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A workshop on swash-zone processes was convened in July of 2014 aimed to present the most recent research advances and to identify topics of future research. This paper presents the most critical topics discussed at the workshop with future perspectives. Recurring themes throughout the workshop (and this paper) are the ability to tie the research to problems of societal interest, focus efforts on understanding extreme conditions, and improve sediment transport and morphodynamic prediction from numerical models. Subsequent papers in the special issue highlight recent advances in swash-zone processes research. [ABSTRACT FROM AUTHOR]

Extensive research efforts in the last decade have improved knowledge of swash-zone processes. This paper reviews and synthesizes from 2004 onward the research advances of small-scale hydrodynamics and sediment transport processes considering field, laboratory and numerical modeling efforts. Swash-zone hydrodynamics are examined by means of shoreline motions, fluid velocities, pressure gradients, flow structure, bed shear stress, friction coefficients and turbulence. Subsequently, advances related to sediment transport mechanisms and morphological changes are described. Detailed descriptions of measuring techniques, novel findings, and the strengths and weaknesses of the different approaches are discussed. The review also acknowledges the major advances in the development of instrumentation to collect highly resolved flow and sediment concentration measurements in direct proximity of the bed and the experimental framework for analyzing local dynamics on a wave-by-wave time scale (e.g., dam break). [ABSTRACT FROM AUTHOR]

We present a comprehensive and critical review of work on the numerical modelling of swash zone processes between 2005 and 2015. A wide range of numerical models has been employed for the study of this region and, hence, only phase-resolving approaches (i.e., depth-averaged and depth-resolving models) are analyzed. The current advances in the modelling of swash zone processes are illustrated by comparing different numerical models against laboratory experiments of a dam-break-driven swash event. Depth-averaged and depth-resolving models describe well the swash flow for both coarse sand and gravel impermeable beach cases. Depth-averaged models provides a practical tool for engineering use, whereas depth-resolving models improve the flow description, especially for the backwash phase, with a significantly higher computational cost. The evolution and magnitude of bed shear stresses predicted by all models is reasonable when compared with laboratory estimates based on the log-law. However, differences between modelling approaches cannot be rigorously evaluated owing to the uncertainty in shear stress estimates while employing such approximation. Furthermore, small-scale processes, such as turbulence evolution, are investigated with depth-resolving models, finding differences between the two-dimensional and three-dimensional approaches. Numerical models allow us to investigate other processes such as beach morphology changes, the evolution of the turbulence coherent structures, and the infiltration/exfiltration effects on the swash flow. A discussion on the advantages and limitations of each model is presented. The future of swash zone modelling depends on the increase of the computational power and, more importantly, on the improvement of the current capability to obtain intra-wave measurements for model validation, calibration, and greater resolution of physical processes. [ABSTRACT FROM AUTHOR]

XBeach, a process-based numerical model designed to simulate morphologic change during extreme storm events, was calibrated and compared to observations from a large-scale laboratory dune erosion experiment. Wave conditions along the tan, wave runup at the beach, and 3-dimensional position of the dune were measured throughout the experiment. Seiching modes in the tank were simulated and matched the observations once the model was calibrated. Simulated waves were sensitive to γ , the ratio of wave height to water depth, and c f. , the coefficient of friction. Simulated runup explained 50–87% of the observed variance in observations. However, the magnitude of simulated wave runup was underestimated throughout the experiment. Errors in simulated runup were composed of a high bias and gain error in mean water level, low bias in the infragravity swash, and low bias and gain errors in incident swash. Observed probability density functions of swash were statistically consistent between times when swash was confined to the foreshore and times when swash interacted with the dune. However, simulated probability density functions of swash were statistically different during the collision regime. Despite the systematic underestimation of wave runup, modeled dune erosion compared well with observations after the sediment transport parameters were calibrated. Modeled dune erosion was sensitive to the critical slope parameters over the wet and dry regions of the beach, the depth of the interface between the wet and dry regions of the beach, and the threshold depth for sediment transport and return flow. [ABSTRACT FROM AUTHOR]

Hybrid 2D/one-line shoreline models are becoming increasingly applied over the mesoscale (101–102 years; 101–102 km) to inform coastal management. These models typically apply the one-line theory to simulate changes in shoreline morphology based on littoral drift gradients calculated from a 2DH coupled wave, flow, and sediment transport model. However, the key boundary conditions needed to effectively apply hybrid 2D/one-line models and their applicability beyond simple planform morphologies are uncertain, which can potentially comprise coastal management decisions. To address these uncertainties, an extensive numerical modelling campaign is carried out to both assess the sensitivity and calibrate an advanced hybrid 2D/one-line model (MIKE21) against six variables in three different sandy coastal system morphologies: (a) a simple planform morphology with a gentle sloping profile, (b) a simple planform morphology with a steep sloping profile, and (c) a complex planform morphology. The six variables considered include nearshore discretisation, bathymetry, bed friction, sand grain diameter, sand porosity, sediment grading, and the weir coefficient of hard defence structures. Five key conclusions are derived from the sensitivity testing and calibration results. First , the optimal boundary conditions for modelling shoreline evolution vary according to coastal geomorphology and processes. Second , specifying boundary conditions within physically realistic ranges does not guarantee reliable shoreline evolution predictions. Third , nearshore discretisation should be treated as a typical calibration parameter as (a) the finest discretisation does not guarantee the most accurate predictions, and (b) defining a discretisation based on process length scales also does not guarantee reliable predictions. Fourth , hybrid 2D/one-line models are not valid for application in complex planform morphologies plausibly because of the one-line theory assumption of a spatially invariable closure depth. Fifth , hybrid 2D/one-line models have limited applicability in simple planform morphologies where the active beach profile is subject to direct human modification, plausibly due to the one-line theory assumption of a constant time-averaged coastal profile form. These findings provide key theoretical insights into the drivers of shoreline evolution in sandy coastal systems, which have practical implications for refining the continued application of shoreline evolution models. • The hybrid 2D/one-line shoreline modelling approach is tested in three coastal morphologies. • Optimal boundary conditions correspond to observed coastal morphology. • The nearshore discretisation in shoreline models should be treated as a calibration parameter. • The hybrid 2D/one-line shoreline modelling approach is limited to simple planform morphologies. [ABSTRACT FROM AUTHOR]

The morphodynamics of orbital vortex ripples due to oscillatory flow and induced sediment transport was studied numerically. The methodology was based on large-eddy simulations of the full coupling of turbulent oscillatory fluid flow, bed and suspended sediment transport, and bed evolution. The Immersed Boundary method was implemented for the imposition of fluid and sediment boundary conditions on the mobile bed surface. Results are presented for ripple creation starting from flat or rippled beds, as well as results of ripples adapting to changing flow (wave) direction. The computed equilibrium ripple dimensions are in agreement with existing empirical predictions; however, the computed ripple profile is not as steep in the vicinity of the ripple crest as the analytical one in Sleath (1984). The time needed for the bed geometry to reach equilibrium, in cases where the initial bed is almost flat or it is formed by ripples that shorten towards equilibrium, is longer than in cases where the initial bed is formed by ripples that widen towards equilibrium. Moreover, the ripple crests are reoriented when the flow changes direction by 45°; the new equilibrium ripple crestline is perpendicular to the flow direction. Finally, the numerical results are compared with corresponding results of flow above fixed ripples. The bed load flux does not present substantial differences between the fixed and mobile bed cases. On the contrary, a strong increase of the suspended load flux is observed for the mobile bed case in comparison to the fixed bed case. • LES Coupling between oscillatory flow, sediment transport, and bed morphodynamics. • Same flow forcing results into same bed equilibrium regardless of initial bed form. • Evolution of ripples longer than equilibrium ones is slower than for shorter ones. • Ripples are reoriented perpendicular to the flow direction. • Suspended load over mobile rippled bed is larger than over fixed ripples. [ABSTRACT FROM AUTHOR]

