Patrick Holmes

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.

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.

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.

PRESSURE GRADIENTS WITHIN SEDIMENT BEDS

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

BEACH EVOLUTION UNDER RANDOM WAVES

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.