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Abstract: The present work analyzes the hydro-morphodynamics characterizing the swash region during the uprush stage. A comparison is illustrated between the sediment transport measured in a series of dam-break experiments and that predicted by the numerical hydro-morphodynamic model of Postacchini et al. (2012). The primary aim is to investigate the differences arising between the weakly coupled or uncoupled model and the measurements, in terms of hydrodynamics, tip celerity and sediment transport. The hydrodynamics are well described by the model and results have been used to calibrate both friction factor and subgrid turbulent viscosity. Comparison of numerically-computed tip celerity with experimental data reveals a fairly good agreement, i.e. a mean error of about 10%, while modeled sediment transport differs by about 40% from the available data. No evident differences are found between results obtained from the coupled and uncoupled model runs (2% for the celerity and 11% for the sediment transport rate at the tip), suggesting that for the specific flow under investigation, at the leading edge of the swash front, hydro-morphological coupling is not an issue of fundamental importance. However, for the special case here of a swash forced by a dam-break, scour occurs at the dam location, and in this case the erosion of the bed is significantly larger in the uncoupled model. [Copyright &y& Elsevier]

Abstract: Numerical cross-shore profile evolution models have been good at predicting beach erosion during storm conditions, but have difficulty in predicting the accretion of the beach during calm periods. This paper describes the progress made in modifying and applying the public domain XBeach code to the prediction and explanation of the observed behaviour of coarse-grained beaches in the laboratory and the field under accretive conditions. The paper outlines in details the changes made to the original code (version 12), including the introduction of a new morphological module based upon Soulsby's sediment transport equation for waves and currents, and the incorporation of Packwood's infiltration approach in the unsaturated area of the swash region. The competence of this modified model during calm conditions for describing the steepening of the profile, and the growth of the beach berm is demonstrated. Preliminary results on the behaviour of the beach subject to both waves and tides are presented. Good agreement is found between the model simulations and large-scale laboratory measurements, as well as field observations from a composite beach in the UK. The reasons for the model's capabilities are discussed. [Copyright &y& Elsevier]

Abstract: This paper presents a simple approximation for assessment of the alongshore sediment transport rate on dissipative weakly curvilinear coasts. The approximation considers spatial variations in the wave angle and energy, sediment size, beach slope and type of wave breaking inside the surf zone. Similar approaches were employed in the past for coasts forced by high oblique waves where shoreline undulations frequently develop. With our approach, we can analyze more realistic situations, i.e., alongshore relations between the sediment size and the beach slope. The general expression for alongshore sediment transport is made explicit using a simple model for wave propagation. The solution is used to examine the implications of the behavior of alongshore sediment transport on shoreline evolution. After the analysis of the effects of variations in the wave conditions, the alongshore beach slope distribution and the shoreline geometry, three different shoreline tendencies were obtained for undulating shorelines: migration for changes in deep-water wave angle; amplitude growth for increases in wave height or period and changes in shoreline geometry; and wavelength changes and asymmetries for coasts with alongshore changes in beach slope. Our results also indicate that wave setup may enhance the formation of undulating shoreline features. [Copyright &y& Elsevier]

Abstract: In this work a detailed hydrodynamic model is presented, which is used for the study of cross-shore sediment transport and morphodynamics in two dimensions. The model is described in the framework of the generally unstructured, finite volume method. Considerable emphasis is put on those subtleties in the morphological formulation, which are required to achieve mass conservation for the amount of sediment in the bed and in suspension. In this first part of two, the hydrodynamic description over the cross-shore profile is presented. The model is validated against an experiment with detailed measurements of the free surface and turbulence over a fixed breaker bar profile. A test matrix covering a large interval of the surf similarity parameter is simulated, and the phase lag between the breakpoint and the initiation of the setup is described. The relation of this phase lag to a cross-shore delay in dissipation of organised energy into turbulence is described. The relation of this phase lag to the distribution of the location of maxima in bed shear stresses and magnitude of the undertow is also described. Furthermore, processes in the hydrodynamics, which will have a smoothing effect on the mean cross-shore sediment transport and morphodynamic response are considered. All simulations are presented for regular waves and for values of the deep-water surf similarity parameter, ζ 0, in the range from 0.08 to 1.19, i.e. covering both spilling and plunging breakers. [Copyright &y& Elsevier]

Abstract: In Part 2 of this work, the hydrodynamic model described in Part 1 is applied for the simulation of sediment transport and the associated morphological development of breaker bars. The sediment description is split into bed load and suspended load, and like the hydrodynamics the sediment transport is phase-resolved in order to get on- and offshore directed contributions to the sediment transport from phase lags between the suspended sediment and the hydrodynamics. First, the sediment transport over a morphologically fixed bed of a constant slope is considered, and the transport rates are discussed in terms of three dimensionless parameters: the surf similarity parameters (ζ 0) and two variants of Dean's parameter (Ω D and Ω HK ). These cover the ranges ζ 0 ∈[0.08, 1.19], Ω D ∈[1.1, 27.7] and Ω HK ∈[0.01, 0.52]. Secondly, the morphological development of the cross-shore profile is considered, where the initial condition is a profile of constant slope. One reference simulation is discussed in detail and the morphological development due to changes in the hydrodynamic forcing (wave period, wave height, wave irregularity and a net cross-shore current) and in other properties (sediment size and beach slope) is discussed relative to the reference simulation. The coupling between sediment transport rate and morphology is analysed and discussed. [Copyright &y& Elsevier]

Abstract: In a closely integrated system, (sub-) littoral sandy sediments, sandy beaches, and sand dunes offer natural coastal protection for a host of environmentally and economically important areas and activities inland. Flooding and coastal erosion pose a serious threat to these environments, a situation likely to be exacerbated by factors associated with climate change. Despite their importance, these sandy ‘soft’ defences have been lost from many European coasts through the proliferation of coastal development and associated hard-engineering and face further losses due to sea-level rise, subsidence, storm surge events, and coastal squeeze. As part of the EU-funded THESEUS project we investigated the critical drivers that determine the persistence and maintenance of sandy coastal habitats around Europe's coastline, taking particular interest in their close link with the biological communities that inhabit them. The successful management of sandy beaches to restore and sustain sand budgets (e.g. via nourishment), depends on the kind of mitigation undertaken, local beach characteristics, and on the source of ‘borrowed’ sediment. We found that inter-tidal invertebrates were good indicators of changes linked to different mitigation options. For sand dunes, field observations and manipulative experiments investigated different approaches to create new dune systems, in addition to measures employed to improve dune stabilisation. THESEUS provides a ‘toolbox’ of management strategies to aid the management, restoration, and creation of sandy habitats along our coastlines, but we note that future management must consider the connectivity of sub-littoral and supra-littoral sandy habitats in order to use this natural shoreline defence more effectively. [Copyright &y& Elsevier]

Abstract: Simulating swash zone morphodynamics remains one of the major weaknesses of beach evolution models. One of the reasons is the limited availability of data on morphological changes at the temporal scales of individual swash events. This paper sets out to present a new hybrid system, consisting of 2D/3D laser scanners and several video cameras, which was designed to monitor swash zone topographic change on a wave-by-wave basis. A methodology is proposed consisting of sensor calibration and several data processing steps, allowing a fusion of different sensors. Such an approach can improve the performance of several field/laboratory, optical technique applications for nearshore hydro- and morpho-dynamic measurements. Digital Elevation Models from a 3D scanner were used in the extrinsic camera calibration procedure and reduced the geo-rectification errors from 0.035m Qt >3.5kgm−1 s−1. Extreme transport swash events became rarer as the morphology was reaching equilibrium. [Copyright &y& Elsevier]

