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This is an Accepted Manuscript of an article published by Taylor & Francis Group in Journal of Hydraulic Research on 2017, available online at: http://www.tandfonline.com/10.1080/00221686.2016.1275047, Most of the coastal hydraulic studies in physical and numerical wave flumes (NWF) require the correct generation of nonlinear, long period waves at intermediate and shallow water conditions. In this paper, the ability to generate such long waves in an NWF based on the particle finite element method (PFEM) is analysed. Wavemaker theories based on the Stokes second-order wave theory, cnoidal wave theory and solitary wave theory are implemented in the NWF through piston type paddle boundary conditions. An assessment of the capability and robustness of generation and propagation of long waves in the NWF is carried out. Numerical results are compared to theoretical results and the data from physical tests. Results indicate that regular long waves of permanent form can be generated in an NWF based on the PFEM using the implemented wavemaker theories. However, when choosing a wavemaker theory a compromise has to be made between the target wave conditions and the validity range of each wavemaker theory., Peer Reviewed, Postprint (author's final draft)

Accurately predicting movable bed roughness is essential to the analyses of transport processes, but when the bottom is rippled, as it commonly is in the coastal environment, characterizing the roughness is less straightforward than when the bottom is flat. The common method of predicting roughness, while effective, unnecessarily predicts ripple geometry and requires a model-dependent factor, which varies widely, relating ripple geometry and bottom roughness. We have therefore developed an alternative, more direct method of predicting bed roughness in the ripple regime: the wave energy dissipation factor is predicted from flow and sediment information and then any desired theoretical friction factor model is used to back-calculate the roughness. This paper describes the common and proposed methods of predicting roughness and presents results of preliminary testing of the methods with field data. Both methods adequately predict current velocities in wave-current field flows, with the proposed method yielding the smaller RMS-error of 3.1 cm/s. Remaining questions concerning the appropriate near-bottom orbital velocity required to describe field conditions must be resolved when additional field data becomes available.

Experimental studies of sediment transport rates due to near shore waves are often conducted in oscillating water tunnels (OWTs). In an OWT, the oscillatory motion produced by the piston propagates almost instantaneously along the entire tunnel. Consequently, unlike the wave motion in the sea or in a wave flume, flow in an OWT is uniform along the tunnel, and second - order wave propagation effects (such as Longuet - Higgins's streaming) are absent. The effect of these hydrodynamic differences between OWT and sea waves on sediment transport rates has generally been neglected. In this paper we present a simple, practical formulation to evaluate bed shear stresses and bed load transport rates due to asymmetric and skewed waves plus a current in an OWT, based on fitting the exact results of a rigorous, analytical model of the OWT wave - current boundary layer. By then accounting for real wave effects we find that wave propagation significantly affects the predicted period - averaged net sediment transport rates. Such real wave effects can therefore not be neglected when comparing near shore transport models with OWT data., Singapore–MIT Alliance for Research and Technology ((SMART) Center for Environmental Sensing and Modeling), Singapore. National Research Foundation

Experimental results for near-bottom current velocity profiles for flows over artificial, definitely 2D ripples made of 1.5 cm high aluminum angle -profile spaced at 10 cm intervals are obtained for the following cases: (i)current alone perpendicular to ripples; (ii) current alone parallel to ripples; (iii) combined orthogonal wave-current flows for current parallel to ripples; and (iv) current alone at an angle of 30° to the ripple axis. The velocity profiles are analyzed by the log-profile method, and show the roughness experienced by the current to increase as the angle between ripple and current direction increases, i.e. demonstrating convincingly the reality of the concept of a direction-dependent roughness for flows over a 2D rippled bottom. Roughness experienced by the velocity component perpendicular to the ripples is, however, found to be independent of the direction of the mainstream flow relative to that of the ripples, and the different roughness experienced by the perpendicular and parallel velocity components gives rise to a turning of the current velocity vector to become increasingly aligned with the ripple crests as the bottom is approached from above. Implications of this feature, in terms of net sediment transport direction in combined wave-current flows in inner-shelf coastal waters, is discussed., Singapore–MIT Alliance for Research and Technology ((SMART) Center for Environmental Sensing and Modeling), Singapore. National Research Foundation