Offshore pipelines are often suspended or partially-buried during operating process. Understanding the scouring mechanism of the suspended and partially-buried pipelines is important for protecting the pipelines. The numerical models of pipelines under steady flow are established, in which Reynolds Averaged Navier-Stocks equation (RANS) with the k-ω turbulence closure, as well as the sediment transport equation, are solved with the finite volume method. After experimental verification, the models are applied to a series of parametric analysis for the suspended and partially-buried pipelines under steady flow. The effect of different variables such as the pipe diameter D , the suspension height e , the pipe shape, the embedment depth, on the scour process of the pipelines are investigated in detail based on more than 150 simulation results. For the suspended pipelines, the gap-to-diameter ratio e / D is the key parameter to predict scour depth. The pipe shape has an influence on the scour depth and the position of the scour hole. A formula to predict scour depth in terms of e/D and the relation between gap velocity and scour depth is obtained. For the partially-buried pipelines, the block height and the curvature of the pipeline is proposed as the key parameters to analyze the flow field around the pipeline. • The numerical model of pipelines under steady flow are established and verified. • Effect of different parameters on scour process of offshore pipelines are studied. • Formula to predict scour depth in terms of e / D is proposed for suspended pipelines. • Block height and curvature are key data to analyze flow of partially-buried pipelines. [ABSTRACT FROM AUTHOR]

We study the migration of natural sand bars that evolve in the nearshore by means of a new process-based wave-resolving, 2DH circulation model. In order to perform reliable and accurate computations, the robust Nonlinear Shallow Water Equations (NSWEs) hydro-morphodynamic solver of Brocchini et al. (2001) and Postacchini et al. (2012) implements a detailed description of the Bottom Boundary Layer (BBL) dynamics and a new predictor for the Suspended Sediment Transport (SST) based on the solution of a Depth-Averaged Advection-Diffusion Equation (DAADE) for the sediment concentration. The robustness and accuracy of the enhanced model are validated against literature theoretical, experimental, and numerical results, all comparisons highlighting good performances and clarifying the role of both BBL and SST contributions, the former one having a larger positive influence than the latter one on the results. Both original and enhanced models are, then, used to predict the evolution of the sand bar system that characterizes the nearshore of Senigallia (AN). The analysis leverages the field observations collected at such a site by means of the Sena Gallica Speculator video-monitoring system. Modeling of the storm-forced sand bar migration patterns reveals that: 1) the enhanced model can adequately reproduce the seaward migration of the sand bars of the system; 2) the process of shoreline retreat in coincidence with the generation of a new-born bar is well described; 3) inclusion of the BBL improves quantitative prediction of the bar crest migration; 4) the SST, beyond improving the prediction of the bar crest location, induces some smoothing of the bar profile, in line with the literature findings of SST being a stabilizing factor for the bar emergence. • Sand bar evolution in the nearshore zone is modeled using a process-based 2DH wave resolving NSWE model. • The model includes BBL dynamics and predicts the SST based on the depth-averaged advection-diffusion equation. • The model is validated against theoretical, experimental, and numerical results. • The model is used to predict the sandbar migration in the nearshore of Senigallia (AN) during a storm. • The developed model can accurately reproduce the sandbar evolution in both field and laboratory conditions. [ABSTRACT FROM AUTHOR]

Engineered and natural submerged coastal structures (e.g., submerged breakwaters and reefs) modify incident wave fields and thus can alter hydrodynamic processes adjacent to coastlines. Although submerged structures are generally assumed to promote beach protection by dissipating waves offshore and creating sheltered conditions in their lee, their interaction with waves can result in mean wave-driven circulation patterns that may either promote shoreline accretion or erosion. Here, we analyse the mean flow patterns and shoreline water levels (wave runup) in the lee of idealised impermeable submerged structures with a phase-resolved nonhydrostatic numerical model. Waves propagating over submerged structures can drive either a 2-cell mean (wave-averaged) circulation, which is characterised by diverging flows behind the structure and at the shoreline, or 4-cell circulation, with converging flows at the shoreline and diverging flows in the immediate lee of the structure. The numerical results show that the mode of circulation can be predicted with a set of relationships depending on the incoming wave heights, the structure crest level, and distance to the shoreline (or structure depth). Qualitative agreement between the mean flow and proxies for the sediment transport using an energetics approach suggest that the mean flow can be a robust proxy for inferring sediment transport patterns. For the cases considered, the submerged structures had a minimal influence on shoreline wave setup and wave runup despite the wave energy dissipation by the structures due to alongshore wave energy fluxes in the lee. Consequently, these results suggest that the coastal protection provided by the range of impermeable submerged structures we modelled is primarily due to their capacity to promote beach accretion. • A phase-resolved model simulates hydrodynamic interactions with submerged structures. • Waves can drive 2-cell or 4-cell mean circulation and sediment transport patterns. • Bottom shear stresses presented an overall similar pattern to the mean currents. • We develop a predictor for the flow patterns based on wave and structure parameters. • The submerged structures had limited influence on shoreline setup and wave runup. [ABSTRACT FROM AUTHOR]

Fast ice affects many coastlines in high- and mid-latitude regions, shaping the coastal morphology by acting as both a protective barrier from winter storms and as an erosive agent through sediment entrainment and transport. It has been hypothesized that when sediment-rich ice is dislodged and transported beyond the depth of closure by storm waves, sediment is permanently removed from the nearshore, starving the littoral system and facilitating future erosion. However, despite this large potential impact on coastal sediment budgets, the rates and mechanisms of sediment-rich ice transport are poorly constrained. This study investigates the sediment entrainment process by fast ice, the mechanisms needed for ice mobilization, and the force needed to breakup and transport sediment-rich ice. We find that the Coulomb frictional strength of the sediment regulates the fast ice system and that the viscous rheology of the ice has no direct mechanical effect. Importantly, these results show that only the frictional coefficient of the beach sediment needs to be measured to estimate the resistive strength of the fast ice, significantly simplifying the requirements for evaluating the potential for fast ice mobilization and associated sediment transport. • Investigations on sediment entrainment and mobilization processes for fast ice. • Experiments reveal the force needed to breakup and transport sediment-rich ice. • The frictional strength of the sediment, not ice, regulates the fast ice system. • Blocks of ice with adhered sediment can be treated as a Coulomb material. [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]