Abstract: The paper examines the dependency between total sediment transport, q, and grain size, D (i.e. q ∝ D p ) under dam break generated swash flows. Experiments were performed in a dam break flume over a sloping mobile sand bed with median grain sizes ranging from 0.22mm to 2.65mm. The total sediment transport was measured by truncating the flume bed and collecting the sediment transported over the edge. The experiments were designed to exclude pre-generated turbulence and pre-suspended sediment so as to focus solely on the swash flow. The magnitude and nature of the grain size dependency (i.e. p value) were inferred for different flow parameters; the initial dam depth, do , the integrated depth averaged velocity cubed, ∫ u 3 dt, and against the predicted transport potential, qp , using the Meyer-Peter Muller (MPM) transport model and variations of that model. The data show that negative dependencies (p <0) are obtained for do and qp , whilst positive dependencies (p >0) are obtained for ∫ u 3 dt. This indicates that a given do and qp transport less sediment as grain size increases, whereas transport increases with grain size for a given ∫ u 3 dt. The p value is found to be narrowly ranged, 0.5≤ p ≤−0.5. On average, the incorporation of a pressure gradient term via the piezometric head into the MPM formulation reduces qp by 4% (fine sand) to 18% (coarse sand). The measured total transport for fine and coarse sands is best predicted using MPM and MPM+ dp*/dx respectively. However, the inferred optimum transport coefficient in the MPM formulation is about 30, much higher than the standard coefficient in a steady flow and this is not due to the presence of the pre-suspended sediment. The optimum transport coefficient indicates some sensitivity to grain size, suggesting that some transport processes remain unaccounted for in the model. [Copyright &y& Elsevier]

Abstract: During storm events wave setup in shallow regions can contribute significantly to the total water elevation, and radiation stress can also generate alongshore drift influencing sediment transport. In low lying coastal regions this generates the potential for flood inundation and morphological change. A coupled tide–surge–wave modelling system is therefore required for accurate forecasting. Liverpool Bay, UK, is taken as a case study because it has a resource of observations and incorporates three estuaries, thus providing conditions to assess the model performance both at the open coast and within estuarine environments. The model covers a region encompassing depths from about 50m below the mean tidal level to shallow wetting and drying regions, and has previously given good wave and surge hindcasts both for individual storm events and multi-year studies. The present study builds on an already accepted model, to include and assess the spatial influence of 2D radiation stress when implemented in a 3D circulation model. The results show that the method is computationally efficient, so relevant for operational use, and also provides a plausible solution. The varied influence of radiation stress across a coastal domain is demonstrated, with larger impact at an estuary mouth and along the open coast, while having lesser impact within an estuary and further offshore. [Copyright &y& Elsevier]

Abstract: This paper describes a unique new physical testing facility for studying ocean-structure–seabed interactions, and in particular pipeline on-bottom stability on erodible seabed under hydrodynamic loading. The facility, named the O-tube due to its shape, is a fully enclosed flume in which ambient and storm-induced near-seabed flows are generated by a computer-controlled flow pump. Combined steady and oscillatory flow can be generated by alternating pump flow directions in a controlled manner, and computer control also allows irregular flow to be generated. The design of the O-tube combines the capabilities of a conventional open channel flume (which provides steady current) with a U-tube (which provides oscillatory flow). The facility is designed to physically model severe storm conditions, as well as ambient or tidal flows. When studying pipeline stability on erodible seabed under severe hydrodynamic loading conditions, tests can be performed at a relatively large scale (typically 1/5) for large diameter pipelines (e.g. 40in. gas trunklines) and at full scale for small diameter pipelines (<8in.) to minimize potential scaling effects associated with movable bed model tests. The specifications of the O-tube, a model pipe and an actuator system that supports the model pipe are given in detail. Preliminary model testing results show that the facility has met its design expectations. [Copyright &y& Elsevier]

Abstract: Potential physical impacts of an anthropogenic effect on a tidal basin evolution are investigated applying the Delft3D model suite under both tidal and wave boundary forcings. Study area is based on a peninsula construction of the Ley Bay in the East Frisian Wadden Sea. Model simulation spans from 1975 to 1990 in two stages of which the second stage begins with the implemented peninsula on the 1984 predicted morphology. The model bed consists of initially distributed three sediment fractions. Sensitivity of the Ley Bay evolution is analysed under three different sediment transport formulations: 1) Van Rijn, 1993 (VR93), 2) Soulsby, 1997 (SVR) and 3) Van Rijn et al., 2004 (VR04). Offshore tides and waves are transformed up to the model boundaries via a nested modelling approach and a statistically derived highly schematised wave climate is adopted in the simulations. Predicted morphologies indicate lower agreement with the measured morphology due to including very sparse data. Despite this discrepancy, they reproduce the major changes in the Ley Bay caused by the peninsula construction while each formula results in a slightly different channel/shoal pattern. Predicted evolution under the SVR shows the strongest sediment exporting system and therefore the lowest agreement with the 1990 measured morphology. Both VR93 and VR04 formulas resulted in marginal exporting systems and more or less similar morphologies. In fact, only the VR04 prediction indicates a fair agreement with the 1990 data. Temporal evolution under the VR04 shows concentrated velocity patterns at the bay entrance and in the eastward bay channel resulting in the development of this channel and sedimentation in the southern part of the bay as found in the data. [Copyright &y& Elsevier]

Abstract: Thus far various numerical models have been developed and improved to aid understanding of the sediment transport process due to tsunamis. However, the applicability of these models for the field-scale bathymetric change remains a major issue due to the scarcity of measured bathymetric data immediately before and after tsunamis. This study focuses on assessing the applicability of the sediment transport model by comparing the model results with measured bathymetry data obtained one month before and two months after the 2004 Indian Ocean tsunami at Kirinda Fishery Harbor, Sri Lanka. Obtained model results were compared with measured data along four different transects. In particular, similar to the measured data, the model reproduced the bed level change at the harbor mouth well, although it shows some discrepancy on bathymetric change along the shoreline, which is directly affected by littoral drift. Therefore, it is noted that the divergence of reproducing the local bathymetry change is due to the normal wind wave effect on measured data and the model limitations. Hence we included the wind wave effect in modeled data and the discrepancy between measured and modeled data was reduced. Furthermore, the modeled bed level change indicates a dynamic behavior in terms of the net variation during the tsunami flow, such that deposition dominates in the inflow and erosion dominates in the backflow. Both bed level variation and the suspended load concentration reveal that the large amount of eroded sediment attributable to tsunami waves was in suspended form and was deposited in the nearshore area after the water fluctuation had abated. The model results further indicate that eroded sediment at the initial depth deeper than 11m might be brought by the incoming tsunami waves and deposited in the nearshore area where the depth is shallower than 7m. [Copyright &y& Elsevier]

Modeling hydrodynamics and sediment transport inside a multi-inlet wetland system is a challenging task due to constraints on model efficiency, accuracy and representation of physics, the scarcity of field data for model validation, and more importantly, the availability of high-resolution data sets of channel bathymetry. Lack of field data sets, model assumptions, and limitations often lead to wrong interpretation of the system's nature. To correctly predict the changes in governing hydrodynamics and morphology of shallow wetland environments, it is essential to resolve the channel properties with the best possible accuracy. In this study, we consider the importance of two factors, numerical model grid resolution around channel shorelines, and bathymetry surveys, in improving the interior channel and mudflat hydrodynamics. Channel shoreline element size is observed to be an essential factor in time-varying channel volume estimation. Lower model resolution in channels inside a marsh can provide good agreement with in-situ surface elevation data, even though the channel surface-velocity phase lag, and velocity magnitude and asymmetry are inaccurate. A higher grid resolution at the channel berms and the high-density channel survey help define the time-varying lateral shoaling bathymetry, cross-section area, and bottom friction, and improve the corresponding model skill and tidal wave properties significantly. We have selected Bombay Hook National Wildlife Refuge, a rapidly eroding salt-marsh system in Delaware Bay, to compare model skills for different hypsometric conditions. Model performance is evaluated during two storm conditions; Hurricane Sandy (2012) and Hurricane Joaquin (2015), using tide gauges located on the marsh interior channels. • Poor grid resolution around marsh-channel berm creates artificial channel convergence and inaccurate tidal wave characteristics. • Topo-bathymetry field survey and bias correction improves channel hypsometry and numerical model reliability significantly. • Inaccurate channel bathymetry and inadequate resolution might lead to acceptable surface elevation but poor tidal velocity field. • The conventional grid generation methods based on sparse cross channel surveys is not sufficient for wetland models. [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]

