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Numerical simulations were conducted to investigate steady current scour around rectangular and square subsea caissons. The caissons, which are representative of subsea structures, have a submerged height ranging from 0.5 to several times their longest base dimension. The flow model used is based on the Reynolds Averaged Navier-Stokes equations, and the scour model simulates bed load and suspended load transport to predict the evolution of the seabed profile. The aim of this study is to investigate the mechanisms of local scour, specifically the contribution of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour. The model is validated through comparisons with experimental measurements, indicating reasonable agreement. Based on the numerical model results, the mechanisms of local scour around square and rectangular caissons are found to be qualitatively similar. If the flow is directed perpendicular to one of the caisson faces, both the horseshoe vortex and the local streamline contraction contribute to scour. The maximum scour depth is located at the two upstream corners of the caisson due to the combined effects of the horseshoe vortex and streamline contraction. When the flow approaches the caisson with a diagonal attack angle of 45°, the two upstream faces act like a streamlined wedge that splits the incoming flow. The study found that the horseshoe vortex almost disappears and the scour is primarily caused by the local velocity amplification at the two side corners. If the flow attack angle is 45°, the scour does not initiate near the upstream corner without the contribution from the horseshoe vortex. These findings are expected to serve as a valuable theoretical foundation for the design of scour protection. • Scour around subsea caissons is studied by fully three-dimensional simulations. • The contributions of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour are identified. [ABSTRACT FROM AUTHOR]

Existing prediction formulas for clear-water scour development are typically empirical fittings of lab-scale results. However, it is unreasonable to evaluate the in-situ clear-water scour process around a pile based on small-scale flume tests due to inherent scale effect. This study proposes a time-dependent model of clear-water scour development around a pile foundation under steady currents. A scaling expression of shear stress acting on sediment particles at the front of a circular pile is established based on the phenomenological theory of turbulence. By applying the sediment transport model of flat-bed to local scour around a circular pile, a physics-based ordinary differential equation for predicting the scour depth development is derived. The analytical solution of scour depth development is generally more consistent with the experimental data compared with previous models. The probability density function distribution of the proposed model's error mainly concentrates within the range of ±10%, which is significantly superior to previous models. The proposed model integrates all pertinent parameters that govern the scour process using fundamental principles, rendering it free from scale issues and applicable to prototype conditions. The present model is applied to evaluating clear-water scour development around typical prototype piles with diameters ranging from 2.0 m to 10.0 m. The predicted variations of equilibrium scour time with pile diameter, flow velocity and sediment particle size aligns closely with previous experimental observations. • A time-dependent model of clear-water scour development around a pile foundation is proposed based on the phenomenological theory of turbulence. • The derived analytical solution of scour depth development is more consistent with the experimental data compared with previous models. • The proposed model is free from scale issues and can be applied to prototype conditions. [ABSTRACT FROM AUTHOR]

Scour around a circular monopile in coastal regions has been investigated extensively over the past decades, but the time development of scour depth around an offshore-wind monopile under large current–wave ratio still lacks a predictive model. By considering the conservation of sand volume and adopting the conventional exponential law for temporal variation, a semi-empirical model, which has three parameters, i.e., an equilibrium scour depth, a shape coefficient and a scour characteristic time scale, is developed for predicting the time development of scour depth around a monopile under live-bed conditions of combined wave–current flows. A series of laboratory experiments was conducted in current-only and wave–current flows to obtain data for model calibration and validation. Experimental results indicate that adding weak waves on current accelerates scour development, which is successfully captured by the proposed model through using the far-field bottom shear stress as a key model input. The overall model inaccuracy is within 25%, and the model's applicability is further confirmed by field measurements from an offshore wind farm in east China. This model can help to determine the timing of installing scour protection around offshore monopiles, especially for the circumstances with very strong local sediment transport (live-bed). • A new model for scour time development around a circular pile is proposed. • A set of scour tests was conducted in live-bed conditions for calibration and validation. • A field study was carried out for an offshore monopile wind farm in eastern China. [ABSTRACT FROM AUTHOR]

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

Foredunes provide many environmental and ecosystem services including protection from wave erosion and flooding hazards during storm events. While the impact of human interventions on the short-term evolution of coastal dunes is reasonably well understood, less is known about their contemporary influence on current wind and sediment dynamics several decades after implementation. The coastal dunes in Leucate (SE, France) have been anthropogenically constructed and are dominated by offshore wind conditions. Since their construction 20 years ago, a distinct variation in their longshore morphology has developed that is inherited from its original construction. The northern part of the dune has a symmetrical profile with a 28° degree stoss slope, 30° lee slope and a single crest. The southern part is asymmetric, with a gentler stoss slope (12°), 26° lee slope, and a double crest. To explore the potential geomorphic impacts of this distinct difference in morphology, several numerical simulations with varying wind speeds and direction were conducted. We used Computational Fluid Dynamics (CFD) to explore the spatial variations of the near surface wind flow, shear stress and aeolian sediment transport. show that for each scenario, near surface wind speed accelerated toward the dune crest on the windward slope of the dune. The maximum wind speed varied with incident wind direction, the highest speeds occurring when incident wind flow was perpendicular to the dune crest. The double crest in the southern section of the dune affected the wind flow by inducing two consecutive speeds-up zones, with a greater maximum wind speed than on the single crested dune. Wind flow separation was observed where a steep lee slope was present (single crested dune), and only during perpendicular winds. This suggests that the shape of the dune and the direction of the wind are key parameters rather than wind speed. The area affected by reversed separated flow was spatially limited and did not extend beyond the dune toe but was below the threshold for aeolian sediment transport. Elsewhere in the lee of the dune, the wind was only deflected by an order of 15° for the oblique winds (310 and 330°), and less than 10° for the perpendicular wind (290°) on the southern part. In these locations, the shear velocity exceeded the threshold, notably on the southern dune, which coincided with the formation of undulating aeolian deposits in the lee of vegetation on the dune crest. The spatial differences observed in wind flow and aeolian sediment transport processes on this human-made dune were directly inherited from differences in their construction two decades ago. These results demonstrate the importance of the constructed dune profile, due to its potential impact on the long-term evolution of the landscape and the sediment budget of the system. • Dune construction methods impact their morphology even after several decades. • A relatively small coastal dune can lead to airflow separation. • The pattern of wind flow perturbation depends on dune shape and wind direction. [ABSTRACT FROM AUTHOR]

The swash zone is an important region for the coastal morphodynamics. Often, model studies of the swash zone use depth-averaged models. These models typically assume a vertically uniform velocity and sand concentration for calculating the sand transport flux. However, this assumption is not always accurate in the swash zone. In order to investigate the vertical distribution of velocity and sand, we use a depth-resolving model that is able to capture these vertical variations. We simulate the flow and suspended sediment transport induced by bichromatic waves using a 2DV depth-resolving RANS model. Our verification of the model shows that special care needs to be taken to deal with bubbles in 2DV simulations. Furthermore, we show that turning off the (Wilcox, 2006, 2008) limiter for turbulence, increases the modelled turbulent kinetic energy that is induced by wave-breaking, resulting in improved predictions of sediment concentrations. Using the depth-resolving model, we show that the vertical distribution of velocity and sand is far from uniform in the swash zone. The results show that if one assumes vertically uniform depth-averaged velocities and concentrations, one can overpredict the sediment flux by 50%. • Air bubbles can stay unrealistically long near the bed in 2DV swash simulations. • Bubbles near the bed lead to lower shear stresses and lower sediment pick up. • Increased wave-breaking turbulence leads to increased onshore sand transport. • A model assuming a vertically uniform flux of sand can underpredict the flux by 50%. [ABSTRACT FROM AUTHOR]

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

Catastrophic breaches of water and mine waste retaining impoundments in recent history have emphasized the severity and consequences to the downstream environment. In this study, the applicability and suitability of the XBeach numerical model for modelling earth dam failures driven by overtopping with water is explored by detailed comparison with controlled experimental data. The XBeach model is applied to simulate laboratory-scale breaches caused by overtopping failure of dams constructed with uniform sand at the Queen's University Landslide Flume by Walsh et al. (2021a). The model sensitivity to boundary conditions for different virtual flume sizes, different sediment transport equations that influence erosion rates, and the process of avalanching are investigated. The model results are in best agreement with laboratory measurements when the Soulsby-van Rijn sediment transport formulation is used, which depends on grain size, and the avalanching module is activated, indicating the importance of these processes. The model is also run for different dam geometries to investigate the influence of upstream slope angle, which is relevant for tailings dam breach analyses. The results indicate that for flatter slopes the model is in closer agreement with observations of the peak discharge, while for steeper slopes the model results are better for breach duration time. Overall, XBeach provides promising results for the prediction of the dam breach outflow hydrograph at the laboratory scale and has potential to improve understanding of earth dam failure outcomes with further investigation. • XBeach is applied to simulate the failure and morphological evolution of earth dams. • Results are compared to laboratory measurements for a range of upstream slopes. • XBeach is a promising tool for predicting the breach outflow hydrograph response. [ABSTRACT FROM AUTHOR]

