relative to observations, a parameterized relationship between directional spread and crest characteristics based on theory for non-breaking, refracting waves generally over-estimates breaking crest lengths and is similar to or underestimates the total number of crest ends observed in the surf zone. The wave-field-dependent breaking-wave characteristics examined in the laboratory with remote sensing techniques can inform future investigations of depth-limited short-crested wave breaking and resulting surfzone eddy processes. • Wave basin generated directionally spread waves were measured with remote sensing. • Stereo reconstructions and LiDAR measurements are similar to in situ observations. • Breaker along-crest length and number of ends were estimated with remote sensing. • Average breaker along-crest length decrease with increasing directional spread. • Parameterized crest length and number of ends trend similarly to observations. [ABSTRACT FROM AUTHOR]

The performance of a recently developed "stabilized" turbulence (k − ω) closure model, which avoids un-physical over-production of turbulence prior to wave breaking, is investigated in the computational fluid dynamics (CFD) simulation of cross-shore sediment transport and breaker bar morphology. Comparisons are made with experiments as well as results from simulations employing (otherwise identical) "standard" turbulence closure. The stabilized turbulence model is demonstrated to result in major (qualitative and quantitative) improvements of the predicted breaker bar position and height. Conversely, the established over-production of turbulence in the standard closure, coupled with associated inaccurate undertow structure in the outer surf zone, contribute to erroneous offshore migration of the breaker bar. By correcting these shortcomings, the stabilized turbulence closure model rightly predicts initial onshore morphological migration of the breaker bar without any calibration. This work thus establishes proper turbulence modeling as a prerequisite for accurate CFD prediction of cross-shore sediment transport and profile morphology. • A recently developed "stabilized" turbulence model is used to simulate morphology. • The stabilized turbulence model leads to accurate breaker bar position and height. • A standard turbulence model leads to erroneous offshore breaker bar migration. [ABSTRACT FROM AUTHOR]

In this work, a recently developed numerical model, capable of solving the hydro- and morphodynamics of the cross-shore beach profile, is used to gain insight into the relevant processes driving the generation of a breaker bar. The bedload and suspended sediment transport contributions are analysed separately. It has been shown that the bedload transport tends to accumulate sediment on the onshore side of the undertow detachment point, at a distance that depends on the skewness of the waves and the magnitude of the friction velocity, shaping the onshore face of the breaker bar. In contrast, the suspended transport contributes to the growth of the offshore side of the breaker bar. Besides, a comparison between the sediment transport rates produced by different types of breakers shows a faster bathymetric evolution and an offshore displacement of the position of the breaker bar for high Iribarren numbers. Differences are consistent with the proposed mechanisms driving the evolution of the beach profile. The findings of this work provide a better understanding of the processes driving the formation of breaker bars, how they interact with each other and the relative importance of bedload and suspended sediment transport at each location of the cross-shore profile. [ABSTRACT FROM AUTHOR]

Plunging waves generate vortices that suspend significant amounts of sediment in the key sediment transport mechanism in the surf and swash zones of beaches, causing major changes in the beach morphology over time. The intermittency and locality of vortices and sediment transport make it difficult to measure the flow fields and sediment concentrations in laboratory experiments or field investigations, making such studies of limited value for studying the sediment transport at beaches or as a basis for predicting changes in beach morphology. An attempt is made in this study to overcome these shortcomings through a particle-scale numerical investigation of sediment transport mechanisms under breaking waves. The numerical simulation was performed using three-dimensional coupling of the discrete element method (DEM) and moving particle semi-implicit (MPS) method, with the validity of the numerical model confirmed by comparison with experimental results obtained in a small wave flume. The numerical results revealed the intra-wave sediment motion and the contribution of the wave-generated pressure gradient to the sediment transport near the ripple crest. Sediment transport mechanisms was investigated at the spatial scale of the individual sediment grains by associating it with the vortices, turbulence, and exerted forces near the ripple trough. The results revealed that vortices generated at the water surface by plunging waves affected the bed material motions mainly through the action of a large pressure gradient force that arises near the bed surface owing to a local minimum of pressure, which can be associated with a vortex region. • Bedload and sediment suspension under breaking waves are experimentally observed. • Sediment transport processes are numerically reproduced by 3D-DEM-MPS coupled scheme. • Sediment motions of individual sediment particles are numerically tracked. • A meshless Lagrange method simulates a translation of a surface-generated vortex. • The pressure gradient due to surface-generated vortex contributes to the sediment suspension. [ABSTRACT FROM AUTHOR]