Although there have been many efforts in the literature to hindcast the patterns of longshore sediment transport (LST), they mainly disregarded uncertainty issues. Forcing datasets, wave transformation methods, and LST models are among the main sources of uncertainty in LST estimations. The combination of the aforementioned sources of uncertainty makes the estimation of LST patterns challenging for non-straight coastlines as the uncertainty ranges might vary from site to site. In this paper, a simple ensemble modeling framework was employed to investigate LST rate uncertainty at seven sites along a non-straight coastline (Gold Coast, Australia). The ensemble was formed by two different forcing datasets (i.e., integral parameters of total wave energy from two hindcast datasets of ERA5 and CAWCR), two different wave transformation methods (i.e., inclusion and exclusion of local wind effects), and eight LST models (i.e., bulk formulae and process-based models). Moreover, the relative importance of each source of uncertainty was ranked using the ANOVA-variance-based model. Finally, the weighted ensemble mean was used to investigate intra- and inter-annual variability of LST rates. The results showed that the range of uncertainty of LST rates for open coasts of Gold Coast is much higher than that of semi-sheltered coasts. On annual scale, for open coasts, 40% to 50% of total uncertainty was due to the choice of wave transformation methods, while for semi-sheltered coasts, it was 20%–30%. Moreover, almost for all sites, 30% to 50% of total uncertainty was controlled by the choice of LST models and the interaction of wave transformation methods and LST models. Although the weighted ensemble mean could provide an estimate of LST patterns along the coast, addressing the residual uncertainties (arising from other sources, discussed at the end of this paper), in future works, would help increase the certainty of the estimations. • Forcing datasets, wave transformation methods, and sediment transport models are the main sources of LST uncertainty. • The effect of local wind on wave transformation is usually overlooked that can be a significant source of LST uncertainty. • Ensemble modeling enables modelers and coastal planners to capture a broader range of uncertainty in the estimation of LST patterns. • Uncertainty ranking reveals which source of uncertainty should be addressed more to yield reliable estimations for LST. [ABSTRACT FROM AUTHOR]

Wind waves breaking at an angle with the shoreline force the drifting of littoral sediments, which is known for contributing to the formation and growth of barrier spits. Intriguingly, increased rates of longshore wave power have also been associated with the erosion of some barrier spits on the updrift margin of tidal inlets. Therefore, a numerical experiment was designed and is presented here, which investigates the possible links between the longshore wave power and the shortening of these elongated coastal barriers. Based on a process-based model, the experiment provides new insights into the forces at play in the redistribution of sediments between a sandspit and its adjacent inlet, respectively the Cap Ferret and the Bay of Arcachon's tidal inlet, in SW France. More particularly, model scenarios were defined that show how combined waves and tide create gradients of residual sediment transport responsible for a sediment deficit at the spit – inlet boundary. The deficit was also found to deepen with increasing longshore wave energy, as if the transfer of sediment from the spit to inlet shoals was accelerated. This physically explains the previously observed retreat of the spit's distal end during periods dominated by the positive phase of North Atlantic Oscillation (NAO) in winter. Indeed, according to model results, higher and/or more oblique waves associated with the positive phase of the NAO are expected to increase the transfer and storage of the drifting sediments to and by the inlet shoals, and this at the expense of the spit. While these conclusions remain valid, we noticed that the sensitivity of model results to the bottom friction enhanced the importance of accurately representing the spatio-temporal distribution of bed roughness when investigating the morphodynamic interactions between real-world tidal inlets and their margins. • Residual sediment transport is modelled at a mixed energy tidal inlet. • Waves with tides create transport gradients near the inlet's updrift spit extremity. • Larger or more oblique waves deepen this gradient and reinforce the sand deficit. • Physically sounded explanation to enhanced spit erosion during NAO + winters. • Bed roughness models need to be improved for 2DH inlet-spit morphodynamic modelling. [ABSTRACT FROM AUTHOR]

Scour beneath piggyback pipelines laid on a sandy seabed is numerically investigated by solving two-dimensional (2D) Unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The fluid flow and the sediment transport are resolved using a two-phase flow Eulerian-Eulerian numerical solver SedFoam based on the open-source code OpenFOAM. A parametric study is performed for different Shields parameters defined as θ = U f 2 / (s − 1) g d 50 of 0.068, 0.18 and 0.33 where U f denotes the shear friction velocity on the seabed, the specific density of the sediment grains is s = ρ s / ρ f where ρ s and ρ f are the density of the solid and fluid phases, respectively, g is the gravitational acceleration and d 50 is the median grain diameter. Scour processes beneath piggyback pipelines under steady current flow condition are studied for different gap ratios of G / D = 0,0.15 , 0.25 and 0.35 between the main cylinder and the additional ones with a diameter ratio of d / D = 0.3 between them. Mesh convergence studies based on the scour depth and the sediment profile are performed to obtain an appropriate grid resolution. A validation study is performed by comparing the scour depth of the single cylinder cases with the published experimental results obtained by Mao (1986). The effects of different θ and geometry configurations on the scour depth and the sediment profile are obtained and discussed. • Scour beneath piggyback pipelines is investigated. • The fluid flow and the sediment transport are resolved using a two-phase flow solver. • Influences of different Shields parameters on the scour depth and the sediment profile are investigated. • Influences of different piggyback geometry configurations on the scour depth and the sediment profile are investigated. [ABSTRACT FROM AUTHOR]

This paper presents a bottom boundary layer model for the unsteady and non-uniform flow in the swash zone, by extending the momentum integral method so as to include spatial gradients. The developed model is further incorporated into a hydrodynamic model based on the Nonlinear Shallow Water Equations. Two swash zone cases are examined to investigate the effect of the inclusion of spatial gradients. In the first of the two, boundary layer development under non-breaking periodic waves formulated by Carrier and Greenspan (1958) is investigated (wave-driven swash). Results show that the spatial gradients have the most pronounced effect in the lower swash, and in the region just seaward. In both these regions the spatial gradients enhance (diminish) onshore (offshore) bed shear stress, thus potentially contributing to onshore sediment transport under non-breaking waves. The second case investigated is the Kikkert et al. (2012) dam-break swash event (bore-driven swash). The model results are qualitatively and quantitatively accurate when compared against the laboratory measurements, and the velocities in the later backwash agree more closely with the measurements than those of Briganti et al. (2011). Results show that the inclusion of spatial gradients also favours onshore sediment transport in the lower swash. In addition, the bottom boundary layer is more fully developed in the uprush tip, resulting in smaller bed shear stress in the upper swash. The extended momentum integral method thus appears to capture more comprehensively the swash boundary layer, and the approach, therefore, offers a way forward in more accurate reproduction of swash dynamics in computational modelling. • A logarithmic bottom boundary layer model is developed for the swash flow. • Spatial gradients are included in the boundary layer model. • Two cases are examined to investigate the effect of spatial gradients. • The model results compare reasonably well against the laboratory measurements. • The developed model captures the fully developed boundary layer in the uprush tip. [ABSTRACT FROM AUTHOR]