Sand waves are found on shallow, sandy seabeds throughout the world and their dynamics may pose an imminent threat to offshore construction. Therefore, there is a pressing need to understand bed level dynamics in sand wave areas. These bed level dynamics lead to variations in sand wave shape and migration rate over time. However, these variations cannot be explained with the present-day process-based sand wave models, which all include a purely periodic tidal forcing. To explain these fluctuations a more intricate description of the hydrodynamics is necessary. The aim of this study is to explore the importance of time-varying, non-tidal currents for sand wave dynamics in the North Sea. We adopted the three-dimensional Delft3D-Flexible Mesh model, and were able to reconstruct time-varying, non-tidal currents on top of the periodic tidal forcing, while significantly reducing computation times. The simulated currents and water levels showed a good agreement with in-situ measurements. Compared to the situation with only tidal forcing, the simulated sedimentation and erosion rates were amplified up to 15 times due to time-varying, non-tidal currents. Additionally, periods of net erosion were found at locations in the sand wave transect where tidally forced models only showed net-sedimentation. It is therefore important to consider time-varying, non-tidal currents when predicting future sand wave dynamics in the field. • The reproduction of observed hydrodynamics is improved significantly compared to previous sand wave modelling approaches. • Effects of time-varying, non-tidal currents on sediment transport in sand wave fields are quantified. • Net sedimentation rates due to time-varying, non-tidal currents are be up to 15 times larger than the tidally induced rates. • First application of Delft3D FM software to study sand wave dynamics leads to a significant decrease in computation time. • Excluding time-varying, non-tidal currents can lead to underestimation of sand wave migration and trench infill rates. [ABSTRACT FROM AUTHOR]

Beach loss is a growing global challenge that threatens the safety of coastal communities, the health of coastal ecosystems, recreational amenities, and regional economies dependent on tourism. Spatial gradients in longshore sediment transport, or divergence of drift (DoD), is a primary driver of beach width change over multi-annual time scales, but the response of any particular beach can be challenging to characterize and predict. Here we present a new method to characterize DoD using a non-uniform segmentation of the coastline informed by the spatial distribution of longshore transport potential, including the location of physical barriers identified using satellite imagery, and both maxima and reversals in longshore transport potential derived from nearshore wave data. The method demonstrates improved capacity to predict beach width trends at sandy beaches compared to methods that rely on a uniform segmentation of the coastline. In an application to southern California where satellite data documents two decades of beach width change, the non-uniform segmentation method correctly predicts the sign of beach trends at 93% of transects within a littoral cell and achieves a linear correlation between DoD and beach width trends exceeding 0.8. Moreover, we find that a minimum of five years of data are required to establish consistently strong correlations between DoD and beach width changes. Conversely, use of a uniform segmentation is shown to be unreliable for estimating beach width trends due to strong sensitivities to shoreline segment size. This work shows the potential to leverage nearshore wave data and satellite-based beach width data for improved characterization of shoreline dynamics relevant to beach erosion management. • A new shoreline modeling method is presented for estimating divergence of drift. • Segment boundaries are identified using longshore transport characteristics. • Strong correlations were shown between divergence of drift and beach width trends. • A littoral Peclet number characterizes the dominant longshore transport mechanism. [ABSTRACT FROM AUTHOR]

A Reynolds-averaged two-phase Eulerian model for sediment transport, SedFoam, is utilized in a two-dimensional domain for a given sediment grain size, flow period, and mobility number to study the asymmetric and skewed flow effects on the sediment transport over coarse-sand migrating ripples. First, the model is validated with a full-scale water tunnel experiment of orbital ripple driven by acceleration skewed (asymmetric) oscillatory flow with good agreement in the flow velocity, net sediment transport, and ripple migration rate. The model results showed that the asymmetric flow causes a net onshore sediment transport of both suspended and near-bed load (the conventional bed load and part of the near-bed suspended load, responsible for ripple migration). The suspended load transport is driven by the "positive phase-lag" effect, while the near-bed transport is due to the large erosion of the boundary layer on the stoss flank, sediment avalanching on the lee flank, and the returning flux induced by the stoss vortex. Together, these processes result in a net onshore transport rate. In contrast, for an energetic velocity skewed (skewed) flow, the net transport rate is offshore directed. This is due to a larger offshore-directed suspended load transport rate, resulting from the "negative phase-lag" effect, compared to the onshore-directed near-bed load transport rate. Compared to the asymmetric flow, the onshore near-bed load transport (and migration) rate is limited by the larger offshore directed flux associated with returning flow on the lee side, due to a stronger lee vortex generation during the onshore flow half-cycle. In the combined asymmetric-skewed case, the near-bed load and migration rate are higher than in the asymmetric flow case. Moreover, the offshore-directed suspended load is much smaller compared to the skewed flow case due to a competition between the negative (due to velocity skewness) and positive (due to acceleration skewness) phase-lag effects. As a result, the net transport rate is onshore directed but slightly smaller than the asymmetric flow case. • Vortex generation-ejection processes lead to different phase-lag effects for asymmetric and skewed flows. • The onshore ripple migration rate is limited (enhanced) by the lee (stoss) vortex induced returning flux for skewed (asymmetric) flows. • Combined asymmetric-skewed flow has the highest onshore migration rate (near-bed load). [ABSTRACT FROM AUTHOR]

Storms and extremely energetic events may significantly impact the form and structure of beaches, and so cause erosive processes and coastal damages. Efficient management actions require an up-to-date and accurate knowledge of beach morphological changes, with the shoreline position being a good indicator of such changes. This work proposes the use of the open-source Shoreline Analysis and Extraction Tool (SAET) software for the definition of satellite-derived shorelines (SDSs) from L8 and Sentinel-2 imagery to reveal the shoreline position changes at the beaches of the Ebro Delta, NE Spain. Spatial-temporal models (STMs) of shoreline changes enable a characterisation of how the beaches responded to the storms of 2020. In conjunction with wave data, STMs enable an analysis of the erosive response to storm events, as well as a monitoring of subsequent beach recovery in the short and medium term (<1 year). Results show how Storm Gloria (January 2020, Hs max = 7.62 m) acted as a disruptive event and shifting point in the shoreline trend. As a response to that storm, major erosive processes occurred along the delta that caused an average shoreline retreat of 47 m. A progressive recovery during the spring and summer was mainly associated with periods of low wave energy. Nevertheless, by the end of the year a complete recovery had been achieved for about half of the coast, while the other half showed an average erosion of more than 10 m when compared to the pre-storm situation. Both the erosive and the recovery processes took place unevenly on different sections of the coast, probably dependent on factors such as the orientation of the beach and the pattern of longitudinal sediment transport along the coast. • Automatic shoreline extraction from Sentinel 2 and Landsat 8 imagery using SAET. • Pre and post-storm characterisation of SDS position in time and space using STMs. • Storm Gloria (January 2020) caused an unprecedented and heterogeneous impact on the Ebro Delta. • Slow recovery occurred in the months following the event conditioned by orientation and wave patterns. • About half of the delta showed a significant shoreline retreat by the end of 2020. [ABSTRACT FROM AUTHOR]

Quantitative predictions of marine and aeolian sediment transport in the nearshore–beach–dune system are important for designing Nature-Based Solutions (NBS) in coastal environments. To quantify the impact of the marine-aeolian interactions on shaping NBS, we present a framework coupling three existing process-based models: Delft3D Flexible Mesh, SWAN and AeoLiS. This framework facilitates the continuous exchange of bed levels, water levels and wave properties between numerical models focussing on the aeolian and marine domain. The coupled model is used to simulate the morphodynamic evolution of the Sand Engine mega-nourishment. Results display good agreement with the observed aeolian and marine volumetric developments, showing similar marine-driven erosion from the main peninsula and aeolian-driven infilling of the dune lake. To estimate the magnitude of the interactions between aeolian and marine processes, a comparison between the simulated morphological development by the coupled and stand-alone models was made. This comparison shows that aeolian sediment transport to the foredune, i.e. 214,000 m3 over 5 years, extracts sediment from the marine domain. As a result, the alongshore redistribution of sediment from the main peninsula by marine-driven processes decreased by 70,000 m3, representing 1.7% of the total marine-driven dispersion. From the aeolian perspective, marine-driven deposition and erosion reshape the cross-shore profile, controlling the supply-limited aeolian sediment transport and the magnitude of sediment deposition in the foredunes. In the region with persistent accretion along the Sand Engine's southern flank, a higher than average foredune deposition was predicted due to morphological development of the region where sediment is picked up by aeolian transport. Including these marine processes in the coupled model resulted in an increase of 1.3% in foredune growth in year 1 and up to 6.7% in year 5 along this accretive section. At the northern flank, where the developing lagoon and tidal channel provided increased shelter to the supratidal beach, predicted foredune deposition reduced up to −11.5% over the evaluation period. Our findings show that both aeolian and marine transports impact reshaping the nourished sand, where developments in one domain affect the other. The study findings echo that the interplay between aeolian- and marine-driven morphodynamics could play a relevant role when predicting sandy NBS. • Predicting nearshore–dune morphodynamics is enabled with a new 2D coupling framework. • The model reproduces the Sand Engine's dynamics in both marine and aeolian domains. • Alongshore varying marine sediment transport influences foredune growth patterns. • Marine alongshore transport is slightly reduced by aeolian transport into the dunes. [ABSTRACT FROM AUTHOR]