Reduced-scale mobile-bed flume tests are conducted on the evolution of a submerged berm under the effects of regular waves in low-energy conditions. Twenty-three flume tests are performed on a profile model of a submerged berm under different combinations of wave heights and wave periods. The profile shapes and water elevations along the profile are recorded during the tests. Comprehensive analyses are conducted on the profile evolution and the wave parameters along the profile of the submerged berm model. In all cases the crest of the submerged berm moves onshore and the profile shape become asymmetrical. Overall analyses are conducted on the evolution-related parameters such as the moving distance of the centroid (D lc), the change of skewness of the profile shape (C sk), the change of the crest elevation of the submerged berm (C cl) and the irregularity parameter of the profile shape (K rms). The effects of wave height and wave period on these parameters are discussed. An Empirical Orthogonal Function (EOF) analysis of the test results indicates two major evolution patterns of the submerged berm, onshore migration and onshore inclining of the crest, both of which are controlled by wave height. The velocity skewness, wave height and local water depth are the major factors that impact the onshore sediment transport under low energy conditions. No obvious connections can be found between acceleration skewness and sediment transport in this experimental study. However, large acceleration skewness would reduce the values of velocity skewness and impact the sediment transport indirectly. This paper proposes new insights on the morphodynamics of submerged berms under low-energy conditions and provide validation data for numerical models. • The evolution of a submerged berm under low-energy conditions are studied via a series of flume tests. • A general portrait on the feature of the evolution of a submerged berm under low energy conditions is given. • The effects of wave height and wave period on the shape-changing of the submerged berm are analyzed quantitatively. • Two major evolution patterns of the submerged berm in this case are onshore migration and onshore inclining of the crest. • The velocity skewness, wave height and local water depth dominate the onshore sediment transport under low energy conditions. [ABSTRACT FROM AUTHOR]

The interaction between a forced vibrating pipeline and an erodible seabed in unidirectional flows is experimentally investigated. The experiments were conducted under clear-water scour conditions with a water depth of 0.3m and averaged approach velocity of 0.261 m/s. The pipeline model with a diameter (D) of 3.5 cm, was subjected to a vertical sinusoidal motion of varying amplitudes (A 0 = 2∼6 cm) and frequencies (f 0 = 0.1–0.6Hz). The initial gap (G 0) between the lower pipe surface and the undisturbed flatbed level was fixed at 1 D. The results show that the maximum scour depth increases significantly with both vibration amplitude and frequency, with the former having a more dominant influence. However, frequency tends to exacerbate the scour hole development more in the early scour stage. Distinct scour mechanisms are observed based on the different combinations of vibration amplitude and frequency. In the high A 0 conditions (A 0 > G 0), the combined effect of pounding (when the pipe hits the seabed) and piston actions (rising and falling of the pipe) during the pipe descending period dominates the earlier stages of scouring, and the pick-up of sediment particles during its rising period controls the subsequent scour process. In the low A 0 conditions (A 0 < G 0) when pounding does not occur, vortex shedding during the pipe-falling stage controls the development of scour hole when the frequency is low. When the frequency is high, however, the flow field associated with the pipe-rising period dominates the scour development. Different empirical formulas for the prediction of the maximum scour depth and width are proposed and compared in the present study. • 2 and 3 scour stages with different scour mechanisms are identified for the low and high amplitude conditions, respectively. • The pounding effect, which is related to pipeline-scour subjected to forced vibration, is discussed. • Vibration amplitude has a dominant influence on the maximum scour depth while frequency influences the early scour stages. [ABSTRACT FROM AUTHOR]

In the swash zone, an assessment of swash flow properties requires an understanding of swash depth and swash velocity, which can affect sediment movement, as well as the design of coastal structures. The method of characteristics provides a useful tool to model unsteady shallow water flows, such as dam-break flow, swash flow and tsunami run-up. In this paper, bottom resistance was incorporated into the nonlinear shallow water equations to extend the application of the method of characteristics to account for frictional effects in the swash zone. The new characteristic equations are solved by the specified time interval scheme. The bottom resistance has little influence on the seaward swash hydrodynamics (initial swash stage), e.g., water depth and flow velocity, while its influence becomes significant in the small water depth region near the swash front, which reduces the maximum run-up height. At a specified time and location, and comparing with a frictionless swash solution, the uprush flow velocity reduces due to the bottom resistance, while the flow velocity increases during the initial phase of the backwash owing to the earlier reversal time with the bottom resistance. However, the differences with the inviscid solution are small within the swash lens. Conversely, the maximum backwash velocity is reduced significantly as a result of the shorter duration of the backwash. Consistent with previous field observations, friction does not significantly alter the gradient of the water surface elevation and a seaward-dipping water surface elevation is observed during the majority of the swash process. The dominant effect of bottom resistance is to reduce the range of inundation, particularly with larger resistance coefficients. The incorporation of the bottom resistance extends the application of method of characteristics in the swash zone to more practical problems, particularly where run-up elevation is critically important, e.g. overtopping flows. [ABSTRACT FROM AUTHOR]

In this paper, large-scale experimental data are presented showing the beach profile morphological evolution induced by four different bi-chromatic wave conditions characterized by very similar energy content between them but varying the modulation period. Important differences were observed in the resultant beach profiles as a function of the wave group periods. Larger variability in the profile evolution is generally observed for larger wave group periods and, more importantly, as the wave group period increases the distance between the generated breaker bar and the shoreline increases. The measured primary wave height to depth ratio ( γ ) increases with the wave group period, which is consistent with the observed larger wave height at the breaking location. The primary wave breaking location is also observed at increasing distances with respect to the initial shoreline as the wave group period increases. The variation in γ with wave group period is related to the selective energy dissipation of the higher primary frequency component ( f 1 ) during the wave group shoaling. Broad bandwith conditions (reduced wave group period) lead to larger dissipation of wave heights at the f 1 component relative to f 2 resulting in a reduction in the wave modulation and primary wave height at the breaking location. Suspended sediment fluxes obtained from collocated velocity and sediment concentration measurements in the surf zone showed a consistently larger contribution of the mean return flow to the suspended sediment fluxes compared with the wave group and primary wave components. The distinct beach profile evolution in terms of bar location is interpreted from an increasing distance of the mean breakpoint location and the location of maximum return flow with respect to the shoreline as the wave group period increases. [ABSTRACT FROM AUTHOR]

The Optical Backscatter Sensor (OBS) has been widely used to measure suspended sediment concentration in both field and laboratory conditions, even though it is very sensitive to many factors. The most significant factor suggested is the grain size. In order to enhance the quality of OBS data, an improved approach is proposed, based on the “mixture of linear component response” method (Green and Boon III, 1993) to account for the effect of grain size. In addition to an original sediment sample, which commonly serves as a single calibration material, an accompanying sediment sample is necessary to calibrate OBS sensors. A multi-fraction sediment model is used to predict the grain size distribution in suspension. Compared with existing methods, the improved approach does not require a sieving procedure nor the assumption that the sediment fractions of the calibrated sediment sample exhibit the same sensitivities as those of the suspended sediments. The applicability of our method has been verified by a series of laboratory experiments over silt–sand mixtures. The results show that this method successfully yields continuous concentration profiles, which agree well with the measurements using a suction method. The converted concentrations of the time-averaged OBS measurements by traditional method and by the improved method, respectively, are compared with the suction measurements. The results of linear regression analyses show that the coefficient of determination increases (e.g. from 0.55 to 0.92 for wave–current conditions) and the Root Mean Square Error decreases (e.g. from 0.97 to 0.39 for wave–current conditions). It demonstrates that improved method enhances the quality of OBS conversion. Furthermore, suggestions on selecting the accompanying sediment sample (i.e. on grain size and composition) are given. [ABSTRACT FROM AUTHOR]

Sediment transport-induced changes to the embedment of three 26 km long sections of subsea pipeline are analysed and subsequently explained using model scale experiments. Rather than the scour and scour-induced sinking and sagging traditionally thought to dominate post-lay pipeline spanning and embedment change, the change for these pipelines is shown to be caused by sedimentation. The pipelines traverse a range of metocean and soil conditions; the variation in embedment correlates well with the variation in metocean conditions, with most change occurring in an area where multidirectional high-velocity short-duration flows associated with internal waves propagate at near-perpendicular angles to the pipeline. To understand the mechanism driving these changes, a series of model scale tests in O-tube flumes have been completed under flow conditions mimicking those recorded in the field. Good agreement is found between the field and laboratory results, both in terms of the process timescale and the post-sedimentation profile. The consistency of the embedment changes between the pipelines, their correlation with metocean conditions, and the ability to replicate these changes in model scale tests suggests that such changes can be accounted for in more effective pipeline design. Spans are relatively rare along the pipelines but where they do occur fish rather than scour are shown to be the principal agent of span formation. [ABSTRACT FROM AUTHOR]