Abstract: The turbulent velocity field associated with the breaking of plunging regular waves on a 3% plane slope was measured in a plane running parallel to the slope using a particle image velocimetry (PIV) system. The measurement plane was located within the wave bottom boundary layer. The horizontal distance from the point of incipient breaking to the center of the measurement area was approximately 12 times of the breaking depth. The same wave train was generated 36 times and in each trial three consecutive wave cycles were recorded at a sampling rate of 15Hz. The measured velocity fields were separated into a mean flow and a turbulence component using the ensemble averaging technique. The impingement process of breaking-wave-generated vortices on the bottom was investigated. The results showed that the impact of a plunging wave vortex on the bottom was a highly transient and three-dimensional phenomenon. The vortex arrived at the bottom around the instant of maximum positive wave-induced velocity. The surge of turbulence continued for a time of about , where Hb is breaker height and g is acceleration due to gravity. The impingement region was not stationary, but continued to travel onshore with an initial speed close to the wave celerity. The distributions of turbulent velocity fluctuations and related momentum fluxes depended on the types of vortices produced. Plunger vortices generated at incipient breaking in deeper water had the characteristics of a three-dimensional vortex loop with counter-rotating vorticity. Large apparent shear stresses were measured in the flow attachment and detachment zones in front and behind the vortex loop. Transverse vortices generated in the subsequent splash in shallower water produced an asymmetrical impingement pattern similar to that of an inclined jet; the downburst of turbulent fluid was deflected outward and shoreward resulting in large onshore fluxes of turbulence energy near the bottom. Large apparent shear stresses were measured in the impingement zone and wall jet region. The motions of glass spheres sliding along the bottom were investigated. It was found that the velocities of glass spheres impacted by downbursts could significantly exceed the wave-induced velocities. It was also found that glass spheres could be trapped by counter-rotating vortices and carried for considerable distances onshore. The measured data suggested that compared to spilling waves, downbursts in plunging waves would enhance onshore sediment transport. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical study of the non-linear development of alongshore undulations up to fully developed quasi-steady equilibrium. A numerical model which describes the longshore sediment transport along arbitrarily shaped shorelines is applied, based on a spectral wave model, a depth integrated flow model, a wave-phase resolving sediment transport description and a one-line shoreline model. First the length of the shoreline undulations is determined in the linear regime using a stability analysis. Next the further evolution from the linear to the fully non-linear regime is described. In the fully non-linear regime down-drift spits and migrating shoreline undulations are described. Three different shoreline shapes are found depending on the wave conditions: undulations with no spits, undulations with shore parallel spit and undulations with reconnecting spits. [Copyright &y& Elsevier]

Abstract: Longshore sediment transport (LST) is one of the main drivers of beach morphology. Bulk LST formulas are routinely used in coastal management/engineering studies to assess LST rates and gradients. Over 50years of research has resulted in several bulk LST formulas that have been tested with varying levels of rigour. In this study, the predictive skill of three of the most commonly used bulk LST formulas (CERC, Kamphuis and Bayram) is rigorously evaluated using the most extensive LST data set presently available. The calibration coefficients in the three formulas are improved using a least-squares optimization algorithm, resulting in a significant improvement in the predictive skill of all three formulas. The generality of the improved formulas is verified via the statistical methods of bootstrapping and cross-validation. While the performance of all three improved formulas is very similar, the improved Kamphuis formula performs best, followed by the improved Bayram formula. [Copyright &y& Elsevier]

Abstract: This paper presents a new numerical model for shoreline change which can be used to model the evolution of shorelines with large curvature. The model is based on a one-line formulation in terms of coordinates which follow the shape of the shoreline, instead of the more common approach where the two orthogonal horizontal directions are used. The volume error in the sediment continuity equation which is thereby introduced is removed through an iterative procedure. The model treats the shoreline changes by computing the sediment transport in a 2D coastal area model, and then integrating the sediment transport field across the coastal profile to obtain the longshore sediment transport variation along the shoreline. The model is used to compute the evolution of a shoreline with a 90° change in shoreline orientation; due to this drastic change in orientation a migrating shoreline spit develops in the model. The dimensions of the spits evolving in the model compare favorably to previous model results and to field observation of the Skaw Spit in the north of Denmark. [Copyright &y& Elsevier]

Abstract: The boundary layer is very important in the relation between wave motion and bed stress, such as sediment transport. It is a known fact that bed stress behavior is highly influenced by the boundary layer beneath the waves. Specifically, the boundary layer underneath wave runup is difficult to assess and thus, it has not yet been widely discussed, although its importance is significant. In this study, the shallow water equation (SWE) prediction of wave motion is improved by being coupled with the k–ω model, as opposed to the conventional empirical method, to approximate bed stress. Subsequently, the First Order Center Scheme and Monotonic Upstream Scheme of Conservation Laws (FORCE MUSCL), which is a finite volume shock-capturing scheme, is applied to extend the SWE range for breaking wave simulation. The proposed simultaneous coupling method (SCM) assumes the depth-averaged velocity from the SWE is equivalent to free stream velocity. In turn, free stream velocity is used to calculate a pressure gradient, which is then used by the k–ω model to approximate bed stress. Finally, this approximation is applied to the momentum equation in the SWE. Two experimental cases will be used to verify the SCM by comparing runup height, surface fluctuation, bed stress, and turbulent intensity values. The SCM shows good comparison to experimental data for all before-mentioned parameters. Further analysis shows that the wave Reynolds number increases as the wave propagates and that the turbulence behavior in the boundary layer gradually changes, such as the increase of turbulent intensity. [Copyright &y& Elsevier]

Abstract: A numerical model coupling the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations with two-equation k − ω turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes. The hydrodynamic model is additionally coupled with bed and suspended load descriptions, the latter based on an unsteady turbulent-diffusion equation, for simulation of sheet-flow sediment transport processes. In addition to standard features common within such RANS-based approaches, the present model includes: (1) hindered settling velocities at high suspended sediment concentrations, (2) turbulence suppression due to density gradients in the water–sand mixture, (3) boundary layer streaming due to convective terms, and (4) converging–diverging effects due to a sloping bed. The present model therefore provides a framework for simultaneous inclusion of a number of local factors important within cross-shore wave boundary layer and sediment transport dynamics. The hydrodynamic model is validated for both hydraulically smooth and rough conditions, based on wave friction factor diagrams and boundary layer streaming profiles, with the results in excellent agreement with experimental and/or previous numerical work. The sediment transport model is likewise validated against oscillatory tunnel experiments involving both velocity-skewed and acceleration-skewed flows, as well as against measurements beneath real progressive waves. Model capabilities are exploited to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity-skewed conditions. For the medium sand grain conditions considered, the model results suggest that streaming effects can enhance onshore sediment transport rates by as much as a factor of two. Moreover, for fine sand conditions streaming (and related convective) effects are demonstrated to potentially reverse the direction of net transport (i.e. from offshore to onshore) relative that predicted in oscillatory tunnel conditions. The developed model is implemented within the popular Matlab environment, and hence may be attractive for both research and educational purposes. [Copyright &y& Elsevier]

Abstract: This contribution describes the development, calibration and verification of a 1-D behaviour-orientated shoreline prediction model. The model primarily encapsulates shoreline displacement forced by wave-driven cross-shore sediment transport. Hysteresis effects are shown to be important and are included in the model, whereby present shoreline change is influenced by past hydro-/morpho-dynamic conditions. The potential magnitude of shoreline change increases with incident wave power and the degree of disequilibrium. The latter disequilibrium term (Ω eq − Ω) is expressed in terms of the time-evolving equilibrium (Ω eq) and instantaneous (Ω) dimensionless fall velocities and dictates the direction of shoreline movement. Following Wright et al. (1985) the equilibrium fall velocity is defined as a function of the weighted antecedent conditions and is a proxy for the evolving beach state. The decay rate of the weighting function used to compute Ω eq is a model free parameter determined by calibration against measured data, which physically reflects the degree of observed ‘memory’ of the system. The decay in amplitude of this weighting function with time is controlled by a ‘memory decay’ term (ϕ), where the weighting reaches 10%, 1% and 0.1% at ϕ, 2ϕ and 3ϕ days prior to the current calculation time. The model is applied to two multi-year (6+ years) data sets incorporating hourly wave and weekly shoreline measurements, from two contrasting energetic sites in SE Australia. The first is the relatively dissipative, straight Gold Coast (QLD) and the second is a more intermediate embayed beach at Narrabeen (NSW). The model shows significant skill at hindcasting shoreline change at both sites, predicting approximately 60% of the total shoreline variability. The Gold Coast shoreline is dominated by a strong seasonal signal. Conversely, at the Narrabeen embayment, shoreline variability (and morphology) is more dynamic, responding at storm frequency. Evidence suggests that there is a strong coherence between the shoreline position and morphodynamic state and that both have response times characterised by ϕ. It is hypothesised that optimised ϕ values in the shoreline model physically relate to the efficiency of sediment exchanges between the shoreface and offshore bars and the prevalence of one- or two-dimensional horizontal circulation. The general success of this new shoreline model for hindcasting the observed shoreline behaviour at two distinctly different open-coast sites suggests that this approach may be suitable for broader application. [Copyright &y& Elsevier]

