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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]

Field-tested numerical model simulations are used to estimate the effects of an inlet, ebb shoal, wave height, wave direction, and shoreline geometry on the variability of bathymetric change on a curved coast with a migrating inlet and strong nearshore currents. The model uses bathymetry measured along the southern shoreline of Martha's Vineyard, MA, and was validated with waves and currents observed from the shoreline to ~10-m water depth. Between 2007 and 2014, the inlet was open and the shoreline along the southeast corner of the island eroded ~200 m and became sharper. Between 2014 and 2015, the corner accreted and became smoother as the inlet closed. Numerical simulations indicate that variability of sediment transport near the corner shoreline depends more strongly on its radius of curvature (a proxy for the separation of tidal flows from the coast) than on the presence of the inlet, the ebb shoal, or wave height and direction. As the radius of curvature decreases (as the corner sharpens), tidal asymmetry of nearshore currents is enhanced, leading to more sediment transport near the shoreline over several tidal cycles. The results suggest that feedbacks between shoreline geometry and inner-shelf flows can be important to coastal erosion and accretion in the vicinity of an inlet. [ABSTRACT FROM AUTHOR]

This paper treats the numerical modelling of the behaviour of a sand core covered by rocks and exposed to waves. The associated displacement of the rock is also studied. A design that allows for erosion and deposition of the sand core beneath a rock layer in a coastal structure requires an accurate prediction method to assure that the amount of erosion remains within acceptable limits. This work presents a numerical model that is capable of describing the erosion and deposition patterns inside of an open filter of rock on top of sand. The hydraulic loading is that of incident irregular waves and the open filters are surface piercing. Due to the few experimental data sets on sediment transport inside of rock layers, a sediment transport formulation has been proposed based on a matching between the numerical model and experimental data on the profile deformation inside an open filter. The rock layer on top of a sand core introduces a correction term in the Exner equation (the continuity equation for sediment and change in bed level). The correction term originates from the fact that the sand can only be deposited in the pores of the filter material. The numerical model is validated against additional data sets on the erosion and deposition patterns inside of an open filter. A few cases are defined to study the effect of the sinking of the filter into the erosion hole. The numerical model is also applied to several application cases. The response of the core material (sand) to changes in the wave period and wave height is considered. The effect of different layouts of the filter is studied in order to investigate the effect of different filter profiles on the resulting erosion. Finally, it is studied how much the design of a hydraulically closed filter can be relaxed to obtain a reduction in the design requirements of the filter thickness, while the deformation to the sand core remains acceptably small. [ABSTRACT FROM AUTHOR]

Follet's Island (FI) is a sediment-starved barrier island located on the Upper Texas Coast; a stretch of coastline along the Gulf of Mexico that experiences on average four hurricanes and four tropical storms per decade. During Hurricane Ike, water levels and wave heights at FI exceeded the 100-year and 40-year return values, respectively, leading to significant overtopping and morphology changes of this low-lying barrier island. The physical processes governing the real-time morphodynamic response of the beach and dune system during 96 h of hurricane impact were modeled using XBeach (2D) and CSHORE (1D). Hydrodynamic boundary conditions were obtained from ADCIRC/SWAN model runs validated with measured buoy and wave gauge data while LiDAR surveys provided pre- and post-storm measured topography. XBeach displayed a decent model skill of 0.34 and provided numerical outputs of the entire 2D domain such as topography, suspended sediment load and bed load which was very useful in visualizing erosion and deposition patterns. CSHORE also displayed a decent model skill of 0.33 and was able to accurately predict the post-storm beach slope and shoreline, but was less effective at simulating the foredune morphology. Modeling results show that the complete morphodynamic response of FI to Hurricane Ike was governed by a sequence of impact regimes, including swash, collision, overwash, inundation, and storm surge ebb. [ABSTRACT FROM AUTHOR]

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

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

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

This paper addresses the role of the phase difference between the peak discharge and the tidal level on the development of nearshore bars at river mouths during extreme river discharge events. We use a coupled hydrodynamic and morphodynamic numerical model (Delft3D) to reproduce extreme river discharge events on an idealized river mouth debouching on a nearshore shelf, characterized by a concave cross-shore profile. Different peak river discharges and lags between the peak discharge at the upstream boundary and the high tide at the offshore boundary are tested. The analysis of the water levels, currents, bed shear stresses, sediment transport rates and bed level changes shows that the lag between tidal conditions and river discharge triggers significant changes in (1) the instant in which the maximum currents at the outlet are reached, (2) the period for which the sediment is mobilized through the outlet, and (3) the maximum sediment transport rates during the events. These changes significantly modify the final characteristics of the river mouth bars, doubling their final extension and quadrupling the final bar volume for the same river discharge conditions and different phase lag, varying also their plan shape and the development of lateral subaqueous levees. These results emphasize the complex interplay between river and coastal hydrodynamics, being of major interest for researchers and managers dealing with projections of the consequences of coastal extreme events and plans to mitigate their effects. • The lag between tide and hydrograph peak modifies the outlet discharge timing. • Discharge-generated velocities tend to be coupled to the tidal ebb at the outlet. • Currents and sediment transport are determined by lag/peak discharge combination. • Final bar extensions double for the same discharge with different lags. • Bar plan shape and lateral levee formation are clearly determined by the lag. [ABSTRACT FROM AUTHOR]

Abstract: A 2DH numerical, model which is capable of computing nearshore circulation and morphodynamics, including dune erosion, breaching and overwash, is used to simulate overwash caused by Hurricane Ivan (2004) on a barrier island. The model is forced using parametric wave and surge time series based on field data and large-scale numerical model results. The model predicted beach face and dune erosion reasonably well as well as the development of washover fans. Furthermore, the model demonstrated considerable quantitative skill (upwards of 66% of variance explained, maximum bias −0.21m) in hindcasting the post-storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied. The prediction skill ranged between 0.66 and 0.77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied. The sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay. The model sensitivity to the sheet flow sediment transport limiter, which served as a proxy for unknown factors controlling the resistance to erosion, was significantly greater than the sensitivity to the hydraulic forcing parameters. If no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response. [Copyright &y& Elsevier]

Abstract: We present a two-dimensional, two-phase model for non-cohesive sediment transport. This model solves concentration-weighted averaged equations of motion for both fluid and sediment phases. The model accounts for the interphase momentum transfer by considering drag forces. A collisional theory is used to compute the sediment stresses, while a two-equation (k–ε) fluid turbulence closure is implemented. A benchmark sediment transport problem concerning the scouring downstream of an apron is carried out as an example and numerical results agree with existing experimental data. [Copyright &y& Elsevier]

Abstract: Numerical modeling of a beach nourishment project is conducted to enable a detailed evaluation of the processes associated with the effects of nearshore dredge pits on nourishment evolution and formation of erosion hot spots. A process-based numerical model, Delft3D, is used for this purpose. The analysis is based on the modification of existing bathymetry to simulate “what if” scenarios with/without the bathymetric features of interest. Borrow pits dredged about 30 years ago to provide sand for the nourishment project have a significant influence on project performance and formation of erosional hot spots. It was found that the main processes controlling beach response to these offshore bathymetric features were feedbacks between wave forces (roller force or alongshore component of the radiation stress), pressure gradients due to differentials in wave set-up/set-down and bed shear stress. Modeling results also indicated that backfilling of selected borrow sites showed a net positive effect within the beach fill limits and caused a reduction in the magnitude of hot spot erosion. [Copyright &y& Elsevier]