Sediment motion on rippled bed under wave action plays an important role in coastal landforms, but the application of traditional numerical model is limited by large computational work or bottom sediment condition due to unsteady effects. A single-phase model is proposed to maintain low computation and high accuracy for the vortex structure development and suspended sediment motion on vortex rippled bed in wave-induced oscillatory flow. The 2D bottom sediment condition takes into account the acceleration effect and asymmetric boundary layer development under nonlinear waves as well as the sediment phase-lag. The particle inertia and wake flow around sediment particle are considered in sediment turbulence, and the SST k - ω turbulence model which is suitable for inverse pressure gradient flow on rippled bed is selected to close the Reynolds stress. The single-phase model well predicts the velocity on vortex ripples, periodic concentration at fixed points and vertical distribution of averaged sediment concentration. The results show qualitative formation, development and ejection of vortex, and demonstrate quantitative development of vortex center, radius and strength in symmetric and asymmetric flows. In addition, the model well performs the development of sediment cloud referring to vortex structure. • Efficient numerical model is developed for oscillatory sediment transport on vortex ripple and sheet flow. • Bottom sediment condition performs acceleration effect, asymmetric boundary layer and phase-lag. • Particle inertia and wake flow around particle are considered in sediment turbulence. • SST k - ω turbulence model is applied for inverse pressure gradient flow on vortex ripple. • Results show quantitative development of vortex structure and associated sediment cloud. [ABSTRACT FROM AUTHOR]

This paper presents the use of an Eulerian–Eulerian multi-phase model to examine sediment transport over a gravel beach under regular waves. A previously published full-scale experiment is reproduced numerically; the simulated wave deformation, flow velocity, pore pressure, and morphological evolution are consistent with measurements. The multi-phase model can produce berm formation for smaller waves and beach erosion for larger waves as observation in previous field studies. The sensitivity analysis suggests that the nonlinear component of drag has significant effect on the berm formation through infiltration and exfiltration. The two simulated forces applied on the sediment (drag and buoyancy) are examined. The drag is the main force directly for sediment transport; therefore the sediment transport rate has high correlation with depth-averaged flow velocity. The buoyancy that is caused by plunging flow impacting beach face can push the sediment landward in the bore front, where the flow acceleration has its peak. The flow acceleration is considered in some previous sediment transport models empirically for simulating morphological evolution of gravel beach under waves. The buoyancy links the flow acceleration and sediment transport rate in the bore front. • Sediment transport in a gravel beach is simulated using a multiphase model. • The drag is the main force directly for sediment transport. • The buoyancy links the flow acceleration and sediment transport rate in the bore front. • The nonlinear drag has the effect on berm formation. [ABSTRACT FROM AUTHOR]

The problem of local scour around a submarine pipeline is vital for oil and gas projects. Conventional scour models around pipeline practically include the flow and sediment transport characteristics, but they ignore the interactions between flow and seabed soil in the scouring process. In this study, a new two-dimensional model for simulating the scouring process is established while considering the impact of the wave-induced soil response and the associated seepage. With the present model, the influence of seepage and soil response on the modified Shields number, repose angle and prediction of the local scour depth and profile are examined. The numerical results indicate that the influence of repose angle when modified by seepage in the whole scour process is small. Meanwhile, the wave-induced dynamic soil response promotes the development of the scour profile and eventually increases the maximum scour depth to some extent, regardless of the change in the soil characteristics. However, at smaller KC numbers, the variations in the maximum scour depth with seabed properties become more significant. [Display omitted] • A novel scour model considering soil response is numerically established in OpenFOAM. • The shape of the scour hole is asymmetrical due to the asymmetrical horizontal flow velocity. • The maximal scour depth with the soil response is larger than that without the soil response. • The difference in S / D with and without soil response would decrease with increasing in the KC number. • The soil response should be considered for a smaller shear modulus, lower soil permeability, and lower saturation. [ABSTRACT FROM AUTHOR]

Storm surge, a major disaster in coastal areas, is generated by tropical cyclones. Strong currents and waves during storm surges often cause sediment transport and beach erosion, significantly damaging the coastal ecosystem. This paper presents a flow-wave-sediment-morphodynamic coupled model to predict morphodynamic changes caused by overwash during storm surges. In the model, the flow field is calculated using the generalized shallow water equations coupled with the Simulating Waves Nearshore (SWAN) model, while sediment transport and bed changes are computed using a non-equilibrium total-load sediment transport model. The model solves the governing equations by employing the explicit finite-volume approach based on a rectangular mesh. The Godunov-type central upwind scheme is adopted to calculate the interface fluxes, thereby solving the complex Riemann problem. A series of numerical experiments considering wave-induced longshore current and short-term bed erosion are conducted to validate the established model and demonstrate its ability to simulate waves, flow fields, and bed changes during storm surge. Specifically, we used the model to simulate the morphological evolution of the Santa Rosa barrier island caused by Hurricane Ivan's storm surge, and the results indicate that the model accurately predicts the real-case erosion process caused by storm surge and overwash during the hurricane. The simulated results clearly explain overwash development, including washover fans, foredune erosion, and back-barrier deposition. It demonstrates that the front dune significantly eroded with the Santa Rosa barrier island coastline retreating by a maximum of about 80 m during the storm surge. More than three breaches with a width of up to 135 m were formed on the sandy barrier due to wave overwash, while the topography at the breach changed significantly. • A new flow-wave-sediment-morphodynamic coupled model is developed to predict morphodynamic changes caused by overwash during storm surges. • A series of numerical experiments conducted using the new model demonstrates its ability and it successfully predicts the overwash erosion of the Santa Rosa barrier island during Hurricane Ivan's storm surge. [ABSTRACT FROM AUTHOR]

To mitigate land losses in the Mississippi River Delta, sediment diversions are being employed to enable the flow of river water and sediments into wetlands experiencing degradation. A two-dimensional coupled flow-wave Delft3D model was used in this study to explore the hydrodynamics and sediment transport in Fourleague Bay (FLB), Louisiana, USA, which has been considered an analog site for studying the efficiency of sediment diversion projects. In-situ measurements of sediment accretion and hydrodynamic characteristics from 2015 to 2016 were utilized to calibrate and validate the morphodynamic model. The validated model was then applied to quantify sediment transport in FLB and surrounding marshes between May 2015 and May 2016. The results show that more sediment could be deposited to the surrounding marshes with high river discharges and strong winds. Thus, by strategically aligning the timing of pulses of river water from the diversion with the seasonal intensification of atmospheric forcing, it is possible to sustain and promote the growth of the surrounding wetlands. Moreover, we found that multiple sediment transport processes occurred during the entire study period, including the deposition of riverine sediment into the bay floor, direct deposition of riverine sediment in the surrounding marshes, resuspension of bay floor sediment, and redistribution of resuspended sediment to adjacent marshes and the Gulf of Mexico (GoM). The results indicate that the riverine sediment tended to be directly deposited in the marshes when the river discharge was high. During calm weather conditions and normal river discharge, FLB acted as a reservoir, storing sediment from the upper river, and later acted as a sediment source to the nearby wetlands and the GoM during energetic atmospheric conditions. This suggests that using the bay floor as a reservoir can extend the distance over which wetlands can benefit from the sediment diversions, as the supply of sediment to the wetlands becomes a multi-step process. Thus, it is important to retain sediments from river diversions in shallow bays and allow storms to redistribute them to adjacent wetlands. • A morphodynamic model was developed to understand the fine sediment dispersal process from a river to coastal wetlands. • The model was calibrated and validated using unique measurements of hydrodynamics and sediment accretion rates. • More sediment could be deposited to the surrounding marshes with high river discharges and strong winds. • Sediment deposited on the surrounding marshes was primarily from the river and bay floor. • Riverine sediment tends to be directly deposited onto the marshes during high discharge stages. [ABSTRACT FROM AUTHOR]

Dune erosion during storm surges can lead to excessive damage to the dune system with devastating floods as a potential consequence. A risk assessment of areas protected by dunes can be facilitated by an understanding and description of the physical processes that take place. Field measurements, knowledge of underlying processes and numerical modelling have developed with time, which enabled a more comprehensive description and new predictive techniques. This review concerns dune erosion in the collision regime, and summarises relevant observations, describes underlying processes and explains existing models predicting dune erosion. Observations of dune erosion consist of field observations, laboratory experiments and manipulative field campaigns. The underlying physical processes that contribute to dune erosion are divided into processes that contribute to sediment transport due to hydrodynamic forcing, which occurs in the surf and swash zone, and sediment transport due to avalanching, which occurs in the swash zone, on the dune face and on the dune crest. The existing dune erosion models that are discussed here contain (empirical) equilibrium profile models and process-based models, which can both be a valuable tool for the risk assessment of storm surges. However, model uncertainties still remain, as specific processes are not yet fully understood and described. Examples are the influences of wave obliquity, sediment grain size, and vegetation on the dune face. By improving our knowledge through research and reducing these uncertainties, we can further improve our predictive models. This could eventually lead to more accurate predictions, more complete risk assessments, and sandy coastlines which are more resilient to excessive dune erosion and possible floods. • Reviews dune erosion during storm surges in the collision regime. • Summarises relevant field and laboratory observations • Summarises underlying hydrodynamic and morphodynamic processes • Summarises existing dune erosion models. [ABSTRACT FROM AUTHOR]