Abstract: Indian River Inlet is located at roughly the mid-point of the Atlantic coast of Delaware and connects the ocean to two Delaware inland bays. Jetties constructed in 1940 have maintained the inlet for navigation purposes but have also acted as a barrier to net northerly alongshore sediment transport causing downdrift erosion. A mobile, land-based bypassing system was initiated in 1990 in an effort to counteract this erosion. Beach profile data from 1985 (pre-bypassing) until 2008 are used to investigate the effect of the sand bypassing system on beaches adjacent to the inlet. The downdrift beach experienced horizontal shoreline erosion between 10 and 60m during the pre-bypassing period but accreted 10–20m during the bypassing period. The mean shoreline location on the updrift beach during bypassing is 10–20m landward (erosion) of its position during the pre-bypassing period. Empirical orthogonal function (EOF) amplitudes from analyses performed on mean-removed elevation surfaces during the periods of highest bypassed volume (average of 83% of design rate) showed that the influence of the bypassing system on the downdrift beach extends to about 1500m of the inlet. An EOF analysis showed that different morphologic responses were evident following the initiation of bypass operations. Temporal variations of shoreline and beach morphology were correlated to the temporal variations in bypassing rates on the downdrift beach only. The downdrift beach response was greatest near the inlet for larger bypassing volumes. Correlation in these instances occurred with a roughly 1-year time lag suggesting that the beach quickly redistributes the bypassed sand. EOF amplitude and shoreline response are weakly correlated to bypassed volumes when the system bypassed smaller volumes (average of 56% of design rate) of sand suggesting that there is a minimum bypassing rate, regardless of yearly variability, below which the effect on the downdrift beach is obscured. [Copyright &y& Elsevier]

Abstract: The paper describes the results of a numerical and experimental investigation of flow and scour around a half-buried sphere exposed to a steady current. Hot-film bed shear stress and Laser Doppler Anemometer measurements were made with a half sphere mounted on the smooth bed in an open channel. The hydrodynamic model is a 3-D general purpose N–S flow solver. The k-omega SST turbulence model was used for closure. The flow model was used to study the horseshoe vortex and lee-wake vortex flow processes around the sphere. The flow model was coupled with a morphologic model to calculate scour around the half-buried sphere in currents. The morphologic model includes a sediment-transport description, and a description of surface-layer sand slides for bed slopes exceeding the angle of repose. The sediment transport description includes, for the first time, the effect of externally-generated turbulence (induced by the horseshoe-vortex flow and the lee-wake flow processes) on sediment transport. The results show that the scour depth increases and time scale decreases when the effect of externally-generated turbulence is incorporated in the calculations. Empirical expressions representing the numerically obtained data on the equilibrium scour depth and the time scale are presented. The results show that the equilibrium maximum scour depth in the live-bed regime can be approximated by 0.5 D in which D is the sphere diameter. [Copyright &y& Elsevier]

Abstract: A modeling scheme based on dynamic coupling of a high-resolution 1D cross-shore model to a 2DH area model is developed to calculate the total longshore sediment transport (LST) rate in wave-dominated coasts. The purpose of this coupling strategy aims at resolving the LST with a high-resolution (both temporally and spatially) inside the surf-zone and with a coarser spatial resolution seaward of the surf-zone. The 2DH area model operates on a fixed pre-designed regional grid (parent grid) and the 1D cross-shore model is dynamically coupled to the boundary of the parent grid with a time-varying domain, starting from the first wave breaking point and ending at the maximum wave set-up point. The time-varying domain is generated in the 1D model by resolving the landward wave propagation from the offshore conditions provided by the 2DH area model at every time step. With a high-resolution cell size the 1D model resolves the wave propagation processes and resulting LST along the profile. The coupled model is applied to study the LST in the Pomeranian Bight at the southern Baltic Sea. Simulation results are compared with three other different hierarchical modeling methods (from empirical formulas such as CERC and Kamphuis to a 2DH area simulation). The comparative study indicates that the dynamically coupled model can be a reliable tool in practical applications, especially for the areas where hydrodynamics is controlled by complex bathymetry (e.g., multiple longshore bars) or morphologically induced circulation patterns. [Copyright &y& Elsevier]

Abstract: Prototype scale physical model tests were conducted to investigate the sheetflow sediment transport of uniform sand under different skewed-asymmetric oscillatory flows with and without the presence of relatively strong currents in the opposite direction against wave propagation. Experiments show that in most cases with fine sands, the “cancelling effect” which balances the on-/off-shore net transport under pure asymmetric/skewed oscillatory flows and results a moderate net transport was developed for combined skewed-asymmetric shaped oscillations. However, under certain conditions (T >5s) with coarse sands, the onshore sediment transport was enhanced for combined skewed-asymmetric flows. Additionally, the new experimental data under collinear oscillatory flows and strong currents show that offshore net transport rates increase with decreasing velocity skewness and acceleration skewness. Sediment movement behaviors were investigated through analysis of experimental data obtained from the image analysis technique and attempts were made to estimate and formulate the sheetflow layer thickness. Accordingly, sediment transport under oscillatory sheetflow conditions was studied and successfully explained by comparing the bed shear stress and the phase lag parameter at each half cycle. Consequently, these parameters were incorporated in an improved Dibajinia and Watanabe''s type sediment transport model. The formula is calibrated against a comprehensive experimental data (331 in total). Good agreement obtained between predictions and measurements shows that the new formula is fulfilled for practical purposes. [Copyright &y& Elsevier]

Abstract: A predictive model of time-dependent sediment suspension was developed with artificial neural network, using velocity and integrated sediment concentration (i.e., suspended load) data observed from a large-scale laboratory experiment under erosive and accretive beach conditions. Low-frequency motion, wave-induced motion, and turbulent kinetic energy per unit mass at lower (1cm above the bed) and upper (50cm above the bed) ADV were used as inputs and suspended load near the bed was used as output. To account for the history prior to sediment suspension, five time steps (corresponding to 2s) of data prior to sediment suspension were included in the inputs. The artificial neural network model shows a predictive capability of the time-dependent sediment suspension in the surf zone for both erosive and accretive conditions. The correlation coefficient (r) between the measured and predicted suspended load reached up to r =0.79 at the bar crest in the accretive beach condition, and typical average correlation coefficients over all subsets (r̄) were 0.53 over the bar crest. The results from various input cases showed that turbulent kinetic energy near the bed is most important to predict the sediment suspension in the surf zone for both erosive and accretive conditions. Also, the best predictive performance was shown at the bar crest, supporting the hypothesis that turbulence induced by intense wave breaking at the bar crest is a dominant mechanism for sediment suspension. Outside the surf zone, the wave-induced motion was the highest contributing factor for the sediment suspension. Furthermore, when the information from the upper ADV was included as inputs, the model improved the prediction capability by 40% at the bar trough in the erosive beach condition. The increased prediction at this location may possibly be attributed to the effect of vertical shear motion in the low-frequency range due to strong undertow from wave breaking. [Copyright &y& Elsevier]