In this paper, the two common approaches to account for wave randomness, the spectral approach and the wave-by-wave approach, are compared through numerical experiments conducted with the coupling of a surf zone hydrodynamic model and a bedload sediment transport model. Special attention is paid to the wave nonlinearity and net cross-shore bedload transport predictions. The two approaches are found to have negligible difference in their predictions of certain average hydrodynamics, such as wave heights, set-up and undertow. However, the wave-by-wave approach outperforms the spectral approach in the wave nonlinearity prediction, and the two approaches differ significantly in their predictions of wave-induced net cross-shore bedload transport which strongly depends on wave nonlinearity. This suggests the necessity of using the wave-by-wave approach. The computational efficiency of the wave-by-wave approach is also discussed., Singapore-MIT Alliance. Center for Environmental Sensing and Monitoring

Open boundaries (OBs) are usually unavoidable in numerical coastal circulation simulations. At OBs, appropriate open boundary conditions (OBCs) are required and a good OBC should be able to let outgoing waves freely pass to the exterior of a computational domain without creating reflections at the OBs. In the present study, a methodology has been developed to predict two parameters, phase speed c_r and decay time T_f, in a standard OBC formulation, so that the OBC is significantly improved compared to commonly used existing OBCs with specified c_r and T_f. For the conditions where wave period is unknown, the OBC with approximated c_r and T_f may be applied and a test reveals that this OBC is able to yield good results in typical coastal flow conditions. In addition, a Swing-Door Boundary Condition (SDBC) is proposed and tested for application at an offshore open boundary where both incoming and outgoing waves exist., Singapore-MIT Alliance for Research and Technology Center

Open boundaries (OBs) are usually unavoidable in numerical coastal circulation simulations. At OBs, an appropriate open boundary condition (OBC) is required so that outgoing waves freely pass to the exterior without creating reflections back into the interior of the computational domain. In this paper, the authors derive, based on the shallow-water equations including bottom friction and neglecting Coriolis effect and by means of nonlinear characteristic analysis, an OBC formulation with two predictive parameters, phase speed c[subscript r], and decay time T[subscript f]. Simple idealized tests are performed to demonstrate the proposed OBC’s excellent skills in elimination of unwanted reflections at OBs when the motion is periodic, as assumed in its theoretical derivation. It turns out that the formulas for the two OBC parameters become independent of period in the limit of small friction and/or short period. This feature is used to derive an OBC applicable when information about the typical period of the motion to be simulated is unavailable. Simple, idealized tests of this period independent OBC demonstrate its ability to afford excellent results, even when the limitations inherent in its derivation are exceeded. Finally, the OBC is applied in more realistic simulations, including Coriolis effects of 2D tidal flows, and is shown to yield excellent results, especially for residual flows.

Oscillating water tunnels are experimental facilities commonly used in coastal engineering research. They are intended to reproduce near-bed hydrodynamic and sediment transport phenomena at a realistic scale. In an oscillating water tunnel, a piston generates an oscillatory motion that propagates almost instantaneously to the whole tunnel; consequently, flow is uniform along the tunnel, unlike the propagating wave motion in the sea or in a wave flume. This results in subtle differences between the boundary-layer hydrodynamics of an oscillating water tunnel and of a propagating wave, which may have a significant effect in the resulting sediment transport. In this paper, we present a zeroth-order analytical model of the turbulent boundary-layer hydrodynamics in an oscillating water tunnel. By using a time-varying eddy viscosity and by accounting for the constraints arising from the tunnel's geometry, the model predicts the oscillating water tunnel hydrodynamics and yields analytical expressions to compute bed shear stresses for asymmetric and skewed waves, both in the absence or presence of an imposed current. These expressions are applied to successfully quantify bedload sediment transport in oscillating water tunnel experiments., Singapore-MIT Alliance for Research and Technology (Center for Environmental Sensing and Modeling (CENSAM)), Singapore. National Research Foundation