Overwash is an important process that controls short-term barrier dynamics, as well as long-term barrier migration, but this process is difficult to study in the field due to its rare occurrence and the challenging conditions under which it occurs. This paper uses data collected during the BARDEX II experiment in the Delta Flume, the Netherlands, where a proto-type barrier was subjected to a range of wave and water level conditions. The objectives of this research are to: (1) compare the morphologic response to overwash on a gravel barrier (BARDEX 2008 experiment) with that on a sandy barrier (BARDEX II); (2) understand the influence of wave period on overwash characteristics and sediment transport; and (3) improve current knowledge of overwash hydrodynamics. The comparative analysis shows that barrier overwash can be affected by negative feedback that stabilises the barrier through barrier crest accretion on gravel barriers, and by submerged bar development on sandy barriers. An increase in the wave period induced a reduction in overwash frequency over the crest, but no significant relation was found between wave peak period and overwash discharge. Nevertheless, overall water discharge during an overwash episode significantly correlates with overall overwash sediment transport rate. Overwash flow depths during the experiment were relatively shallow and velocities were similar compared to those measured during previous studies and reported in the literature. Despite the controlled laboratory conditions, collection of reliable and accurate measurements of overwash velocities remains challenging. [ABSTRACT FROM AUTHOR]

Swash zone sheet flow and suspended sediment transport rates are estimated on a coarse sand beach constructed in a large-scale laboratory wave flume. Three test cases under monochromatic waves with wave heights of 0.74 m and wave periods of 8 and 12.2 s were analyzed. Sediment flux in the sheet flow layer exceeds several hundred kg m − 2 s − 1 during both uprush and backwash. Suspended sediment flux is large during uprush and can exceed 200 kg m − 2 s − 1 . Instantaneous sediment flux magnitudes in the sheet layer are nearly always larger than those for suspended sediment flux. However, sediment transport rates, those integrated over depth, indicate that suspended load transport is dominant during uprush for all cases and during the early stages of backwash except in the case for the 12.2 s wave case when the foreshore was steeper. Results could not be obtained for an entire swash event and were particularly truncated during backwash when water depths fell below the elevation of the lowest current meter. [ABSTRACT FROM AUTHOR]

Despite the increased sophistication of numerical models and field techniques for investigating wave-induced nearshore sediment transport and ensuing beach morphological response, there remains a significant demand for large-scale laboratory experiments to address this research topic. Here, we describe the Barrier Dynamics II Experiment ( BARDEX II ), which involved placing a near prototype-scale sandy barrier in the middle of the Delta Flume in the Netherlands and subjecting the structure to a range of wave, tide, and water level conditions. A unique aspect of the experiment was the presence of a lagoon behind the barrier, as often occurs in natural barrier settings, providing a convenient means to experimentally manipulate the groundwater hydrology within the barrier. The overall aim of the BARDEX II was to collect a large-scale data set of energetic waves acting on a sandy beach/barrier system to improve our quantitative understanding and modeling capability of shallow water sediment transport processes in the inner surf, swash, and overwash zone. In this paper, we introduce BARDEX II and provide a detailed description of the experiment, including the experimental design, instrumentation, test program, and data set, as well as presenting some examples of the morphological and hydrodynamic data set. We also reflect objectively on the strengths and weaknesses of the data set. This paper serves as an introduction to a special issue of Coastal Engineering , solely devoted to the results of BARDEX II . [ABSTRACT FROM AUTHOR]

The paper reports on dambreak-type swash experiments in which intra-swash hydrodynamics and sediment flux are measured for swash on a coarse sand beach and a gravel beach. Flow velocity and depth are measured using PIV and LIF respectively; the intra-swash sediment flux is measured using sediment traps. Comparison of measured hydrodynamics with the immobile, permeable bed experiments of Kikkert et al. (2013) indicates that bed mobility impacts on the swash hydrodynamics, reducing the maximum run-up by approximately 8% for both beaches, compared to the maximum run-up on the corresponding immobile beach. The measured intra swash sediment flux at a given location is characterised by high flux at the moment of bore arrival, followed by rapid decay during uprush, becoming zero at some time before flow reversal. For the gravel beach, the backwash sediment flux is negligibly small, while for the sand beach the backwash flux increases slowly as the flow accelerates down the beach, and peaks at about the time of maximum backwash velocity. Intra-swash sediment flux calculated using the Meyer-Peter and Müller bed load transport formula, with measured hydrodynamics as input and bed shear stress estimated using both the Swart and Colebrook formulae, is within a factor 2 of the measured intra-swash flux. The agreement between the calculated and measured flux is better for the sand beach than for the gravel beach, and better for uprush than for backwash. For the sand beach there is good agreement between calculated and measured total uprush and total backwash sediment volumes. The agreement is less good for the gravel beach, for which calculated and measured uprush volumes show a similar trend but the calculated backwash volumes over-estimate the (negligible) volumes observed in the experiments. [ABSTRACT FROM AUTHOR]

This paper concerns numerical modelling of the impact on the littoral drift and the shoreline from groynes forming a field of equidistant and identical groynes. The most important effect of a groyne on the shoreline morphology is that the littoral drift is blocked completely or partially. A local reduction in the littoral drift around the groyne introduces alongshore gradients in the alongshore sediment transport and sedimentation and erosion around the groyne which will cause re-orientation of the bed contours towards the prevailing wave direction until an equilibrium is reached. A discussion of this mechanism is presented including effects of scales, e.g. the effect of the relative length of the groynes (compared to the width of the surf zone). The model results indicate a strong dependency of the reduction in littoral drift on the initial geometric bypass ratio (Q geo *), which is defined from the groyne length and the littoral transport on the undisturbed coastline; Q geo * is the transport occurring outside the tip of the groynes divided by the total transport. It is found that the sensitivity of the littoral drift to variations in groyne spacing and the angle, of the approaching waves, is inversely proportional to Q geo *. [ABSTRACT FROM AUTHOR]

Groins interfere with coastal dynamics and sediment transport, leading to sediments accumulation at the updrift side, while at downdrift, the erosive process is anticipated due to the lack of sediments. To improve numerical modeling capacity to simulate the groins impacts, it is necessary to totally understand the shoreline evolution along time and its relationship with the cross-shore profiles shape. The main goal of this work was to analyze and compare the performance of physical and numerical studies on evaluating the evolution of updrift cross-shore profiles geometry and shoreline position after the construction of a groin. This study analyzed a coastal stretch updrift of the groin, at a prototype and model scales, considering the analytical formulation of Pelnard-Considère, the numerical model LTC (Long-term Configuration) and the laboratory tests. The laboratory tested scenario was designed with the aim to gather results, which could be analyzed and compared with numerical simulations from LTC (Coelho, 2005), allowing its improvement, and with the Pelnard-Considère (1956) analytical formulation, both at model scale. The developed study shows an important difference between LTC and Pelnard-Considère (1956) approaches because the analytical solution for the shoreline in equilibrium does not include the wave refraction effects over the updated bathymetry. LTC observed trend of the equilibrium shoreline is not parallel to the initial shoreline, and this behavior was confirmed in the laboratory tests. It was also observed that the sediment transport capacity has very small impact on the LTC shoreline configuration, despite the refraction effects over the updated bathymetries along time. The profile shape obtained in laboratory includes bed forms difficult to reproduce in long-term numerical modeling evaluation of cross-shore profiles because LTC transversal behavior is only based on geometrical considerations and does not represent cross-shore sediment transport and its impact on profile geometry. [ABSTRACT FROM AUTHOR]