Abstract: Hydrodynamics and sediment transport in the swash zone are unique and very complex phenomena. Acquiring a robust model for predicting sediment motion in the swash zone requires in-depth insight about the behavior of sediment particles in highly asymmetric and turbulent waves that continuingly uprush the beach face and retrieve in backwashes. In order to consider a physical system of the coarse particle motion in the swash zone with a reduced number of external parameters and capture some of the realism of the situation, the experiments are conducted in rather simple conditions to provide some information about the particle responses in terms of jump length against asymmetric waves to be implicated for the swash zone. The results show that despite reduction of many external parameters, jump length is still a highly skewed stochastic parameter. The probability distribution of jump length is quite wide (large variance) and skewed, different from that of saltation length in steady unidirectional flow, in which the distribution is close to normal. Considering integral of excess energy of an arbitrary single wave over its duration as a rational and meaningful representative of flow condition, the average jump length incremental trends reduces in the higher energy condition, resulting in a linear trend in logarithmic scale. The s due to higher turbulent kinetic energy follow similar trend of less turbulent flow conditions. [Copyright &y& Elsevier]

Abstract: In the present paper a general longshore transport (LT) model is proposed after a re-calibration of the model originally introduced by Lamberti and Tomasicchio (1997) based on a modified stability number, N s ⁎⁎, for stone mobility at reshaping or berm breakwaters. N s ⁎⁎ resembles the traditional stability number (Ahrens, 1987; van der Meer, 1988) taking into account the effects of a non-Rayleighian wave height distribution at shallow water (Klopman and Stive, 1989), wave steepness, wave obliquity, and nominal diameter of the units. Nine high-quality data sets from field and laboratory experiments have been considered to extend the validity of the original model for a wider mobility range of the units: from stones to sands. The predictive capability of the proposed model has been verified against the most popular formulae in literature for the LT estimation of not cohesive units at a coastal body. The comparison showed that the model gives a better agreement with the physical data with respect to the other investigated formulae. The proposed transport model presents a main advantage with respect to other formulae: it can represent an engineering tool suitable for a large range of conditions, from sandy beaches till reshaping breakwaters. [Copyright &y& Elsevier]

Abstract: We present a new type of model for calculating morphological changes induced by the presence of breakwaters. The model combines a process based area model, used to calculate the sediment transport field in the two horizontal dimensions, with a simplified morphological updating scheme where the evolving cross-shore profile is described by a limited number of parameters. The hybrid morphological model is a strong tool for medium and long term modelling because it is cost effective while containing important features of the sediment transport description. Two versions of the model are developed in order to study the evolution of beach morphology: one suited for offshore breakwaters (1D model) and one mainly dedicated to coastal breakwaters (“1.5D” model). The version for offshore breakwaters is first presented and tested against field observations of salient evolution. The model is then applied to a model study of the principle correlations between evolving salients (spatial and temporal scales), the characteristic dimensions of the breakwater (distance to shore and alongshore length) and wave climate (wave height, normal and oblique wave incidence). The second version is applied to investigate in more detail the evolving morphology behind coastal breakwaters. It is demonstrated how the model is able to calculate the evolution of either salient or tombolo planforms, and furthermore it is shown that the results are in reasonable agreement with existing rules. [Copyright &y& Elsevier]

Abstract: This paper uses results obtained from the large-scale BARDEX experiments undertaken in the Delta flume to investigate the morphological response of a prototype gravel barrier to wave and tidal forcing during overwash conditions. Gravel barrier behaviour depends upon a number of factors such as sediment properties (porosity, permeability, grainsize), geological setting and wave climate. Since overwash processes are known to control short-term gravel barrier dynamics and long-term barrier migration, the development of a robust quantitative method to define the critical conditions leading to gravel barrier overwashing is important both for scientific and practical management purposes. It is known that when wave runup exceeds the barrier crest elevation, three outcomes are possible: 1) insignificant morphological change, when wave runup just overtops the barrier crest and flow velocities are very weak; 2) overtopping, resulting in accretion on the barrier crest region and barrier stabilisation; and 3) overwashing, resulting in erosion, lowering of the crest region and ultimately to barrier inundation. This study provides an insight into the critical conditions that distinguish these two possible outcomes and to the different mechanisms that provide the required positive and negative feedbacks to the sediment dynamics. In order to define the overwash threshold condition, and to predict the morphological outcome of particular overwash events, use is made here of the Overwash Potential (OP), defined as the difference between the wave runup and the barrier crest elevation. To make effective use of OP it is necessary to identify a reliable runup predictor. Following tests using 12 runup equations the Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A., Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53, 573–588] approach has been identified as the best predictor of runup conditions necessary to generate positive values of OP, with overtopping and overwashing predicted to occur for average OP values of 0.2m and 0.5m, respectively. The use of OP values provides a practical means by which to identify potential coastal hazards associated with gravel barrier overwash processes and is considered to have a range of practical coastal management applications. [Copyright &y& Elsevier]

Abstract: It is generally accepted that the effect on beach morphology of the interactions between swash motion and the beach groundwater table depends on the elevation of the latter relative to that of the sea. Specifically, a low beach groundwater table is considered to enhance onshore sediment transport and beach accretion, while the reverse is thought to hold for a high beach groundwater table. Here, we use the results of a carefully designed series of tests conducted in the Delta Flume, the Netherlands, as part of the Barrier Dynamics Experiment (BARDEX), to unequivocally demonstrate the validity of this assertion. During the experiment, a 4-m high and 50-m long gravel barrier (D 50 =11mm) was constructed within the central region of the 250-m long and 7-m deep wave flume, enabling a back-barrier ‘lagoon’ to be located at its landward side. The groundwater table in the beach was manipulated by lowering and raising the lagoon level, and the gravel beach was subjected to variable wave conditions and sea/lagoon levels. Using as the initial beach profile the morphology resulting from 90min of wave forcing with the sea and lagoon at the same level, the identical wave forcing with a low lagoon level consistently resulted in onshore sediment transport in the swash zone and beach accretion, whereas the identical wave forcing with a high lagoon level consistently caused offshore sediment transport in the swash zone and beach erosion. The divergent beach morphological responses for the low and high lagoon levels could not be attributed to changes in the ‘bulk’ swash hydrodynamics (i.e., based on single-point velocity measurements) arising from a difference in the amount of swash infiltration into the unsaturated beach. Instead, application of a sediment transport model that accounts for the effects of in/exfiltration suggested that the onshore sediment transport and beach accretion for the low lagoon tests mainly resulted from enhanced bed shear stresses during the wave uprush due to infiltration into the saturated part of the beachface. Due to the large hydraulic conductivity of gravel (K = O(0.1ms−1)), the effect of altered shear stresses due to in/exfiltration may be an important factor in sediment transport in the swash zone of gravel beaches. [Copyright &y& Elsevier]

Abstract: Very few studies have quantified the wave friction factor, f w , for coarse sediments at field-scale. To address this shortcoming, high-frequency measurements of turbulence obtained within the boundary layer of irregular waves over gravel in the Delta Flume, have been used to calculate values of f w using different evaluation methods. In the field-scale laboratory experiments reported here, three velocimeters were deployed on the seaward side of a 4m-high, 5m-wide and 55m-long gravel barrier subject to a JONSWAP spectrum of waves with significant wave heights ranging from 0.8m to 1.3m, and peak periods of 3.0s to 10s and offshore water depths ranging from 1.75m to 3.75m. The deployment area was essentially flat, with little or no predicted or observed sediment movement under the wave conditions investigated. The turbulent kinetic energy method was found to be the most suitable approach for calculating the bed shear stress, which can be related to f w . Wave friction factor values under the conditions tested here fell in the range 0.01 and 0.27. Although f w predicted by an existing equation agrees well with the mean measured f w value, the application of a new predictor for f w is recommended for improved parameterisation of skin friction over the range of relative roughness values encountered in this study. This approach combines the wave Reynolds number, wave steepness and relative depth to provide a simple expression to assist assessments of coarse sediment transport by waves for uses within a range of practical engineering applications. [Copyright &y& Elsevier]