This paper presents results from numerical simulations of the propagation and run-up of full scale tsunamis, using a Reynolds-Averaged Navier-Stokes model, with emphasis on the resulting boundary layers and bed shear stresses. Spatial distributions of the Shields and Rouse parameters during run-up and draw-down show that for the tsunamis considered, with a sediment grain size corresponding to medium sand, considerable sediment transport (Shields parameter O(10) O(100)) can be expected during run-up and that the sediment transport can be expected to be dominated by suspended load. The results likewise show that the expected sediment transport during draw-down will similarly be considerable and dominated by suspension. The tsunami-induced boundary layers are monitored in time, and the observed boundary layer thickness ranges from spanning only a small fraction of the water depth to spanning the entire depth. The velocity profiles beneath the tsunamis are shown to have good correspondence with a logarithmic profile within the boundary layer. Similarly, the bed shear stresses beneath the tsunamis are investigated and a new and simple engineering model is developed for predicting the temporal variation of the bed shear stress based only on a free-stream velocity signal. The new engineering model is shown give better predictions for the bed shear stress than a standard Manning approach, and is likewise shown (in the Appendix) to produce reasonable predictions for shorter (wind scale) waves. It is finally also shown how the temporal evolution of the boundary layer thickness can be predicted based on the free-stream velocity signal alone. The results presented here are Part 2 of a larger study, where Part 1 presents the model validation and detailed descriptions of the run-up process. • Presented full-scale highly resolved CFD simulations of propagating and inundating tsunamis. • Showed spatio-temporal distribution of Shields parameters and Rouse numbers for inundating tsunamis. • Demonstrated that tsunami-induced boundary layers may span a small fraction of the depth or may span the entire water depth. • Presented a new engineering model for predicting the time varying bed shear stress beneath tsunamis. • Demonstrated that the new engineering model works also for shorter waves. [ABSTRACT FROM AUTHOR]

Depth of Closure (DoC) calculation has been analysed for engineers and researchers during decades due to its relevant importance on marine works, such as beach nourishments or littoral defense projects. Several methods can be found in the literature related to the evaluation of DoC. However, they just focus on the bottom elevation change, leaving aside the cross-shore sediment transport issue. This work deals with all requests of the DoC definition and presents a new method based on the comparison of repetitive beach profiles. The main contribution of this research is the methodology presented to quantify the cross-shore sediment transport by the calculation of the Net Cross-shore Sediment Transport Parameter (NCSTP C). Its application to a complex area of the East Spanish coast concludes that NCSTP C values less than or equal to 10% can be considered as no significant. The differences between several criteria based on the change of the bottom elevation are also studied in order to evaluate if they are relevant for engineering works. The new methodology developed is a great advance in the knowledge of this imaginary boundary (DoC), since it allows to discriminate between different possible DoC values when applying any method based on profile surveys comparison. • New method to obtain the Depth of closure from profile surveys. • Quantification of the net cross-shore sediment transport. • 762 periods of time analysed over 22 years (1992–2014). • Five profile survey criteria have been compared. • The new method permits the selection of the DoC among several possible values. [ABSTRACT FROM AUTHOR]

Scour is recognised as one of the major causes of seawall failure. This paper presents numerical modelling of seawall scour due to wave impact on a vertical seawall. The modelling of waves hydrodynamics is based on the solution of the Reynolds-Averaged Navier-Stokes equations with the k- ω model. Fifth-order Stokes waves are generated in a numerical wave tank (NWT) using the relaxation method. The free surface under the breaking wave is captured with the level set method. The wave field is coupled with a sediment transport algorithm to simulate seawall scour. The model is validated for an accurate wave field and the seawall scour due to the wave impact. The accuracy of the simulations is assured by comparing the numerical results with the theory and the experimental observations. The numerical results give insight into the seawall scour process through different stages. The results show that the seawall scour is governed by the breaking wave impact on the seawall. The development of the standing wave due to the reflected wave energy from the seawall leads to further sediment transport seawards. Finally, the model is used to simulate seawall scour for different scenarios of seawall locations, incident wave height and seabed slope. The study examines the wave impact on the seawall toe and resulting scour. The maximum seawall scour is observed when the seawall is located at the intersection point of the still water depth and the bed slope. A displacement of the seawall from the intersection point leads to a decrease in the wave impact. The surf similarity parameter ξ 0 is recognised as an important factor affecting the seawall scour. • Numerical modeling of the breaking wave-induced seawall scour. • Validation of breaking wave hydrodynamics. • Validation of breaking wave-induced seawall scour. • Seawall scour for different seawall locations, wave steepness and slope. • Effect of surf similarity parameter on seawall scour. [ABSTRACT FROM AUTHOR]

Local scour induced by tidal flow is a major hazard for sea-crossing bridge foundations in coastal areas. The irregular tidal flow and silt-muddy seabed environment generates a very complicated flow turbulence and sediment transport. This paper, for the first time, presents the field measured scour holes around the Hangzhou Bay Sea-Crossing Bridge (HBSB) under irregular tidal flow dominated water regions. The high-resolution seabed topography around 19 bridge foundations in the south waterway of HBSB is measured from 2015 to 2020 by using a multi-beam sonar system. The field measured data are analyzed to investigate the local scour characteristics under irregular tidal flows. Results indicate that under irregular tidal flows with stronger flood tide, the local scour hole around every bridge foundation is slightly asymmetric and the scour depth downstream is larger than that upstream. The slope angles at upstream, downstream and lateral sides of local scour hole are about 13°～18°, which are much smaller than the Angle of Repose (AoR = 35°) due to bidirectional scouring generated by tidal flows. During 2015～2020, the maximum scour depths around all bridge foundations range between 1.2 D e ～2.5 D e (where D e being the effective diameter of bridge foundation). The maximum scour depths vary frequently with a fluctuation within 0.6 De. Comparison between the field measured and predicted scour depth using three available scour depth predication equations show that all prediction equations overestimate the scour depth. As such, considering the discount effect of the varied tidal flow compared with steady current, an improved maximum scour depth prediction equation is proposed to estimate the scour depth under tidal flow on a silty seabed. Results show that the predicted scour depth using the proposed equation has a relative error of ±30%. • High quality field scour data around bridge foundations under irregular tidal flows are presented. • Slope angle of scour hole under tidal flows is smaller than the Angle of Repose. • An improved prediction equation for scour depth around bridge foundations under clear-water and live-bed tidal flow condition on silty seabed is proposed. [ABSTRACT FROM AUTHOR]

In nearshore areas, waves change direction due to refraction in shallowing water. Wave angles spatially evolve across this zone, however local variability is challenging to capture with in-situ sensors. In this study, X-band radar (XBR) remote sensing, a phase-resolving numerical model (SWASH), and in-situ observations with acoustic wave and current sensors are combined to assess nearshore wave angles during three storm events. Two new methods of estimating the spatial distribution of wave angles are developed to enable direct comparison of the model results and the remote sensing observations. The results indicate that wave angle estimates from SWASH and XBR are similar and accurate within the directional resolution of in-situ sensors. Differences between the two estimates can occur in areas of high bathymetric variability or regions of low radar backscatter intensity. When waves are breaking over shallow bathymetric features, such as a sand bar, both SWASH and XBR capture a sharp increase in directional spreading. Different types of offshore wave boundary conditions were also investigated, and it was found that when forced with observed directional spectra, SWASH limits the directional spreading compared to when it is forced with bulk wave statistics and JONSWAP spectra. The high spatial variability of local wave angles observed and simulated in this study suggests that wave directions should be more carefully investigated, and may have implications for nearshore sediment transport, morphological change, wave impact forces and wave power estimates. • A wave model and radar observations are used to investigate wave directions. • New methods calculate spatially-continuous estimates of surface wave angles. • Results agree with in-situ data and reveal high variability in wave directions. [ABSTRACT FROM AUTHOR]

