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Seawalls are commonly constructed to prevent landward erosion of the shoreline and to maintain the configuration of the area behind them against wave action. In order to consider the effect of seawalls on surf zone hydrodynamics, experiments have been performed at laboratory model scale on partially reflective seawalls located in the surf zone. The main objectives of these experiments were to undertake a quantitative comparison of near-bed velocities in two cases (i.e. with and without the reflective structure). The presence of a reflective structure and the influence of reflected waves result in significant changes in the mean flow and the near-bed horizontal velocities in the surf zone. The latter is illustrated by comparing the probabilistic properties of velocities measured with and without a reflective structure. In this paper, a semi-empirical approach based on the measured probability density functions of near-bed horizontal velocities, is presented to predict the short-term response of a partially reflective seawall to random wave attack. The results obtained from the model and comparison with the experimental results, which have been reported previously are promising and encouraging for further developments of the preliminary model.

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In this paper, a new approach is presented for estimating the vertical and horizontal distribution of undertow in the surf zone for reflective beaches. The present model is a modification of the original model presented by Okayasu et al., (1990) for natural, non-reflective beaches to include the effect of partially reflected waves. The nonlinearity of waves, wave-current interaction and nonlinear mass drift of the incident wave are also included in the present model. The results of experimental investigation and model development show that existence of reflective conditions on beaches results in a reduction in the magnitude of undertow and modifies its distribution across the beach profile. Comparison of the results by those obtained from the experiments clearly indicates that by taking the nonlinearity and wave-current interaction, the predictions of undertow in the surf zone are much improved. In particular, due to the effect of turbulence induced by wave breaking for nonlinear waves, the predicted results show more consistence with the measurements.