Abstract: A wave-averaged process-based one-dimensional model for beach profile change was developed to predict the cyclic evolution of longshore bars. The cross-shore sediment transport was assumed to be composed of suspended load due to wave breaking and bed load due to velocity skewness, acceleration skewness, and beach slope. The model''s performance was investigated using the beach profile data obtained every weekday during a 12-year period from 1989 to 2000 along a 400-m long pier at the Hazaki Oceanographical Research Station, located on the Hasaki coast of Japan, where the mean duration of bar evolution is approximately 1year. The model was unable to reproduce bar development from a rather flat profile, possibly because some sediment movement process was missing in the model. However, the model calibrated with a 1-year data set, including the bar evolution cycle consisting of bar generation, seaward migration, decay, and new bar generation, can be used to predict the first cycles of bar evolution at Hasaki. [Copyright &y& Elsevier]

Abstract: The interactions between a solitary wave and a submerged, vertical, bottom-mounted barrier are investigated experimentally and numerically. Numerical results are calculated using the well-validated two-dimensional volume of fluid (VOF)-type model, named COBRAS (COrnell BReaking And Structure), based on the Reynolds-Averaged Navier–Stokes (RANS) equations and the non-linear k–ε turbulence closure model. Experiments are conducted to measure the free surface motion and velocity fields using a particle image velocimetry (PIV) system to provide data for model validation. Fairly good agreements at both the pre- and post-breaking stages are obtained. Then, the verified numerical results are employed to illustrate the free surface motion and the vortex evolution in detail. Wave breaking occurs in the direction opposite to that of the wave after a solitary wave propagates over the barrier. A local maximum value of turbulence intensity is observed for both experimental and numerical results at the impinging point of the breaking wave. An almost linear relationship between the maximum net force and the wave non-linearity is obtained. The effectiveness of the bottom-mounted barrier is estimated by evaluating the wave reflection, transmission, and dissipation coefficients using the energy integral method. Finally, the trajectories of marked fluid particles that are initially located around the barrier help understand the possible sediment transport. [Copyright &y& Elsevier]

Abstract: The impact of a uniform bore on an erodible beach is investigated using a shallow water description and a sediment conservation equation. The solution, which is obtained using the method of characteristics, employs a cubic bed-load formula and a crudely calibrated sediment transport coefficient. It is found that, as with the fixed bed case of Hibberd and Peregrine (1979), a backwash bore forms, which for the mobile bed also comprises a bed step. It is found that this bed step achieves a significant height. The volume of sand deposited above the final still water level is consistent with that observed under certain events on some sandy beaches. [Copyright &y& Elsevier]

Abstract: Multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years. Generally, a bar is generated near the shoreline, grows in height and width, while migrating offshore before finally decaying at the seaward limit of the surf zone. Based on a three-year hindcast of a bar cycle at Noordwijk (Netherlands) and on additional synthetic runs using a wave-averaged cross-shore process model, this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration. The bar amplitude response is particularly sensitive to the water depth above the bar crest, h Xb , and the angle of wave incidence, θ. These variables largely control the amount of waves breaking on the bar and the strength and cross-shore distribution of the associated longshore current. The longshore current has its maximum landward of the bar crest, inducing additional stirring of sediment on the landward bar slope and trough. The enhanced sediment concentration in the trough region shifts the cross-shore transports peak landward of the bar crest forcing bar amplitude growth during offshore migration. For increased h Xb -values wave breaking becomes less frequent, consequently reducing the influence of the longshore current on sediment stirring. Therefore, the resulting dominance of the cross-shore current results in a sediment transport peak at the bar crest causing bar amplitude decay. All four types of bar response (viz. all combinations of onshore/offshore migration and bar amplitude growth/decay) can occur for a single wave height and wave period combination, depending on h Xb and θ. Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that h Xb and θ largely control the transient bar amplitude response. [Copyright &y& Elsevier]

Abstract: This paper deals with field measurements of undulations of the bottom bathymetry along an otherwise straight coast at the Danish West Coast. Two bathymetric datasets and two time series of wave measurements are used in order to determine the following properties: the offshore extent of shoreline undulations, the amount of sediment transported alongshore in the shoreline undulations, the relationship between the shoreline undulations and longshore bars and the relationship between the morphology and the hydrodynamics. In one of the data sets the shoreline undulations are well correlated with undulations on the depth contours between −5m and +2m relative to mean sea level. In the other data set, only undulations on the depth contours between −1m and +1m are well correlated with the shoreline undulations. The main difference in the wave climate between the two locations is the orientation of the shoreline such that the wave incidence angle is more oblique on the section of shoreline with well correlated depth contours, suggesting that the well correlated depth contours are a result of the oblique wave incidence. [Copyright &y& Elsevier]

Abstract: A comprehensive two-dimensional (cross-shore) process-based numerical model of nearshore hydrodynamics (based on the Navier–Stokes equations, k–ε turbulence closure and the Volume-Of-Fluid method), beach morphology, and variable-density groundwater flow (SEAWAT-2000) was developed. This model, which was applied at the field scale, relaxes simplifications in existing models that do not include such detailed mechanistic descriptions. Numerical experiments were conducted to investigate the effects of varying aquifer, beach and wave characteristics (e.g., inland groundwater head, sand grain size, different wave heights and periods) on the coupled system. Spilling and plunging breakers on dissipative and intermediate beaches were simulated. For a given set of boundary conditions, the model was run for 1y without the hydrodynamic sub-model to achieve a realistic salt-/freshwater interface. Then, the hydrodynamic component was run for 15min and the model results analyzed. The main features considered were groundwater circulation, saltwater wedge position, in/exfiltration across the beach face, and beach morphology. The predictions of the numerical model agree well with existing understanding and experimental measurements. For an inland watertable that is lower than the still water level (SWL), such that the groundwater flow is mainly landward, on both coarse and fine sand beaches the addition of wave motion moves the saltwater wedge further landward. For an inland watertable that is higher than the SWL, the opposite behavior occurred. The numerical experiments showed that more sediment transport takes place on intermediate beaches than on dissipative beaches. In addition, beach profile variations are greater under plunging breakers, while coarse sand beaches are steeper than fine sand beaches for the same wave conditions. There is a strong correlation between in/exfiltration and beach face deposition/erosion for the coarse beaches, while in/exfiltration has a slight effect on sediment transport for fine beaches. The model is capable of simulating the short-term evolution of foreshore profile changes, and beach watertable and saltwater wedge movement due to interactions between wave motion and coastal groundwater. [Copyright &y& Elsevier]

Abstract: Understanding of sea bottom boundary layer characteristics, especially bottom shear stress acting on the sea bed, is an important step needed in sediment transport modeling for practical application purposes. In the present study, a new generation system for bottom boundary layer under solitary wave is proposed. Applicability of this system is examined by comparing measured and numerical solution velocities. Moreover, transitional behavior from laminar to turbulence was investigated. It is concluded that the critical Reynolds number in the experiments shows good agreement with DNS result of Vittori and Blondeaux (2008) and laboratory data of Sumer et al. (2010), indicating validity of the generation system. Since the present generation system enables continuous measurement to obtain ensemble averaged quantities, it can be effectively utilized for future experimental studies on solitary wave boundary layers, including sediment transport experiments with movable bed. [Copyright &y& Elsevier]

Abstract: Regional Ocean Modeling System (ROMS v 3.0), a three-dimensional numerical ocean model, was previously enhanced for shallow water applications by including wave-induced radiation stress forcing provided through coupling to wave propagation models (SWAN, REF/DIF). This enhancement made it suitable for surf zone applications as demonstrated using examples of obliquely incident waves on a planar beach and rip current formation in longshore bar trough morphology (Haas and Warner, 2009). In this contribution, we present an update to the coupled model which implements a wave roller model and also a modified method of the radiation stress term based on Mellor (2008, 2011a,b,in press) that includes a vertical distribution which better simulates non-conservative (i.e., wave breaking) processes and appears to be more appropriate for sigma coordinates in very shallow waters where wave breaking conditions dominate. The improvements of the modified model are shown through simulations of several cases that include: (a) obliquely incident spectral waves on a planar beach; (b) obliquely incident spectral waves on a natural barred beach (DUCK''94 experiment); (c) alongshore variable offshore wave forcing on a planar beach; (d) alongshore varying bathymetry with constant offshore wave forcing; and (e) nearshore barred morphology with rip-channels. Quantitative and qualitative comparisons to previous analytical, numerical, laboratory studies and field measurements show that the modified model replicates surf zone recirculation patterns (onshore drift at the surface and undertow at the bottom) more accurately than previous formulations based on radiation stress (Haas and Warner, 2009). The results of the model and test cases are further explored for identifying the forces operating in rip current development and the potential implication for sediment transport and rip channel development. Also, model analysis showed that rip current strength is higher when waves approach at angles of 5° to 10° in comparison to normally incident waves. [Copyright &y& Elsevier]