Empirical formulations for the engineering prediction of equilibrium scour depths, as well as time scales (for both scour and backfilling), are re-visited for monopiles. Following prior work, it is argued on physical grounds that dimensionless time scales ought to scale proportionally to the Shields parameter raised to the − 3 / 2 power, based on scaling common to numerous sediment transport rate formulae. This is in contrast to stronger power dependence found in many existing formulations based on laboratory experiments, which may become unreliable when extrapolated to (storm/strong) field scales. In light of this, existing data sets for scour and backfilling time scales are re-analyzed. Novel parameterizations are developed for their prediction which are in line with the argument above, while maintaining similar coherency with existing data as in previous parameterizations. Re-analysis of existing data sets for equilibrium scour depths is likewise performed. An existing framework valid for slender monopiles is extended to incorporate the so-called large monopile regime (non-negligible diffraction), especially important e.g. for modern wind turbine installations. • Existing parameterizations of scour and backfilling time scales are reparametrized. • Suggested time scales scale according to the Shields parameter to the −3/2 power. • Existing framework for predicting equilibrium scour depth is extended to include large monopiles. [ABSTRACT FROM AUTHOR]

The world's shallow continental shelves are currently experiencing a rapid pace of development from the growth of offshore renewable energy. The emplacement of infrastructure on the seabed can change the morphology of the bed, the nature of the flow above it the transport of sediment, and so complicate the assessment of seabed stability for planning and designing offshore renewable infrastructure. To ascertain how much of an impact these natural processes have on the stability of cables, we present the first field observations made directly over a section of subsea cable, from two deployments in the Eastern Irish Sea at a location of current and planned offshore windfarms. Profiles of flow, turbulence and suspended sediment concentration were measured over a section of typical high voltage electricity cable. Upon deployment our observations showed that sediment was deposited around the cable and self-burial occurred. The rate of deposition varied between surveys dependent on forcing and local bed conditions. Turbulence generated from the cable itself reduced as the embedment depth increased, but the relationship between bed shear stress and suspended sediment concentration was not consistent between surveys. We discuss several processes potentially responsible for the prevalence of deposition around the cable, and the difference in seabed mobility between the surveys. • Field surveys quantified mean flow, turbulence and suspended sediments around a section of seafloor electricity cable. • Deposition around the cable occurred at all initial embedment depths, but rates of deposition varied between surveys. • Seabed mobility varied between surveys beyond any variation due to grain size and affected the rate of cable burial. [ABSTRACT FROM AUTHOR]

The risks posed by local scour around offshore monopile foundations are substantial and threaten the structural safety of these installations. It is therefore essential to study the mechanisms of local scour to mitigate these risks. Numerical simulation is an effective tool to help us understand the mechanisms of local scour. However, the accurate modeling of the local scour process remains to be challenging due to the complexity of sediment transportation, in which the continuity threatment of sediment phase is typically used, the discrete behavior of the sediment is hard to be described. Therefore, in this paper, in order to conduct the deepin investigation into the mechanism of the local scour around monopile, a three-dimensional CFD-DEM model was established by coupling the computational fluid dynamic method (CFD) and discrete element method (DEM). To increase the efficiency and accuracy of the CFD-DEM model, the coarse grain method (CGM) was used to decrease the number of required DEM particles in the simulation, and the angle of repose was used to calibrate the contact parameters between DEM particles. During the validation process of the numerical model, it was found that the development of the scour depth and the morphology of scour pit obtained by the CFD-DEM model showed good agreement with the previously published results. The CFD-DEM simulation revealed that seepage-induced vertical drag force plays a crucial role in the initial stage of local scour. Two sediment transportation processes, "Pile toe erosion-Slope avalanche" and "Push-Accumulation-Wash", were observed and the relationship between bedload sediment flux and the Shields number was quantified. In the end of this paper, the critical evaluation of the CFD-DEM method was conducted. • The first time that the CFD-DEM method is used to research the sediment motion during the local scour around monopile. • The coarse grain method is used to improve the efficiency of CFD-DEM coupling method. • The seepage-induced vertical drag force was proved to be crucial in the early stage of local scour around monopile. • The "Pile toe erosion-Slope avalanche" and "Push-Accumulation-Wash" process of sediment transportation were observed. • A method is proposed to convert the parameters of discrete phase into the continuous phase parameters. [ABSTRACT FROM AUTHOR]

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

Training walls to improve navigability at river mouths alter natural sediment transport regimes, occasioning morphological changes in the nearby littoral zone. In addition, harbor channels and shorelines are susceptible to on-going changes in global circulation conditions and large-scale engineering interventions. Here we present a process-based modeling approach to determine the relationship between spatial patterns of erosion and deposition, seasonal river discharge, and the geometry of coastal defenses. The study uses the Magdalena River delta as an example, where the redistribution of freshwater and sediments after the construction of the "Tajamar" training wall heralded significant morphological changes in the area. The numerical experiments are used to describe how the erosion and deposition patterns within the estuary and delta of the Magdalena River are linked to the seasonal cycle of the Magdalena River and to the geometry of the hard structures. We demonstrate that the construction of the Tajamar and complementary hard structures would have decreased depositional fluxes in the littoral zone, leading to the observed shoreline retreat and disappearance of an extensive coastal lagoon system. It is shown that an aperture in the training walls may help restore the wetlands without compromising navigability. In addition, projected increases in mean sea level are shown to decrease velocities within the lower estuary, potentially causing increased sedimentation within the channel and more complex conditions for the management of safe navigation over the Magdalena River estuary. • Seawall construction on the Magdalena River, Colombia, resulted in the redistribution of freshwater and sediments. • Numerical experiments provide evidence that shoreline retreat was caused by the altered sedimentation regime. • A diversion system in the training walls may help restore sediment supply to the shorelines without risking navigability. • Sea level rise may increase rates of sedimentation within the estuary. [ABSTRACT FROM AUTHOR]

Abstract Understanding the pipeline scour processes under the combined action of waves and current is essential for adequate scour protection measures. This paper presents the numerical modelling of pipeline scour under the co-directional combined action of waves and current including the prediction of the unsteady free surface. The numerical modelling is performed with the open-source CFD model REEF3D. The model solves the Reynolds-Averaged Navier-Stokes (RANS) equations together with the k − ω turbulence model and a sediment transport algorithm. In order to ensure the accuracy of the flow field, the model is thoroughly validated for the hydrodynamics of co-directional combined waves and current. Results for the velocity profiles show a good agreement with the experimental observations. The model is then applied to simulate the different case scenarios of the scour under waves alone, current alone, and combined waves and current. The numerical results for scour below the pipeline show good agreement with the experimental data which confirms the applicability of the model to study pipeline scour under the combined action of waves and current. A series of simulations are run for different values of the non-dimensional parameter for the combined waves and current U c m varying between 0 and 1.0 for the given KC number. The results demonstrate the correlation between values of U c m and (1) the flow field below the pipeline, (2) the scour depth, and (3) the temporal variation of the scouring process. The findings highlight the variation of the maximum scour depth below the pipeline with U c m . Highlights • Numerical modeling of the pipeline scour under the combined action of the waves and current. • The assurance of the wave field quality in NWT using AWA method through grid and time convergence study. • Hydrodynamic validation of combined action of waves and current before sediment transport starts. • Morphological validation for pipeline scour under waves and steady current conditions. • Pipeline scour under different flow conditions of the combined waves and current with free surface featuring. • Variation of the maximum scour below the pipeline with Ucm for given KC number. [ABSTRACT FROM AUTHOR]

Abstract The seabed is sloped in shallow coastal regions, where strong nonlinear shoaling waves can generate sheet-flow sediment transport. Both wave nonlinearity and bottom slope can lead to a wave-averaged (net) sediment transport rate, but the bottom slope effect is often overlooked in existing sediment transport models. A full-scale experimental study of sheet-flow sediment transport under the oscillatory flows with nonlinear features (skewness or asymmetry) was conducted in an inclinable oscillatory water tunnel, so the bottom-slope-induced net transport rate in the context of nonlinear waves was experimentally investigated. The tests cover a variety of nonlinear wave shape, bottom slope (0 to 2. 5 ∘ ) and two sediment diameters (0.24 and 0.51 mm). Since the downslope direction is the "offshore" direction in our tests, it is found that bottom slope reduces the net "onshore" transport rate due to wave nonlinearity. The slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping-bed experiment and the measured net transport rate from the corresponding horizontal-bed experiment. The results suggest that slope-induced net transport rate is insensitive to wave nonlinearity, and its variation with bottom slope can be considered linear in the context of predicting net transport rate. Comparisons between this study and a preceding sinusoidal-flow study of Yuan et al. (2017b) suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate. Subsequently, an empirical model for predicting net sheet-flow transport rate due to bottom slope is established based on dimensional analysis. This model can be simply added to existing sheet-flow sediment transport models to improve their applicability in coastal regions. Highlights • Presented experiments on the sheet-flow sediment transport under skewed and asymmetric oscillatory flows above a sloping bed. • Test results show that the slope-induced net transport rate is insensitive to nonlinear wave shape. • Proposed an empirical model for slope's contribution in net sheet-flow sediment transport rate. [ABSTRACT FROM AUTHOR]