Experimental measurements are obtained to investigate the detailed hydrodynamics of wave–wave interactions in the swash-zone. Two bores are generated using a double dam-break mechanism and interact on a 1:10 impermeable rough slope. Measurements of the hydrodynamics are obtained via acoustic displacement sensors and a combined particle image velocimetry and laser induced fluorescent system. Two types of interactions are investigated: wave capture, with the second bore reaching the first one during the uprush, and weak wave–backwash interactions when interaction happens during the backwash of the first wave. The relative strength of the bores at the initial shoreline and time between their arrivals determines the initial type of interaction, however the type as well as the intensity of the interaction may vary throughout the swash-zone for the same swash event. During wave capture and weak wave–backwash interactions, the fluid of the first bore is advected upwards, then mixed with the fluid of the second bore by intense shearing and highly turbulent vortices generated at the front of the second bore because of the relative velocity differences between the two bores. The details of the hydrodynamics during interaction confirm the potential of swash–swash interactions to suspend sediment and transport it shoreward or interrupt seaward sediment transport. [ABSTRACT FROM AUTHOR]

This study presents the analysis of the water surface elevation, velocity and suspended sediment concentration measurements obtained at a large wave flume under mobile bed conditions. The wave reproduced erosive and accretive conditions, and included monochromatic, short waves perturbed with a free long wave, bichromatic and random conditions. Each tested condition started from a handmade 1/15 slope and lasted for an approximate time period of 2.4 h (6 runs of the selected wave condition), to compare the different beach profile developments and, in particular, the events that control sediment transport in the swash and surf zones. All erosive tested conditions produced a shoreline retreat and a bar at the breaking area whose development in time is directly correlated to the length of the breaking area. On the other hand, not all accretive conditions present a shoreward transport, and random conditions do not seem to alter the initial profile. The processed data show the suspended sediment event control produced by the existence or absence of wave–backwash interactions in the swash zone. The existence of these interactions, and their number within the wave group, will be a key parameter in controlling the sediment stirring, water velocity magnitudes and, therefore, the suspended sediment fluxes in the inner surf and outer swash. [ABSTRACT FROM AUTHOR]

This note presents a new method for high resolution beach profiling in small and medium scale laboratory wave flumes. The combined use of laser distance sensors for elevation ( z ) measurements and a barcode reader for cross-shore ( x ) coordinates provides a non-contact, rapid and accurate method to measure beach profiles through the water surface. The system reduces errors from wheeled or contact sensors and eliminates the need to drain the wave tank to perform measurements. The need for a bed follower or motorised traverse and encoder is also eliminated. The system resolves the beach profile both below and above the waterline, with x and z coordinates provided with 1 mm resolution and repeatability of +/− 1 mm, so also resolving wave ripples to a high level of detail. The method is demonstrated with an array of 8 sensors spaced across a 1 m wide tank, which enables accurate calculation of the cross-shore distribution of the total sediment transport at short intervals using multiple beach profiles to account for minor cross-tank variation in the bed elevation. [ABSTRACT FROM AUTHOR]

The Bay of Fundy in eastern Canada has the world's largest tidal range of over 16 m with tidal currents up to 5 ms − 1 making it an ideal place for tidal power extraction using tidal in-stream energy conversion devices in the Minas Passage. Field observations collected from ship-based and bottom-moored sensors over an 8-day period in 2013 are used to validate a 3D hydrodynamic and sediment transport model of the Minas Basin with measurements of water levels, current profiles, waves, suspended sediment fluxes and suspended sediment concentration (SSC) profiles. The sediment conditions are initialized using a bi-modal sediment distribution map and the model simulates both cohesive and non-cohesive sediments in the Minas Basin. Model results for fine-grained suspended sediment concentrations are compared horizontally, vertically, and temporally to observations and indicate strong data-model agreement for SSC from 5 to 287 mg L − 1 . The implications of constructing a large-scale turbine farm within the Minas Passage and the impacts on suspended sediment within the Minas Basin are investigated using the model. The farm is simulated by adding semi-permeable structures that use an energy loss term in the fluid momentum equations to parameterize turbine regions in the hydrodynamic model. The results emphasize the sensitivity of the system to changes in flow and suggest that a large-scale tidal energy farm that extracts maximum power could reduce SSC by 37% on average across the basin which would influence physical and biological processes particularly on the fine-grained intertidal areas around the macrotidal basin. [ABSTRACT FROM AUTHOR]

Detailed measurements of bed level motions and sheet flow processes in the lower swash are presented. The measurements are obtained during a large-scale wave flume experiment focusing on swash zone sediment transport induced by bichromatic waves. A new instrument (CCM + ) provides detailed phase-averaged measurements of sheet flow concentrations, particle velocities, and bed level evolution during a complete swash cycle. The bed at the lower swash location shows a clear pattern of rapid erosion during the early uprush and progressive accretion during the middle backwash phase. Sheet flow occurs during the early uprush and mid and late backwash phases. Sheet flow sediment fluxes during these instances are highest in the pick-up layer. Sediment entrainment from the pick-up layer occurs not only during instances of high horizontal shear velocities but also in occurrence of wave–backwash interactions. As opposed to oscillatory sheet flow, the pivot point elevation of the sheet flow layer is time-varying during a swash event. Moreover, the upper sheet flow layer concentrations do not mirror the concentrations in the pick-up layer. Both differences suggest that in the lower swash zone the dynamics of the upper sheet flow layer are not only controlled by vertical sediment exchange (such as in oscillatory sheet flows) but are strongly affected by horizontal advection processes induced by the non-uniformity of the flow. [ABSTRACT FROM AUTHOR]

Nowadays there are numerous computational tools to forecast the evolution of the shoreline position in sandy coast environments. However, few studies have waited for the time horizon of the predictions to actually check the accuracy of the forecasted scenarios. Thus, this work proposed to assess the accuracy of medium-term predictions made with Long-Term Configuration (LTC) and GENEralised model for SImulating Shoreline change (GENESIS) numerical models for the time horizon of 15-year and evaluate the models' performance in forecasting the actual status of a complex sandy coastal stretch in 2018, the targeting year. The projection premises were already presented in Baptista et al. (2014) on which observational data used to calibrate and blind hindcast future scenarios of shoreline development was discussed. The case study focused on an area located downdrift of Poço da Cruz groin, on the northwest Portuguese coast, and considered two beach sectors in accordance with Baptista et al. (2014): sector AC, about 3.5 km long, and sector AB, about 2.4 km long and included in the first one. The whole coastal stretch is composed of sandy beaches, and past trends highlight erosion downdrift of Poço da Cruz groin due to the interruption of sediment transport. A monitoring program performed by the University of Aveiro between 2003 and 2018 (corresponding to the period of simulation presented in Baptista et al., 2014), including six topographic surveys, was considered for comparison with the projections previously made. An additional topographic survey performed in 2018, within COSMO programme, was also considered. Simulations results have proved divergent performances of LTC and GENESIS models, despite the equity of inputs parameters and modelling processes. It is concluded that GENESIS and LTC, considering the typical (TYP) wave climate (randomly generated to represent the typical frequencies of wave height and direction during five years), create an envelope (GENESIS below and LTC above) to the real average results. For TYP, GENESIS underestimates the mean retreat rates up to 23 m in sector AB and up to 23 m in AC; LTC overestimates the retreat up to 33 m in sector AB and up to 13 m in AC. Conversely, observed (OBS) wave climate (shorter characterisation period but maintaining the registered data sequence) projections overestimate the shoreline retreat in both coastal stretches, suggesting that a larger characterisation period should be more important than maintaining the sequence of recorded waves. For OBS, the models overestimate the mean retreat rates in both sectors: GENESIS overestimates up to 90 m in sector AB and up to 51 m in AC; LTC overestimates up to 60 m in AB and up to 60 m in AC. Also, the consideration of the artificial nourishment operations executed during the simulation period leads to better results in the LTC, which was overestimating the erosion rate. This showed that if all man-made interventions could be anticipated, the average retreat in the main sector, AC, would only be up to 5 % different from the projections made by Baptista et al. (2014) with LTC model and TYP wave climate. • Checked accuracy of 15-year forecasts. • Checked performance of LTC and GENESIS shoreline evolution models. • The wave sampling period used in calibration has relevant impact on forecasts made. • LTC and GENESIS create an envelope to the real average retreat surveyed. • Better projections would be achieved if planed interventions were known in advance. [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]

