Tom E. Baldock

Hydrodynamics and sediment transport in the swash zone: a review and perspectives

The dominant hydrodynamic forcing and resulting sediment transport mechanisms in the swash zone are reviewed, combined with a discussion of future measurement and modelling requirements. The importance of swash zone processes in the overall behaviour of the nearshore littoral zone are identified, with the paper subsequently focusing on aspects that directly relate to sediment transport on beaches. The review considers analytical and numerical modelling work, and both field and laboratory data. The hydrodynamics of the swash zone are first discussed with reference to the surf zone boundary, swash characteristics, forcing mechanisms and shoreline oscillations. Subsequent sections consider more micro-scale processes such as the internal flow kinematics, turbulence, the bed boundary layer and infiltration/exfiltration through permeable beds. The second part of the paper outlines the fundamental mechanics of sediment transport in the swash zone, followed by a more detailed discussion of sediment transport modes induced by the dominant hydrodynamic conditions. Modelling concepts are reviewed, and the strengths and weaknesses of different approaches identified. A final summary and future modelling perspectives conclude the paper.

A Laboratory Study of Nonlinear Surface Waves on Water

This paper describes an experimental investigation in which a large number of water waves were focused at one point in space and time to produce a large transient wave group. Measurements of the water surface elevation and the underlying kinematics are compared with both a linear wave theory and a second-order solution based on the sum of the wave-wave interactions identified by Longuet-Higgins & Stewart (1960). The data shows that the focusing of wave components produces a highly nonlinear wave group in which the nonlinearity increases with the wave amplitude and reduces with increasing bandwidth. When compared with the first- and second-order solutions, the wave-wave interactions produce a steeper wave envelope in which the central wave crest is higher and narrower, while the adjacent wave troughs are broader and less deep. The water particle kinematics are also strongly nonlinear. The accumulated experimental data suggest that the formation of a focused wave group involves a significant transfer of energy into both the higher and lower harmonics. This is consistent with an increase in the local energy density, and the development of large velocity gradients near the water surface. Furthermore, the nonlinear wave-wave interactions are shown to be fully reversible. However, when compared to a linear solution there is a permanent change in the relative phase of the free waves. This explains the downstream shifting of the focus point (Longuet-Higgins 1974), and appears to be similar to the phase changes which result from the nonlinear interaction of solitons travelling at different velocities (Yuen & Lake 1982).

Recent advances in modeling swash zone dynamics: Influence of surf-swash interaction on nearshore hydrodynamics and morphodynamics

The role of the swash zone in influencing the whole nearshore dynamics is reviewed with a focus on the interaction between surf and swash zone processes. Local and global hydromorphodynamic phenomena are discussed in detail, and a description of the overall swash zone operation is given. The effects of swash zone boundary conditions are highlighted, together with the importance of surf zone boundary conditions. Major emphasis is placed on illustrating the interactions of various hydrodynamic modes which, in turn, control the swash and surf zone morphology. Finally, methods to account for swash zone processes in coastal models with different temporal and spatial resolutions are proposed.

Cross-shore hydrodynamics within an unsaturated surf zone

Abstract This paper concerns the hydrodynamics induced by random waves incident on a steep beach. New experimental results are presented on surface elevation and kinematic probability density functions, cross-shore variation in wave heights, the fraction of broken waves and velocity moments. The surf zone is found to be unsaturated at incident wave frequencies, with a significant proportion of the incident wave energy remaining at the shoreline in the form of bores. Wave heights in both the outer and inner surf zones are best described by a full Rayleigh distribution [Thornton, E.B., Guza, R.T., 1983. Transformation of wave height distribution. J. Geophys. Res. 88, 5925–5938], rather than a truncated Rayleigh distribution as used by Battjes and Janssen (1978) [Battjes, J.A, Janssen, J.P., 1978. Energy loss and setup due to breaking of random waves. Proc. 16th Int. Conf. Coastal Eng. ASCE, New York, pp. 569–588]. A new parametric wave transformation model is outlined which provides explicit expressions for the fraction of broken waves and the energy dissipation rate within the surf zone. On steep beaches, the model appears to offer improved predictive capabilities over the original Battjes and Janssen model. Cross-shore variations in the velocity variance and velocity moments are best described using Linear Gaussian wave theory, with less than 20% of the velocity variance in the inner surf zone due to low frequency energy.

Eulerian flow velocities in the swash zone: Field data and model predictions

Measurements of Eulerian flow velocity obtained within the swash zone on a relatively steep beach face (gradient 1:23) are compared with an extended ballistic swash model. The model only requires a friction factor, beach slope and terminal bore velocity as input. The following model predictions matched well with observations: (1) The maximum Eulerian flow velocity is the shoreline velocity when it arrives at the fixed point of interest on the beach face; (2) at any location the time of flow reversal occurs prior to the shoreline reaching its maximum landward excursion; (3) the maximum flow velocity in the backwash is the velocity recorded as the shoreline recedes past the fixed point of interest (and this is less than the maximum uprush velocity); and (4) the duration of the uprush flow is shorter than the duration of the backwash flow. Previous studies have already confirmed that the ballistic swash model (including friction) can accurately predict shoreline motion and maximum run-up on steep beaches. This study shows that it is similarly successful in predicting Eulerian flow velocities during individual swash events. The model does not presently account for interacting swash, however, and so may be less appropriate on gently sloping beaches.