The long-term goals of this research are: (i) to identify all relevant physical processes that participate in and contribute significantly to sediment transport in near-shore coastal waters; (ii) to investigate each of the identified processes in order to understand the underlying physics in a quantitative manner; (iii) to develop simple predictive models for each process; and (iv) to incorporate the simple predictive process-models in a predictive model for beach profile response to the action of waves and currents. The objective of the present research is to evaluate the effect of fluid accelerations in near-shore waters. The first part of the research intended to determine the importance of the subsurface sediment transport rate induced by the pressure gradient (acceleration) associated with the passage of the front of a forward-leaning, near-breaking or broken wave. We have concluded that this subsurface transport rate is of small importance compared with the surficial transport rate caused by shear stresses acting on the sediment bed. Consequently, the objective of the second part of the research was to develop and verify the accuracy of an easily applicable methodology to compute surficial transport under near-breaking and breaking waves (hereafter referred to as near-shore waves)., The original document contains color images. All DTIC reproductions will be in black and white.

The long-term goals of this research are: (1) to identify all relevant physical processes that participate in and contribute significantly to sediment transport in nearshore coastal waters; (2) to investigate each of the identified processes in order to understand the underlying physics in a quantitative manner; (3) to develop simple predictive models for each process; and (4) to incorporate the simple predictive process-models in a predictive model for beach profile response to the action of waves and currents. The objective of the present research is to evaluate the effect of fluid accelerations in nearshore waters. The first part of the research intended to determine the importance of the subsurface sediment transport rate induced by the pressure gradient (acceleration) associated with the passage of the front of a forward-leaning, near-breaking wave. We have concluded that this subsurface transport rate is of small importance compared with the surficial transport rate caused by shear stresses acting on the sediment bed. Consequently, the objective of the second part of the research, currently in progress, is to develop and verify the accuracy of an easily applicable methodology to compute surficial transport under nearbreaking and breaking waves (hereafter referred to as nearshore waves)., The original document contains color images. All DTIC reproductions will be in black and white.

The long-term goals of this research are: (1) to identify all relevant physical processes that participate in and contribute significantly to sediment transport in near-shore coastal waters; (2) to investigate each of the identified processes in order to understand the underlying physics in a quantitative manner; (3) to develop simple predictive models for each process; and (4) to incorporate the simple predictive process-models in a predictive model for beach profile response to the action of waves and currents. The present research is concerned with the effect of fluid accelerations, which is synonymous to pressure-gradients, on fluid-sediment interaction in near-shore waters, i.e. under near-breaking or broken waves. In particular, the objective is to ascertain the importance of the subsurface sediment transport rate induced purely by the strong pressure gradient (acceleration) associated with the passage overhead of the steep forward-leaning front of a near-breaking or broken wave relative to the surficial sheet flow sediment transport rate induced by shear stresses and pressure gradients.

The objective of this study was to investigate the application of simple eddy viscosity models, developed for wave-current interaction, to the problem of sediment suspension and transport. A review of existing models is given; however, it is pointed out that all models compute suspended sediment transport by interacting the product of mean concentration and mean velocity. Because recent field experiments indicate that time-varying quantities may dominate the sediment flux due to mean components, the model described in this report calculates time-varying components of the concentration along with the mean flow component. The report describes the governing equations and boundary conditions, the process used to select an eddy viscosity model for hydrodynamics of wave-current interaction, and the determination of a resuspension coefficient. Detailed summaries of model development and the solution scheme are provided, as well as example calculations using the computer program WCSTRANS. A source listing of the computer model is included.

Based on a set of approximate equations for long waves over an uneven bottom, the transformation of a solitary wave climbing a slope is studied numerically and compared with available experiments. Results are also obtained for a solitary wave progressing over a slope onto a shelf. The development of a train of solitary waves on the shelf is found and interpreted physically. Supporting experimental evidence is also given. (Author)