New flume experiments with surrogate seagrass meadows are presented. The experiments included the creation of a full-scale and realistic Posidonia Oceanica model to evaluate the effect over wave attenuation, sediment transport and shoreline erosion. A hydrodynamic and morphodynamic comparison between cases with seagrass and without seagrass for two wave energy conditions was performed. Meadow density and submergence ratio were constant for tests under irregular waves. The wave height reduction, bar crest location, total sediment transport and shoreline position were used to evaluate the coastal protection efficiency of the meadow. The measured wave heights suffered a reduction due to the presence of the seagrass. These reductions were persistent in the area located between the onshore edge of the meadow and the depth of closure, being of greater magnitude in the more energetic case. All tests showed the development of a bar and the migration of the crest offshore over time. However, the dissipation of the incoming wave energy on the meadow made the bar crest stay closer to the shoreline and consequently generated a lower freeboard. In addition, the bar migration rate was reduced by the simulated meadow effect, in particular in the lower energetic case. A sediment volume significantly smaller was transported offshore when the seagrass meadow was present in both wave conditions. Additionally, in the higher energy case, a smaller shoreline retreat was observed when the meadow was present. • Flume experiments were carried out with a realistic Posidonia Oceanica model to assess the effect on beach sediment transport. • Two wave condition were analysed comparing the test with and without seagrass meadows. • Wave heights were reduced due to the effect of the Posidonia Oceanica model in areas after the meadow. • The total volume of sediment transported in the seagrass meadows cases was considerably less than without seagrass meadows. • Bar crest morphological parameters and shoreline position were analysed to verify the meadow effect on the coastal protection. [ABSTRACT FROM AUTHOR]

Artificial coastal dykes have been widely built to prevent erosion, or to accelerate sedimentation processes in saltmarshes. To investigate the sediment dynamic changes induced by construction of dykes, three observation stations were set around a dyke within an expanding Scirpus mariqueter saltmarsh, in southern Hangzhou Bay. The hydrodynamics of the study area was dominated by tidal currents. The presence of dyke, in combination with the adjacent tidal creek, altered the hydrodynamic boundary conditions, thereby affecting the flow field. When the tidal flows approaching the dyke, the speed was accelerated at the head of the dyke and then decreased at the rear, due to the narrowing effect of the dyke head. Turbulence intensity around the dyke showed a similar pattern to that of the tidal flows. The net sediment input behind the dyke is approximately 2.3 times of that at the dyke head, illustrating sediment deposition in the back region of the dyke. As a result, flows entrained the sediments and then deposited behind the dyke to cause a local accretion to promote saltmarsh vegetation succession. The construction of dykes should be taken into account for future coastal wetland management. • In situ sediment dynamic observation was conducted around a dyke in saltmarsh. • Dyke altered tidal flows and caused sediment transport towards the rear region. • Construction of the dyke drove vegetation succession in the saltmarsh. [ABSTRACT FROM AUTHOR]

Coastal zones are vulnerable to both erosion and flood risk, which can be assessed using coupled hydro-morphodynamic models. However, the use of such models as decision support tools suffers from a high degree of uncertainty, due to both incomplete knowledge and natural variability in the system. In this work, we show for the first time how the multilevel Monte Carlo method (MLMC) can be applied in hydro-morphodynamic coastal ocean modelling, here using the popular model XBeach, to quantify uncertainty by computing statistics of key output variables given uncertain input parameters. MLMC accelerates the Monte Carlo approach through the use of a hierarchy of models with different levels of resolution. Several theoretical and real-world coastal zone case studies are considered here, for which output variables that are key to the assessment of flood and erosion risk, such as wave run-up height and total eroded volume, are estimated. We show that MLMC can significantly reduce computational cost, resulting in speed up factors of 40 or greater compared to a standard Monte Carlo approach, whilst keeping the same level of accuracy. Furthermore, a sophisticated ensemble generating technique is used to estimate the cumulative distribution of output variables from the MLMC output. This allows for the probability of a variable exceeding a certain value to be estimated, such as the probability of a wave run-up height exceeding the height of a seawall. This is a valuable capability that can be used to inform decision-making under uncertainty. • First use of MLMC (multilevel Monte Carlo) with a coupled hydro-morphodynamic model. • Large drop in computational cost for same accuracy using MLMC instead of Monte Carlo. • Determine the distribution of variables of interest in coastal zone cases using MLMC. [ABSTRACT FROM AUTHOR]

Shear plate measurements of bed shear stress under fixed and loose bed swash uprush flows are presented and contrasted. A shear plate previously developed for fixed bed flows was modified and tested for coarse and fine grain loose beds. Transient flows were generated by a dam-break flow over initially dry horizontal and sloping beds, with flows upslope analogous to swash uprush flows on beaches. Performance of the shear plate varied with grain size. For the coarse grained loose beds only the peak shear stress at the leading edge of the swash flow was consistently obtained; for fine grained loose beds the shear stress was reliably obtained over the whole uprush swash cycle. Data were obtained for varying thickness of loose bed material, commencing with a single grain layer, which correspond to partially starved bed conditions. The direct measurements clearly indicate that the bed shear stress comprises of a fluid shear stress component and a grain shear stress component. The grain shear stress increases linearly with sediment load and is significantly larger for the coarse loose grain bed tests due to the much larger normal stress compared to that for fine grain loose bed tests with the same number of grain layers. The data show a high degree of consistency with the classical theory of Bagnold (1956) and the derived coefficient of dynamic internal friction is similar to that observed by Hanes and Inman (1985). [ABSTRACT FROM AUTHOR]