Abstract: Details are given herein of the refinement and application of a three-dimensional layer integrated numerical model to predict morphological changes in tidal basins. The solution of governing differential equations, which consist of the conservation of mass and momentum for the hydrodynamics, the transport equation for the suspended sediment fluxes and the sediment mass conservation equation for the bed level changes are carried out by the use of Alternating Direction Implicit (ADI) Finite Difference Method (FDM). The model includes different criteria for the initiation of motion namely Shields (1936, Application of Similarity Principles and Turbulence Research to Bed load Movement, Hydrodynamics Laboratory, California Institute of Technology, Pub. No. 167), Kolahdoozan (1999, Numerical Modelling of Geomorphological Processes in Estuarine Waters, PhD Thesis, Department of Civil and Environmental Engineering, University of Bradford, Bradford, UK, 288) and Zanke (2003, On the Influence of Turbulence on the Initiation of Sediment Motion, International Journal of Sediment Research, 18(1), 17–31), to compare different aspects of flow conditions. As the flow is highly turbulent with the random nature of its components, many researchers have tried to express sediment transport processes by using stochastic approaches. In the current study both deterministic and stochastic methods are included in the numerical model to evaluate their accuracy and efficiency. To validate the numerical model results, laboratory measurements are used, with these being obtained from an earlier experimental program undertaken by the authors. Results of a short term bed level changes in a laboratory model harbor are included for the model verification purposes. Comparisons are undertaken using different criteria for the initiation of motion, with the results highlighting that the unsteadiness in the flow parameters included in the numerical model has a major effect on the bed level changes inside the harbor, in compare with the turbulence structure of the flow. The model is then applied to a real case study of the Humber Estuary, located in the UK, with comparisons being undertaken for different criteria for the initiation of motion, using both deterministic and stochastic approaches for the long term bed level predictions. [Copyright &y& Elsevier]

Abstract: A numerical model for the general description of the sediment transport under oscillatory sheet flow conditions is developed based on a two-fluid representation of the two-phase turbulent flows. The governing equations of the model are the Reynolds averaged continuity equations and equations of motion for both the fluid and the sediment phases. The two phases are coupled by the interphase forces including the resistance force, the inertia force, and the lift force. Turbulence closure of the fluid phase is based on a slightly modified k–ε model while an algebraic particle-turbulence model is applied to the sediment phase. The numerical method is based on the modified SIMPLE scheme and an improved time stepping technique. The model is validated by the published data for the symmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at the University of Tokyo and for both the symmetrical and the asymmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at University of Aberdeen. The numerical results on the temporal and spacial variation of the sediment concentration, the horizontal velocities of the two phases, the horizontal and vertical fluxes of the sediment phase, as well as the thickness of the sheet flow layer all show satisfactory agreement with the laboratory data. The model is also shown to predict the net sediment transport rate with a reasonably good accuracy. [Copyright &y& Elsevier]

Abstract: A universal formula for the estimation of equilibrium scour depth around a single cylindrical pile under the action of steady currents, tidal and short waves is presented. Following a dimensional analysis, the controlling parameters of scour was found to be the Keulegan–Carpenter number and the ratio between the flow velocity and the critical velocity for the initiation of motion of the sediment particles, u/uc. Based on this finding a new formula for the prediction of equilibrium scouring depths was derived in this paper. Measurements of the scouring process and the action of waves and/or currents from literature are used with KC and u/uc ranging between 6 and 105, and 0.6 and 4.5, respectively for the parameterization and verification of the newly proposed formula. The predicted scour depths and the investigated measurements show clearly the influence of the bed properties which have not been reported before in the literature. Predictions using the new approach have been compared with previous measurements and other formulations presented in the specialized literature. The proposed empirical formula accounts for the effect of waves, currents and sediment characteristics on the scouring process and shows better results than previous ones. [Copyright &y& Elsevier]

Abstract: A numerical model was developed of beach morphological evolution in the vicinity of coastal structures. The model includes five sub-models for random wave transformation, surface roller development, nearshore wave-induced currents, sediment transport, and morphological evolution. The model was validated using high-quality data sets obtained during experiments with a T-head groin and a detached breakwater in the basin of the Large-scale Sediment Transport Facility at the Coastal and Hydraulics Laboratory in Vicksburg, Miss, USA. The simulations showed that the model reproduced well the wave conditions, wave-induced currents, and beach morphological evolution in the vicinity of coastal structures. Both salient and tombolo formation behind a T-head groin and a detached breakwater were simulated with good agreement compared to the measurements. [Copyright &y& Elsevier]

Abstract: The formation of beach megacusps along the shoreline of southern Monterey Bay, CA, is investigated using time-averaged video and simulated with XBeach, a recently developed coastal sediment transport model. Investigations focus on the hydrodynamic role played by the bay''s ever-present rip channels. A review of four years of video and wave data from Sand City, CA, indicates that megacusps most often form shoreward of rip channels under larger waves (significant wave height (H s )=1.5–2.0m). However, they also occasionally appear shoreward of shoals when waves are smaller (H s ~1m) and the mean water level is higher on the beach. After calibration to the Sand City site, XBeach is shown to hindcast measured shoreline change moderately well (skill=0.41) but to overpredict the erosion of the swash region and beach face. Simulations with small to moderate waves (H s =0.5–1.2m) suggest, similar to field data, that megacusps will form shoreward of either rip channels or shoals, depending on mean daily water level and pre-existing beach shape. A frequency-based analysis of sediment transport forcing is performed, decomposing transport processes to the mean, infragravity, and very-low-frequency (VLF) contributions for two highlighted cases. Results indicate that the mean flow plays the dominant role in both types of megacusp formation, but that VLF oscillations in sediment concentration and advective flow are also significant. [Copyright &y& Elsevier]

Abstract: Field experiments were conducted on a low-gradient, high-energy sandy beach (Truc Vert, France) and a steep, low-energy gravel beach (Slapton, UK) to examine alongshore-directed currents within the swash zone. At Truc Vert, data were collected over 33 tidal cycles with offshore significant wave heights of 1–4m and periods of 5–12s. At Slapton data were collected during 12 tides with wave heights of 0.3–1m and periods of 4–9s. The swash motion was predominantly at infragravity frequencies at Truc Vert and incident frequencies at Slapton. During selected tides, strong alongshore currents of the same order of magnitude as the cross-shore flows were a consistent feature of both swash zones; however, while the cross-shore flows were saw-toothed with a distinct flow reversal, the alongshore flows were unidirectional and continuous. On the sand beach, alongshore momentum was advected into the swash from the surf zone by standing long-wave motion resulting in the strongest flows in the lower swash. In contrast on the gravel beach, highly oblique uprush directly forced the longshore-directed motions and resulted in strong flow velocities in the upper swash. On the sandy beach, the cross- and longshore boundary layers were consistently different with strong vertical shear during the uprush for the cross-shore and during the backwash for the alongshore flows. A logarithmic profile better fitted the longshore flows during the backwash, compared to during the uprush for the cross-shore flows, and enabled the drag coefficient to be computed. The alongshore drag coefficient was C d, v =10−3 and of the same order as for the cross-shore, but was generally larger during the backwash compared to the uprush. Sediment transport efficiency during the backwash is potentially enhanced by longshore-directed swash flows. Significant bed shear stress is maintained throughout that cross-shore flow reversal enabling down-slope transport earlier in the swash cycle. [Copyright &y& Elsevier]