Long-wave forcing by the breaking of random gravity waves on a beach

This paper presents new laboratory data on long–wave (surf–beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long–wave frequencies, from which the correlation between forced long waves and short–wave groups is examined. A detailed analysis of the cross–shore structure of the long–wave motion is presented, and the observations are critically compared with existing theories for two–dimensional surf–beat generation. The surf beat shows a strong dependency on normalized surf–zone width, consistent with long–wave forcing by a time–varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random–wave simulations considered. The seaward–propagating long waves show a positive correlation with incident short–wave groups and are linearly dependent on short–wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.

Simulation and prediction of swash oscillations on a steep beach

This paper presents numerical simulations and analytical predictions of key aspects of swash oscillations on a steep beach. Simulations of the shoreline displacement based on bore run-up theory are found to give excellent agreement with recent experimental data for regular waves, wave groups and random waves. The theory is used to derive parameters that predict the onset of swash saturation and the spectral characteristics of the saturated shoreline motion. These parameters are again in good agreement with the measured laboratory data and are also consistent with previous experimental data. Simulation of irregular wave run-up using a series of overlapping monochromatic swash events is found to reproduce typical features of swash oscillations and can accurately describe both the low and high frequency spectral characteristics of the swash zone. In particular, the low frequency components of the run-up can be modelled directly using a sequence of incident short wave bores, with no direct long wave input to the numerical simulations. This suggests that wave groupiness must be accounted for when modelling shoreline oscillations.

Breakpoint generated surf beat induced by bichromatic wave groups

Abstract This paper presents new experimental data on 2-D surf beat generation by a time-varying breakpoint induced by bichromatic wave groups. The experimental investigation covers a broad range of wave amplitudes, short wave frequencies, group frequencies and modulation rates. The data include measurements of incident and outgoing wave amplitudes, breakpoint position, shoreline run-up and the cross-shore structure of both the short and long wave motion. Surf beat generation is shown to be in good agreement with theory [Symonds, G., Huntley, D.A., Bowen, A.J., 1982. Two dimensional surf beat: long wave generation by a time-varying breakpoint. J. Geophys. Res. 87, 492–498]. In particular, surf beat generation is dependent on the normalised surf zone width, which is a measure of the phase relationship between the seaward and shoreward breakpoint forced long waves, and linearly dependent on the short wave amplitude. The cross-shore structure of the long wave motion is also consistent with theory; at maximum and minimum surf beat generation, the mean breakpoint coincides with the nodal and anti-nodal points, respectively, for a free long wave standing at the shoreline. A numerical solution, using measured data as input, additionally shows that the phase relationship between the incident bound long wave and the outgoing breakpoint forced wave is consistent with the time-varying breakpoint mechanism.

Low frequency swash motion induced by wave grouping

Abstract This paper concerns the low frequency motion of swash directly induced by wave grouping on a steep beach. A new experimental investigation is presented which considers the hydrodynamics of the inner surf zone and swash zone using vertical wave gauges and a run-up wire. Results for regular waves, wave groups and random waves are discussed, with particular reference to low frequency motions. The inner surf zone and swash zone are found to be unsaturated at incident short wave frequencies and, as a result, significant wave grouping is apparent at the shoreline. The low frequency motion in the surf zone is found to be in phase with the incident wave grouping and may therefore be regarded as a time varying set-up (Watson and Peregrine, 1992). The low frequency motion of the swash is shown to be an order of magnitude greater than that in the inner surf zone, inconsistent with cross-shore standing long waves, for which no evidence is found. We demonstrate that the low frequency motion of the shoreline provides an excellent approximation to the run-up of individual bores and therefore describes the run-up envelope. Spectral analysis shows that the low frequency motion of the swash is directly linked to the modulations in offshore wave height i.e. the low frequency energy in the incident wave envelope. In addition, the random wave run-up spectra show an f−4 high frequency roll-off, as found by Huntley et al. (1977). The accumulated data show that, unless the surf zone is totally saturated, a significant proportion of the low frequency swash motion may be directly due to incident wave grouping and not standing long waves.

Measurements and Modeling of Swash Induced Pressure Gradients in the Surface Layers of a Sand Beach

Field measurements of swash-induced hydraulic (pressure) gradients in the surface layers of a sand beach are presented and compared to a one-dimensional diffusion model. The model provides a description of flow through a porous, quasi-saturated sediment and is driven by the time-varying swash depth on the beach surface. The field measurements broadly show four different types of pressure propagation behavior, which appear dependent on cross-shore position in the swash zone. The hydraulic gradients within the surface layers of the beach are typically found to be much greater than those likely to be generated by tidally-induced groundwater flow. In these instances the diffusion model generally provides a good description of the data. However, very large hydraulic gradients are frequently observed just below the beach surface, and these cannot be adequately described by the diffusion model. A mechanism is proposed through which these large hydraulic gradients could be generated.