Abstract The shape of vortex ripples generated by wave boundary layers determines the local hydrodynamics and sediment motions, and therefore is critical for understanding coastal sediment transport in the rippled-bed regime. In this study, coarse-sand vortex ripples were generated by periodic oscillatory flows mimicking full-scale field conditions in an oscillatory water tunnel. A laser-based bottom profiler was applied for measuring the movable bed's profile. The observed ripple development from a flat bed is in agreement with many previous studies. Small ripple marks appeared within a couple of flow periods, and gradually became 3-dimensional (3-D) transient ripples, which eventually evolved to equilibrium 2-dimensional (2-D) ripples after O (100–1000) flow periods. The shape of equilibrium 2-D ripple was studied based on a representative ripple profile obtained by ensemble averaging a periodic ripple train. The shape of ripples formed under sinusoidal flows become increasingly sinusoidal as the flow becomes stronger (larger Shields parameter) or wave period becomes longer, which is possibly because the influence of coherent vortex becomes more significant. Wave nonlinearities have different effects on the ripple shape. Generally speaking, wave skewness makes ripples onshore-leaning while wave asymmetry makes ripples' crests sharper and troughs flatter. Our measurements also revealed some detailed geometric characteristics, i.e., the roundness of ripple crests and the maximum local slope along ripple flanks. Highlights • A full-scale experimental study on the geometry of coarse-sand vortex ripple is conducted in an oscillatory water tunnel. • Ripple development from a flat bed is reported, and the observation is in agreement with previous studies. • Full-scale coarse-sand vortex ripples can be well described by a sinusoidal profile. • Skewed flows make ripples become asymmetric while asymmetric flows make ripples become skewed. [ABSTRACT FROM AUTHOR]

Abstract Mega nourishments are a novel approach to stimulating coastal safety and resilience. Mega nourishments are intended to spread along the coast on a decadal time scale by natural sediment transport processes with a minimum of intrusion into the natural coastal system. The supratidal morphodynamic behaviour of mega nourishments is not well understood due to complexities introduced by limitations in sediment availability to aeolian sediment transport. Consequently, the effectiveness of mega nourishments to stimulate coastal safety and to influence coastal landscape and habitat development remains unknown. In this paper we present a detailed 4-year hindcast of the morphological development of the Sand Motor mega nourishment in The Netherlands. We use the aeolian sediment transport and availability model AeoLiS that focuses specifically on the simulation of spatiotemporal variations in sediment availability. The model includes the recurrence relation between sediment availability and aeolian sediment transport through self-grading and beach armoring. We show that the model is able to reproduce multi-annual aeolian sediment transport rates in the Sand Motor domain in the four years after its construction. The RMSE is 3 ⋅ 10 4 m 3 (7% of the total sediment accumulation) and R 2 is 0.93 when comparing timeseries of total sediment accumulation in the dunes, dune lake and lagoon. The combination of spatial and temporal variations in aeolian sediment availability, due to the combined influence of soil moisture, sediment sorting and beach armoring, is essential for an accurate estimate of the total sedimentation volume. The simulated feedback between aeolian sediment availability and transport is required for accurately describing compartmentalization of the beach and locating the aeolian sediment source areas in the Sand Motor domain. [ABSTRACT FROM AUTHOR]

Abstract Monopile foundations are the most common foundation type of offshore wind turbines. In the ocean environment, in addition to threats related to scour of the foundation, offshore monopiles of wind turbines also are subjected to lateral vibrations induced by waves and winds. Over the past decades, the scour process induced by flow and pile-soil interactions has been independently investigated by researchers from the fields of hydraulic/coastal engineering and soil mechanics, respectively. Very few studies can be found in the published literature on lateral vibration effects on scour at monopile foundations. The objective of this experimental study is to improve understanding of the mechanics of current-induced scour at monopiles subjected to lateral vibrations with a non-cohesive bed. When compared to a non-vibrating monopile, the results show that scouring around its vibrating counterpart is comparatively faster during the first few hours of the test. This is primarily due to soil densification and the subsidence process induced by monopile-seabed interactions. Increasing both the frequency and amplitude of the monopile vibration would decrease the equilibrium scour depths and scour hole slopes. The reduction of equilibrium scour depth likely is due to the effect of sediment ratcheting motions on the surface of the scour hole. An empirical dimensionless equation is proposed to estimate equilibrium scour depth at monopiles subjected to lateral vibrations. Highlights • New experimental results of temporal evolution of scour depth and final scour geometry at vibrating monopiles. • New conceptual model of scour mechanism around monopile foundations subjected to lateral vibrations. • New relationship between scour reduction and vibration properties of a monopile. [ABSTRACT FROM AUTHOR]

Abstract Submarine pipelines are widely used for the conveyance of hydrocarbons, oil or other fluids in the offshore industry. Although much progress has been made on pipeline-seabed interactions in the past decades, an experimental study solely devoted to investigating interactions between the flow fields, scour and forced vibrating pipelines in a quiescent water environment still has not been conducted. In practice, this condition, to a certain extent, resembles a Steel Catenary Riser (SCR) in deep offshore environments in the absence of currents and waves. The present study aims to obtain an improved understanding of the mechanics of scour around a forced vibrating pipeline in quiescent water. Tests with the same initial gap ratio G 0 / D = 1 and two different induced frequencies were conducted. A circular cylinder with a diameter of 3.5 cm was used as the pipeline model. With the help of a high-speed camera, the details of flows, pipeline motion and scour process are vividly presented in this paper. The magnitudes of flow velocity, vorticities, and turbulence fluctuations are found to be strongly dependent on the frequency of the vibrating pipeline. The scour mechanism of the two tests is different because of the complex interactions between the flow fields and pipeline motion. For the low-frequency case, the flow field associated with the falling stage of the vibrating pipeline controls the formation of the scour trench. On the other hand, for the high-frequency test, the scour process is controlled by the flow field during the rising stage of the vibrating pipeline. Highlights • An extensive experimental study of scour around a forced vibrating pipeline in quiescent water using PIV techniques. • Synchronous quantification of pipeline dynamic responses, turbulent flow fields and near bed shear stresses. • The scour process and geometry are strongly linked to the dynamic responses (amplitude and frequency) of the pipeline. • The fluid-pipeline-sediment interaction relating to a forced vibrating pipeline with a non-cohesive seabed. [ABSTRACT FROM AUTHOR]

Abstract This paper presents 25 new laboratory experiments involving the wave-induced backfilling of wave-induced scour holes beneath submarine pipelines (so-called wave-to-wave backfilling scenarios). The experiments complement the previous 8 wave-to-wave backfilling experiments of Fredsøe et al., as well as recent current-to-wave backfilling experiments of Bayraktar et al. It is found that the wave-induced backfilling time scale is generally an order of magnitude larger than for scour, and is relatively insensitive to the initial (current- or wave-induced) pre-backfilling scour profile. Based on this, the data sets involving wave-induced backfilling are collectively analyzed, resulting in a new generalized expression for estimating the wave-induced backfilling time scale beneath pipelines in the live-bed regime. This expression accounts for primary dependence on the Shields parameter, as well as secondary dependence based on the difference between the (estimated or known) initial and expected final equilibrium scour depth due to the backfilling wave condition. Highlights • We study wave-induced backfilling of wave-induced scour beneath submarine pipelines. • 25 new experiments have been conducted at laboratory scale. • New and existing data are analyzed regarding the time scale of backfilling, T b. • T b is relatively insensitive to the initial (current- or wave-induced) scour hole. • A new generalized formula is proposed for the wave-induced backfilling time scale. [ABSTRACT FROM AUTHOR]

Abstract In order to investigate the effect of wave processes on the geotechnical characteristics of topmost sediments, a Portable Free Fall Penetrometer (PFFP) was deployed in the energetic nearshore zone of the U.S. Army Corps of Engineers' Field Research Facility (FRF) in Duck, NC. A total of 335 deployments were conducted from the FRF's 560 m long pier over six non-consecutive days in September and October of 2016. During the surveying period, significant wave heights varied between H s = 0.8–2.4 m measured at a water depth of 17.4 m. The results showed that the sediment strength is affected by the wave climate. The derived average maximum quasi-static bearing capacities (qsbc), which reflects sediment strength, during low energy wave periods (September 22–24 and 26; H s = 0.8–1.3 m) ranged between 67 and 73 kPa with maximum qsbc values of 120–136 kPa, while the average values were 47–59 kPa with a maximum qsbc of 82–98 kPa during the high energy wave periods (October 5–6; H s = 2.4 m). Results also showed that the sediment strength decreased in the shallow water depth regions in spite of the increase in the particle size distribution, due to the increasing wave impact on the seabed within the shallow water region. The area where seabed strength variations were affected by the wave action became wider in the cross-shore direction as the wave height increased. Results further showed that the strength of the topmost sediment layers varied spatially along the profile and temporarily with variations in significant wave height. This likely reflects the impact of waves on sediment strength. Highlights • Sediments strength variations were investigated in an energetic nearshore zone. • Special attention was given to effects of wave breaking and storm conditions. • The lowest sediment strength along the profile was found in the breaking zone. • Mobilization processes on the first day of storm resulted in low measured strength. • Significant erosion resulted in a higher strength on the second day of the storm. [ABSTRACT FROM AUTHOR]