A laboratory experiment was conducted to investigate the dynamics of cross-shore sediment transport across a fringing coral reef. The aim was to quantify how a highly bimodal spectrum of high-frequency (sea-swell) and low-frequency (infragravity and seiching) waves that is typically present on coral reef flats, influences the various sediment transport mechanisms. The experiments were conducted in a 55 m wave flume, using a 1:15 scale fringing reef model that had a 1:5 forereef slope, a 14 m long reef flat, and a 1:12 sloping beach. The initial 7 m of reef flat had a fixed bed, whereas the back 7 m of the reef and the beach had a moveable sandy bed. Four seven-hour irregular wave cases were conducted both with and without bottom roughness elements (schematically representing bottom friction by coral roughness), as well as for both low and high still water levels. We observed that the wave energy on the reef flat was partitioned between two primary frequency bands (high and low), and the proportion of energy within each band varied substantially across the reef flat, with the low-frequency waves becoming increasingly important near the shore. The offshore transport of suspended sediment by the Eulerian mean flow was the dominant transport mechanism near the reef crest, but a wide region of onshore transport prevailed on the reef flat where low-frequency waves were very important to the overall transport. Ripples developed over the movable bed and their properties were consistent with the local high-frequency wave orbital excursion lengths despite substantial low-frequency wave motions also present on the reef flat. This study demonstrated that while a proportion of the sediment was transported by bedload and mean flow, the greatest contributions to cross-shore transport were due to the skewness and asymmetry of the high and low-frequency waves. [ABSTRACT FROM AUTHOR]

Field measurements of hydrodynamics, suspended sediment transport rates and bedform sediment transport rates were made in the intertidal section of a dissipative sandy beach (D50 = 0.26 mm, slope = 1/80) at Perranporth (UK). Pressure transducers, acoustic Doppler velocimeters, optical backscatter sensors and an acoustic sand ripple profiler were deployed for 12 tides, measuring in a range of wave heights from 0.5 to 2.2 m, water depths from 1 to 6 m, and in current strengths up to 0.4 m/s. Data were analysed in terms of the distance to shore (x) normalised by the surf zone width (xs), and spanned the region 0.4 < x/xs < 3. Bedform heights up to 30 cm and wavelengths 0.5 to 2.7 m were recorded. Maximum wavelengths were observed just shoreward of the breakpoint. Bedforms were classified as sub-orbital, vortex ripples. Bedform migration was mostly onshore directed, and correlated with positive (onshore) wave skewness. Migration rates increased through the shoaling zone to a maximum of 1.5 cm/min just shoreward of the breakpoint (x/xs = 0.8). The bedform component of sediment transport was generally onshore directed, and maximum just shoreward of the breakpoint (0.021 kg/m/s). Point measurements showed that the cross-shore suspended sediment transport 25 cm above the bed was dominated by the mean component, with an offshore directed maximum at x/xs = 0.5. Contributions to onshore transport were only made by the incident wave (gravity band) component. The total depth integrated suspended sediment transport was offshore directed and maximum in the mid surf zone (− 0.16 kg/m/s). The depth integrated suspended sediment transport dominated over the bedform sediment transport in the inner to mid surf zone (x/xs < 0.5) and in the outer shoaling zone (x/xs > 1.5). The fractional contribution of the shoreward directed bedform transport to the total absolute transport was up to 100%, and occurred broadly in the region of the breakpoint (0.5 < x/xs < 1.5). However, spatial averaging in the cross-shore indicated that a more realistic bedform contribution was up to 15% of the transport, with a maximum at x/xs = 0.9. Results from this dissipative beach experiment generally agree with previous findings on intermediate beaches, steep beaches, and offshore sandbars. [ABSTRACT FROM AUTHOR]

In modeling the morphological impact of an offshore dredging pit using a process-based model, the hydrodynamic conditions, often derived from regional scale models and sometimes incomplete measurements, may not be in equilibrium with the initial bathymetry profile before dredging. This lack of equilibrium causes a fast profile adjustment toward an equilibrium, which may bias the model results representing the direct effect of the pit. In this study, a method is described which uses the parallel computing technique and evaluates only the morphological changes due to a single factor, i.e., the dredging of a pit. The hydrodynamics and morphodynamics are assumed to be in equilibrium before initial dredging of the pit. The morphological evolution is computed based on the change of sediment flux due to the presence of the pit. The model can avoid the computational demanding spin-up period necessary to reach equilibrium conditions. In particular the concurrent correction method is efficient for scenario testing with a large amount of morphological settings and hydrodynamic conditions. [ABSTRACT FROM AUTHOR]

Existing numerical models of the swash zone are relatively inflexible in dealing with sediment transport due to a high dependence of the deployed numerical schemes on empirical sediment transport relations. Moreover, these models are usually not well-balanced, meaning they are unable to correctly simulate quiescent flow. Here a well-balanced and flexible morphological model for the swash zone is presented. The nonlinear shallow water equations and the Exner equation are discretized by the shock-capturing finite volume method, in which the numerical flux and the bed slope source term are estimated by a well-balanced version of the SLIC (slope limited centered) scheme that does not depend on empirical sediment transport relations. The satisfaction of the well-balanced property is demonstrated through simulating quiescent coastal flow. The quantitative accuracy of the model in reproducing key parameters (i.e., the notional shoreline position, the swash depth, the flow velocity, the overtopping flow volume, the beach change depth and the sediment transport rate) is shown to be satisfactory through comparisons against analytical solutions, experimental data as well as previous numerical solutions. This work facilitates an improved modeling framework for the swash hydrodynamics and sediment transport. [ABSTRACT FROM AUTHOR]

Turbulence and sediment transport models are incorporated into a three-dimensional hydrodynamics model to investigate the mechanisms of morphologic evolution of scour holes within the Indian River Inlet, Delaware, USA. The inlet bed had eroded slightly since stabilizing the channel walls in late 1930s through 1976. The mean rate of bed erosion roughly doubled as a response to anthropogenic activities within the inlet such as the removal of ~ 80 piles remaining from an old bridge. Severe erosion near the in-water bridge supports and cofferdams for the replacement bridge necessitated channel bed stabilization that along with the remained debris from the removal of old bridge piles enhanced the growth of two deep scour holes on the bayside and oceanside of the bridge cofferdams. Scour hole and channel bed evolution has decreased drastically since 1994. The present investigation suggests that, initially, flow concentration through the cofferdams of the replacement bridge was the main reason for scour hole development. Bed shear stress over the forward-facing slope of the scour hole entrains sediment from the bed and extends the scour hole along the inlet and in the vertical direction. Flow separation within the developed scour holes after channel bed stabilization enhances turbulence and appears to be the dominant mechanism for further scour hole development. [ABSTRACT FROM AUTHOR]

Seven years of field survey measurements of a subsea pipeline obtained using sonar profilers and underwater video indicate significant lowering of the pipeline into the seabed due to sediment mobility and scour. The majority of the lowering occurs within 2 years of pipeline laying and appears to result from sustained ambient tidal and soliton currents as opposed to large storms. The lowering results in an increase in pipeline embedment relative to the far field seabed of up to 0.8 times the pipe diameter (referenced at a distance ± 8 D from the pipeline). At most locations along the pipeline, this increase in far-field embedment is uniform and occurs after the formation of many closely spaced scour holes. This suggests that the pipeline lowered mainly through sinking into the seabed at span shoulders, rather than sagging into widely spaced scour holes, for much of the pipeline length. A beam bending analysis confirmed the dominance of sinking, but did show some evidence of pipeline sagging, calculating deflections of up to 0.3 pipeline diameters at the time of surveying. In contrast to the traditional conception of span growth and self-burial, which conceives of complete pipeline burial as an endpoint, this pipeline primarily appears to exhibit ‘self-lowering’ towards a mature state that consists of a pseudo-static profile of alternating spanning and embedded sections that are distributed at regular intervals. The observed changes appear to be predictable given sufficient pipeline setting data, which suggests that they can be quantified in the stability design of new pipelines. This opens up the possibility of more efficient on-bottom stability design, as the beneficial shielding and support provided by the self-lowering process is not usually accounted for. [ABSTRACT FROM AUTHOR]