A series of laboratory experiments and high-fidelity numerical simulations were carried out to investigate the scour induced by a solitary wave around a non-slender, vertical structure of a square cross-section on a sandy berm. Various wave and water level combinations, structure layouts, and dimensions were studied. The flow separation at the sharp edges resulted in counterclockwise rotating out-of-plane vortices that extended from the free surface to the sandy berm. These energetic vortices were found to be the primary driving mechanism of the scouring through entraining, and entrapping the sand particles into their cores, keeping them in suspension, carrying them, and releasing them as the vortices propagate along a spiral trajectory. The analyses also showed that the flow blockage by the structure could result in flow field modulation and exacerbate the scour processes. The structure dimension appeared to be the most influential factor in the scouring process, drastically altering the characteristics of the non-equilibrium scour holes. Irrespective of the structure dimension or layout, the scour depth was greater around the seaside edge of the structure while the scour width was larger on the leeside edge. The uncertainties associated with the maximum non-equilibrium scour depth were quantified via Monte Carlo simulations which showed that the impact of the structure dimension on the maximum scour depth was almost twice more significant than that of the layout. The three-dimensional (3D) Eulerian two-phase flow numerical model, SedWaveFoam, was shown to be able to simulate sediment transport and scour around sharp-edged square structure at a low Keulegan-Carpenter (K C) number. • Interactions between a solitary wave and an impermeable structure placed on an erodible berm were studied experimentally. • Energetic vortices, formed at the corners of the structure, were found to be the primary driving mechanism of the scouring. • Structure dimension was the most influential factor in the scouring process. • Monte Carlo simulation was employed to assess the sensitivity of the maximum non-equilibrium scour depth to flow conditions. • SedWaveFoam model simulated scour around sharp-edged square structure at low Keulegan-Carpenter (KC) number. [ABSTRACT FROM AUTHOR]

In this work, a new numerical model for cross-shore beach profile evolution, IH2VOF-SED, is developed. It consists in the bidirectional coupling of a 2D RANS hydrodynamic solver and a sediment transport module. The resulting model is extensively validated against three benchmark cases at different scales, attending to the hydrodynamics, bottom shear stress and bathymetry evolution. Comparisons between experimental and numerical results show a good agreement for both the flow variables and the seabed evolution in all the validation cases without making use of calibration parameters. Additionally, the qualitative analysis of the results is in accordance with previous experimental observations of sediment transport induced by breaking waves. The computational cost is greatly reduced to about 1/10 of other available RANS models. As a novel aspect regarding RANS models, the model is able to simulate the swash zone and changes in the position of the coastline. A good compromise between precision and computational cost is achieved, allowing for an in-depth analysis of the processes leading to the cross-shore profile evolution. • A new, efficient RANS model for cross-shore beach morphodynamics is presented. • It includes bedload and suspended sediment transport and the effect of landslides. • The hydrodynamic and sediment transport features are validated at different scales. • The model is able to reproduce the evolution of the beach profile accurately. • The computational cost is 10 times lower than that associated with other RANS models. [ABSTRACT FROM AUTHOR]

The performances of phase-averaged parametric nearshore wave transformation models depend significantly on a reliable estimate of the wave breaker index γ (the breaker height-to-depth ratio), a free model parameter that essentially controls the amount of breaking energy dissipation. While the previous studies have suggested separate relationships between γ and the offshore wave steepness (s 0) or the normalized local water depth (kh), the parameterization of γ still requires further investigation considering a wider variety of conditions and a sounder physical basis. In this study, we use the field datasets of wave height and the inverse modelling approach to reveal a composite dependence of γ on both s 0 and kh. Specifically, the results show a positive dependence of γ on kh for larger s 0 , and a negative dependence of γ on kh for smaller s 0. Based on such composite relationships, a new γ formula is proposed, and its performance is verified against the available datasets of wave height in three coasts and 14 laboratory tests. Implementation of this new formula in a parametric wave model leads to the error reduction of wave height prediction by 10–24% (mean = 19%) relative to seven widely used models in literatures. In particular, a remarkably higher model accuracy is obtained under wave conditions with small offshore wave steepness, which is important for studying onshore sediment transport and beach recovery. Two counteractive physical mechanisms for wave nonlinearity effects, namely the breaking intensification mechanism and the breaking resistance mechanism, are suggested to explain the opposite γ - kh relationships within different ranges of s 0. • The composite dependence of wave breaker index on both the offshore wave steepness and the normalized local water depth. • A new formula for wave breaker index used for parametric wave transformation modelling leads to improved wave height prediction especially under wave conditions with small offshore wave steepness. • Two counteractive physical mechanisms to explain the composite relationships. [ABSTRACT FROM AUTHOR]