Abstract This paper examines the numerical prediction of the sediment transport and bed evolution for a single swash event on a coarse sediment beach. In these conditions bed load is the dominant mode of sediment transport. Laboratory experiments of a single bore-driven swash event are simulated numerically using a fully-coupled model that solves the system of Non Linear Shallow Water Equations and the Exner sediment conservation formula. The analysis focuses on two aspects: the optimal choice of parameters for the Meyer-Peter and Müller sediment transport formula and the model used for computing the shear stress. The methods tested for the bed shear stress are the momentum integral method and the Chezy formulation in which the friction factor is computed using two different formulae. Infiltration into the beach and its effects on the shear stress and sediment transport are also modelled. Results show that the basic Meyer-Peter and Müller sediment transport formula provides good results in the run-up. On the other hand, the sediment transport in the early stage of the backwash is overestimated. A reduction of the sediment mobility constant in the formula in the backwash marginally improves the results. However, the causes of the overestimation of the sediment transport at the early stage of the backwash is the overestimation of the shear stress, while at later stages there are several contributions that are identified, i.e. modelling of the sediment transport and infiltration. It is also suggested that the Meyer-Peter and Müller sediment transport formula might not capture the complexity of the processes involved during the backwash. The comparison of the methods for the estimate of the bed shear stress show that comparable results can be obtained using the momentum integral method and the Chezy formulation with time and space varying friction factor. The resulting bed evolution is also described. In the predicted final profile, deposition is found in the upper part of the beach and erosion in the lower part. A bed step is formed just below the position of the initial shoreline. This feature is determined by the onset of a hydraulic jump during the backwash. Highlights • A single swash event is simulated numerically to study the sensitivity to the Meyer-Peter-Müller formula parameters and bed shear stress model. • Comparison with laboratory experiments suggests that sediment transport in the uprush is well modelled, unlike in the backwash phase. • An adjustment of the Meyer-Peter-Müller formula is suggested. [ABSTRACT FROM AUTHOR]

Abstract In the present study a new multi-fractional, depth-averaged sediment transport module was developed and embedded into a morphodynamic model for a sand-silt mixed shallow water environment. Subsequently, the model was applied to the case of the Jiangsu coast, which features a silt enriched sedimentary environment bordered by two large-scale geomorphological units: the Old Yellow River Delta (OYRD) in the north and the Radial Sand Ridge Field (RSRF) in the south. Based on this case, the predictive abilities of the present model are assessed on both the short-term and the long-term. Comparisons with measurements over two successive tidal cycles indicate that the present model produces very good results on short-time scales. The model performance is extended and further validated by comparing the overall annual Suspended Sediment Concentration (SSC) pattern, the annual morphological changes, the annual sediment budget and the evolution trend of the bed composition. Also, these long-term results agree well with existing observations over the past several decades. Hence, an essential feature of the present modelling approach is the ability to simulate sediment transport and morphological changes over a relatively long time span (i.e., time scale of years) in a sand-silt mixed sedimentary environment, based on its validated short-term performance. Highlights • A multi-fractional sediment model is developed for a silt-enriched environment. • The model is validated for its ability on successively simulate sediment transport over period from days to years. • The simulated long-term sediment dynamics is consistent with observation-based estimates. [ABSTRACT FROM AUTHOR]

Abstract Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (T g = 15.0 s and 27.7 s). For the shortest wave group, due to bore focusing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of wave–swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave–swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave–backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra–swash cross–shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration is influenced by horizontal advection, leading to large sheet-flow layer thickness. However, for the longer wave groups, the local vertical exchange of sediment in the sheet–flow layer is dominant, with the presence of a pick–up and mirroring upper layer similar to oscillatory sheet–flow measurements. These results reaffirm the important effects of the wave group structure and the wave–swash interactions on the swash zone sediment dynamics and beach face evolution. Highlights • The swash zone of short groups oscillates at the group period while for long groups it oscillates with individual waves. • Short groups induced sediment transport is dominated by horizontal advection with important wave-swash interactions. • The sheet flow layer (SFL) of short groups is thickest and it develops at the group period time scale. • The long groups SFL is thinnest and dominated by vertical sediment exchange occurring at the time scale of individual waves. [ABSTRACT FROM AUTHOR]

Abstract The purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinders – such as pipelines, cables or idealised chains – close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (K C number). For K C ≲ 4 sediment motion results in bedload transport that is symmetric about the cylinder centreline. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4 ≲ K C ≲ 9 sediment motion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9 ≲ K C ≲ 12 the sediment motion is characterised by localised transport events and asymmetric bedload transport driven by overall vortex dynamics. In terms of trench size, the maximum (equilibrium) depth is found to increase with K C and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with K C number and ψ , with an additional dependency on the cylinder β number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the K C number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue. Highlight • Experiments investigate sediment transport and trench formation beneath a cylinder oscillating normal to a sandy bed. • Transport mechanisms and trench geometry linked to pumping of fluid beneath the cylinder and to vortex dynamics. • Control volume arguments combined with an oscillatory boundary layer assumption predict initiation of trenching for small oscillation amplitudes close to the bed. • Rate of trench development and maximum trench depth increase with oscillation amplitude and maximum cylinder velocity. [ABSTRACT FROM AUTHOR]

Abstract The large-scale littoral sediment drift along barrier island coasts involves alongshore sand transport at the islands’ surf zone and across tidal inlets. Mechanisms of sediment bypassing at non-migrating, mixed-energy tidal inlets are discussed based on process-based numerical modeling of sediment dynamics at an exemplary barrier island system in the southern North Sea. Residual transport pathways of distinct sediment grain-size fractions and morphological changes reveal the complexity of processes at tidal inlets driven by the non-linear interaction of competing tidal and wave forces. Sediment bypassing from the updrift to the downdrift shoreline of a tidal inlet is evaluated and the ratio of bypassed to recirculated sand volumes is estimated. Common inlet bypassing schemes are discussed and verified. It is shown that the different bypassing mechanisms can happen simultaneously and are related to the transport behavior of particular sediment grain-size fractions: Very fine to fine sand bypasses the inlet along the entire ebb-tidal delta periphery by wave-induced transport processes. Fine to medium sand is transported by the ebb-directed tidal currents in the inlet throat to the ebb-tidal delta where wave-driven transport processes and bar welding bypasses the sediments to the downdrift shore. For fine to medium and medium sands, a sediment recirculation cell at the downdrift ebb-tidal delta and reversal to the main inlet throat is identified. Highlights • Contradictory schemes of sediment bypassing at tidal inlets are explained by modelled residual transport patterns. • Net pathways of tidal inlet sediment bypassing are strongly dependent on sediment grain-size. • Sediment recirculated at the tidal inlet surmounts the bypassed sand volume with increasing grain-size. [ABSTRACT FROM AUTHOR]

Abstract Laboratory experiments have been carried out to investigate the erosion behaviour of a number of marine sediments which were reconstituted and sieved in various ways to ensure a large variation in soil properties such as (i) particle size distribution (including median grain size and fines content), (ii) bulk density, and (iii) hydraulic permeability. Based on the experimental results, it was found that for finer marine sediments the erosion rate at a given shear stress was sensitive to changes in these soil properties. No unique relationship was found between changes in the erosion rate and changes in the bulk density or fines content. More specifically, we find that the erosion rate at a given shear stress reduces with increases in density and fines content, but by an amount that is different for different sediments. In contrast, we show that there appears to be a unique relationship between permeability and erosion rate, such that permeability may be used to predict the erosion rate for the marine sediments at any density. By reinterpreting existing experimental results in the literature, we find that this same relationship between permeability and erosion rate is apparent for quartz sediments. We propose an empirical relationship, which fits well the erosion rate behaviour of finer sediments close to the threshold shear stress. Highlights • The effect of soil properties on the erosion behaviour of marine sediments. • Unique relationship between permeability and erosion rate discovered. • Permeability may be used to predict the erosion rate for fine-grained sediments at any bulk density. • Novel empirical relationship proposed to predict the erosion rate behaviour of finer sediments close to threshold conditions. • Reinterpretation of experimental data from literature shows a similar relationship between permeability and erosion rate. [ABSTRACT FROM AUTHOR]

Abstract Experiments and field observations have revealed that when silt and very fine sand are subject to oscillatory wave motion, a high shear flow layer and a high concentration layer (HCL) exist near the bottom. The behavior of the HCL is still under researched. Firstly, an intra-wave process based 1DV model was established for fine sediment transport under the combined action of waves and currents. Some key processes that were included in the model are represented through approaches for different bed forms (rippled bed and 'flat bed'), hindered settling, stratification, reference concentration and critical shear stress. A number of experimental datasets were collected to verify the model, which shows that the model is able to properly simulate the flow and sediment dynamics. Secondly, sensitivity analyses were carried out on some factors which would impact the suspended sediment concentration (SSC) profile of the HCL by the 1DV model, such as bed forms, flow dynamics, stratification effects, mobile bed effects and hindered settling. Results show that bed forms play a significant role in the HCL and determination of the shape of the concentration profile. When a current is imposed, the SSC profiles become smoother; however, sediment concentration in the lower HCL is still dominated by the wave motions. For finer sediment, the stratification effects and the mobile bed effects strongly impact the HCL. In conclusion, this paper provides a tool for the study of the HCL and an evaluation of several impact factors on the HCL. Highlights • An intra-wave process based 1DV model was established for silty sediment movement under the combined action of waves and currents. • Important factors impacting the high concentration layer (HCL) are bed forms, flow dynamics, stratification and mobile bed effects. • Bed forms play a significant role in the HCL and determination of the shape of the concentration profile. • For fine sediment, the stratification effects and the mobile bed effects strongly impact the HCL. [ABSTRACT FROM AUTHOR]