Nearshore submerged structures made of sand-filled geosystems are an interesting use for this type of coastal protection technology. These systems have an advantage over more traditional materials such as concrete units or rock, which is the limited and non-permanent impact on natural coastal processes. On top of that are also some important research achievements and improvements at the level of the materials. However, at more exposed hydraulic conditions under high wave energy its application is still hampered by the lack of knowledge about their performance. This paper describes research results based on a series of laboratory measurements of the hydrodynamic and morphodynamic change produced in the vicinity of submerged nearshore structures. An analysis of the different model outputs produced, with respect to sediment transport, wave reflection and wave-induced pressures, has been made and is discussed throughout the paper. Such a comparative analysis provides insights into the efficiency of each defence scheme in maintaining a beach and in protecting the shoreline. The sand-filled geosystems as nearshore submerged structures proved to be efficient in retarding the offshore movement of sediments and in maintaining the shoreline, even if instabilities due to elements' displacement and local scour have been observed. [ABSTRACT FROM AUTHOR]

Rivers supply sediment to estuaries and alter estuarine tidal wave regimes. However, the long-term (> century) impact of seasonally varying river discharges on estuarine morphodynamics is not well understood. Process-based models are useful tools to analyze these morphodynamic processes. This study aims to explore modeling techniques to efficiently include the impact of discharge seasonality on long-term estuarine morphodynamic development by a process-based morphodynamic model. Inspired by the dimensions and hydraulic forcing of the Yangtze River estuary, we evaluate three methodologies to schematize a river discharge hydrograph in a 1-D morphodynamic model. These are a representative constant discharge, a non-compressed hydrograph and a compressed hydrograph. To shorten computational time, the latter methodology decreases the timescale of seasonal river flow variations compared to the tidal timescale. Sensitivity analysis suggests that the yearly averaged sediment transport controls long-term morphodynamic development. Morphodynamic equilibrium is reached after millennia when deposition during a high river flow period is balanced by erosion during a low river flow period. An important finding is that a yearly hydrograph can be squeezed in time up to a factor of 20, without leading to unrealistic river–tide interactions. Applying a constant river discharge may not produce equilibrium bed profiles that are similar to that under hydrograph forcing due to the non-linearity in sediment transport. The long characteristic timescales in defining morphodynamic equilibrium indicate that a long-term view is needed to interpret large scale estuarine morphodynamics. Though simple and highly schematized, the modeling experiments in this study provide conceptual understanding of morphodynamics in river-influenced estuaries. [ABSTRACT FROM AUTHOR]

A morphodynamic model for weakly dispersive wave and shallow flows in horizontal two-dimensional (H2D) space is proposed. The horizontal density variation and the bottom evolution are incorporated into a fully nonlinear Boussinesq equation including vorticity effects. By coupling the flow equation with the depth-integrated sediment transport equation, morphodynamic simulation is made possible. A fourth-order finite volume method with an approximate Riemann solver is used to solve the governing equations. For the hydrodynamic test under strong wave–current interaction environment, the accuracy of the computed velocity and water surface was verified. For the three typical morphological benchmark tests, such as tsunami, regular wave and dam break flow, good agreement was obtained when proper parameters were provided. Therefore, the proposed morphodynamic model can be a useful tool for a wide range of shallow and wavy flows under dispersive and nonhydrostatic environments associated with wave, current, and sediment interaction. [ABSTRACT FROM AUTHOR]

A coupled wave–current–sediment transport beach profile model is used to simulate cross-shore sandbar evolution on the time scale from days to months comprising both rapid offshore and slow onshore migrations. The discrimination of four modes of sediment transport driven by velocity and acceleration skewness, mean currents and slope effects allows addressing the dominant hydrodynamic processes governing cross-shore sandbar behavior. Acceleration–skewness-induced transport systematically results in a slow onshore sandbar migration together with a slow bar growth. Velocity–skewness-induced transport can drive onshore and offshore bar migrations with substantially larger rates. Mean–current-induced sediment transport systematically drives an offshore bar migration with either bar growth or decay. Slope effects essentially act as a damping term. The water level above the sandbar crest mainly influences the sandbar migration direction, while wave obliquity regulates the magnitude of the migration rates and is crucial to accurately simulate offshore sandbar migration during energetic obliquely incident waves. The inclusion of acceleration skewness is a necessary requirement to accurately reproduce the onshore migration of shallow sandbars. Detailed inter-site comparison of best-fit model parameters shows large differences meaning that free parameters attempt to compensate some mispecifications of the physics in the model. Although this also applies to other existing beach profile models, this suggests that this model needs further improvements including, for instance, the contribution of the injection of breaking wave turbulence onto the bed to sand stirring. [ABSTRACT FROM AUTHOR]

A fully-coupled hydrodynamic/morphodynamic numerical model is presented and utilized for the simulation of wave-induced scour and backfilling processes beneath submarine pipelines. The model is based on solutions to Reynolds-averaged Navier–Stokes equations, coupled with k − ω turbulence closure, with additional bed and suspended load descriptions forming the basis for sea bed morphology. The morphological evolution is updated continuously, rather than being based e . g . on period- or other time-averaging techniques. Simulations involving wave-induced scour over the range of Keulegan–Carpenter number 5.6 ≤ KC ≤ 30 demonstrate reasonable match with previous experiments, both in terms of the equilibrium scour depth as well as the scour time scale. Wave-induced backfilling processes are additionally studied by subjecting initial conditions taken from scour simulations with larger KC to new wave climates characterized by lower KC values. The simulations considered demonstrate the ability of the model to predict backfilling toward expected equilibrium scour depths based on the new wave climate, in line with experimental expectations. The simulated backfilling process is characterized by two stages: (1) An initial re-distribution phase involving re-organization of sediments in the immediate vicinity of the pipeline, potentially followed by (2) a more lengthy backfilling evolution toward equilibrium scour depth. The simulated backfilling time scales are of the same order of magnitude as in experiments, though the multi-stage process complicates a more systematic characterization. The simulated sequences of scour and backfilling achieved within the present work are estimated to represent temporal durations of up to approximately 12 h at full practical scales. [ABSTRACT FROM AUTHOR]

The newly developed nearshore circulation model, SHORECIRC, using a hybrid finite-difference finite-volume TVD-type scheme, is coupled with the wave model SWAN in the Nearshore Community Model (NearCoM) system. The new modeling system is named NearCoM-TVD and the purpose of this study is to report the capability and limitation of NearCoM-TVD for several coastal applications. For tidal inlet applications, the model is verified with the semi-analytical solution of Keulegan (1967) for an idealized inlet-bay system. To further evaluate the model performance in predicting nearshore circulation under intense wave-current interaction over complex bathymetry, modeled circulation patterns are validated with measured data during RCEX field experiment (MacMahan et al., 2010). For sediment transport applications, two sediment transport models are applied to predict three sandbar migration events at Duck, NC, during August to October 1994 (Gallagher et al., 1998). The model of Kobayashi et al. (2008) incorporates wave-induced onshore sediment transport rate as a function of the standard deviation of wave-induced horizontal velocities. The modeled beach profile evolution for two offshore events and one onshore event agrees well with the measured data. The second model investigated here combines two published sediment transport models, namely, the total load model driven by currents under the effect of wave stirring (Soulsby, 1997) and the wave-driven sediment transport model due to wave asymmetry/skewness (van Rijn et al., 2011). The model study with limited field data suggests that the parameterization of wave stirring is appropriate during energetic wave conditions. However, during low energy wave conditions, the effect of wave stirring needs to be re-calibrated. [ABSTRACT FROM AUTHOR]

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]