Despite offshore wind farms being mostly situated in tidally dominated waters, only limited research on the effects of tidal flow on the scour process around offshore foundation structures has been carried out so far. To further improve the prediction of scour around those structures, systematic laboratory tests were conducted in a closed-circuit flume on the processes and time development of scour around a monopile in tidal flow. The tidal currents were adapted by bidirectionally reversing currents with continuously changing flow velocity. Therewith, multidirectional flow aspects were simplified into flow components in tidal main direction. Tidal cycles and flow intensities were scaled with regard to field measurements at the FINO 1 platform in the North Sea. The model tests were conducted at a length scale of 1:40 and scour depths were measured at multiple positions around the monopile by a camera system. Novel insights on the intrinsic progression of sediment displacement and time scale of the scour process were gained and in the following presented as a function of flow intensity for clear-water and live-bed conditions. In addition, baseline tests with unidirectional currents were conducted, in which the constant flow velocity was either based on the maximum peak or the root mean square velocity of the tidal currents. Significantly slower scour progression and smaller scour depths in tidal currents were found, compared to unidirectional currents based on the maximum peak velocity. In contrast, scour depths in tidal currents exceed those for unidirectional currents by up to 51%, if the unidirectional currents are based on the root mean square value. The comparison further implies that a flow velocity of around 15–20% larger than the root mean square flow velocity of the tide is needed to accurately estimate tidal current induced scour depth by unidirectional current. Thereby, the results underline the importance of selecting suitable conditions for the design process of offshore structures. For design purposes, the evolution of tidally induced scour could be well predicted with a time discretised stepping approach. [ABSTRACT FROM AUTHOR]

Velocity-skewed oscillatory sheet flow transport is studied by an approximate model for instantaneous sediment transport resulting from exponential profiles of sediment concentration and velocity over a mobile bed. The phase lag effect, and asymmetry in wave friction factor, oscillatory flow orbital amplitude, roughness height, bed shear stress and wave boundary layer thickness are modelled so that the net boundary layer stream of pure velocity-skewed oscillatory flow can be obtained. The proposed model is found consistent with the previous classical formulas, and qualitatively refines a series of exponents of velocity power function for the approximation of instantaneous sediment transport rate under different flow conditions. The sediment fluxes can be well predicted with accuracy as that of a two-phase numerical model. The net boundary layer stream and sediment transport rates are also well predicted, especially the negative net stream and rates with large phase residual. The phase residual is shown a useful but insufficient factor in the net sediment transport in pure velocity-skewed flows. In addition, the net boundary layer stream resulting from velocity-skewed wave boundary layer development difference between onshore and offshore flow durations is an essential factor as well. [ABSTRACT FROM AUTHOR]

Differences between breaker bar evolution under high energy (erosive) and subsequent mild energy (accretive) wave conditions are studied based on three data sets from large-scale morphodynamic experiments. Under erosive wave conditions, a clear linear relationship between the height and the cross-shore location of the breaker bar is observed. The similar cross-shore shape of the sediment transport magnitudes underlines the similarity of the bar evolution under erosive wave conditions. Under the subsequent accretive wave conditions, two major evolution patterns are observed during onshore migration (decaying or non-decaying breaker bar). In the presented data, the feedback between the wave conditions and the beach morphology determines if a bar decays during onshore migration. This feedback involves that the wave breaking location depends on the morphology and on the wave conditions resulting in differences in sediment transport patterns between the two types of onshore bar migration. A comparison against numerous other experimental beach profile data sets strongly supports the linear relationship between bar height and bar location under erosive wave conditions. For accretive wave conditions, the number of comparable data sets is very limited. The comparison underlines that breaker bar evolution under accretive wave conditions cannot be condensed to a single pattern as under erosive wave conditions. [ABSTRACT FROM AUTHOR]

A reduced-scale mobile-bed model is employed in a series of flume experiments to investigate the evolution of a submerged berm under low-energy irregular waves. Sixteen flume tests are conducted on the same submerged berm model under different wave conditions, and the bed elevations and water surface elevations are recorded along the profile during the tests. The results are analyzed comprehensively and compared to those under regular wave conditions from a previous study. Generally, no bulk evolution of the submerged berm is observed, and large wave periods lead to wavy profile shape and offshore sediment transport. The wavy profile can be found under both regular and irregular wave conditions, while the offshore transport is only observed under irregular wave conditions. Two major evolution patterns are extracted with the Empirical Orthogonal Function (EOF) analysis, including the decrease of the crest elevation and offshore sediment transport to the seaward-side slope, both of which are impacted to a larger degree by wave period than wave height. A cross-shore analysis is conducted on the wave hydrodynamic parameters and sediment transport parameters, based on which the differences in the evolution of the submerged berm under small and large wave periods are explained. This paper proposes new insights on the evolution of submerged berms under low-energy irregular waves and provides a new perspective to explain different behaviors of submerged berms via a comparison with the test results under low-energy regular waves from previous studies. • The evolution of a submerged berm under low-energy irregular waves are studied via a series of flume tests. • The behaviors of the submerged berm under irregular wave and regular wave conditions are compared. • Large wave period leads to offshore sediment transport under low-energy irregular waves. • The submerged berm exhibits two major evolution patterns: a crest elevation decrease and the offshore sediment transport. • The variations in cross-shore H s , Sk v and Sk a account for the differences in the evolution of the submerged berm. [ABSTRACT FROM AUTHOR]

In this study the numerical model Delft3D is implemented on high-resolution grids to evaluate beach morphological response to groins and submerged breakwaters under sea-breeze conditions. The numerical model is compared with field measurements from three sites on the northern Yucatan coast of Mexico. At these sites, relatively low energy waves with significant wave heights arrive at a steep oblique angle to the coast that drives high sediment transport rates. After calibration of the model, by adjustment of model free parameters, the model results show accurate representations of the diurnal variation of hydrodynamic conditions (wave height and current velocity) driven by the daily sea-breezes. Moreover, the morphodynamic model shows a good representation of the rapid morphological changes, particularly of the accretion pattern caused by a temporary groin and the subsequent recovery following the structure removal. However, erosion patterns at the downdrift areas of structures were not reproduced, owing to the numerical model limitations of simulating swash morphodynamics. Model validation with field observations of the beach response behind submerged breakwaters enabled the model to be used for a parametric study. Following previous work, new equations are developed in the present study that show an improvement for predicting shoreline changes near submerged breakwaters under sea-breeze conditions. Despite the limitations in reproducing downdrift erosion, overall the high-resolution modelling approach satisfactorily simulates the hydrodynamic and the updrift accretion responses to coastal structures on sandy beaches with oblique waves. • Simulations of beach change near groins and submerged breakwaters are performed. • Results are compared to measurements at three sites with high sediment transport rates. • New equations show improved prediction of shoreline change near submerged structures. [ABSTRACT FROM AUTHOR]

Rising sea levels are expected to cause widespread coastal recession over the course of the next century. In this work, new insight into the response of sandy beaches to sea level rise is obtained through a series of comprehensive experiments using monochromatic and random waves in medium scale laboratory wave flumes. Beach profile development from initially planar profiles, and a 2/3 power law profile, exposed to wave conditions that formed barred or bermed profiles and subsequent profile evolution following rises in water level and the same wave conditions are presented. Experiments assess profile response to a step-change in water level as well as the influence of sediment deposition above the still water level (e.g. overwash). A continuity based profile translation model (PTM) is applied to both idealised and measured shoreface profiles, and is used to predict overwash and deposition volumes above the shoreline. Quantitative agreement with the Bruun Rule (and variants of it) is found for measured shoreline recession for both barred and bermed beach profiles. There is some variability between the profiles at equilibrium at the two different water levels. Under these idealised conditions, deviations between the original Bruun Rule, the modification by Rosati et al. (2013) and the PTM model predictions are of the order of 15% and all these model predictions are within ±30% of the observed shoreline recession. Measurements of the recession of individual contour responses, such as the shoreline, may be subject to local profile variability; therefore, a measure of the mean recession of the profile is also obtained by averaging the recession of discrete contours throughout the active profile. The mean recession only requires conservation of volume, not conservation of profile shape, to be consistent with the Bruun Rule concept, and is found to be in better agreement with all three model predictions than the recession measured at the shoreline. [ABSTRACT FROM AUTHOR]