CROSS-SHORE HYDRODYNAMICS WITHIN AN UNSATURATED SURF ZONE

This paper concerns the hydrodynamics induced by random waves incident on a steep beach. New experimental results are presented on surface e levation and kinematic probability density functions, cross-shore vari ation in wave heights, the fraction of broken waves and velocity momen ts. The surf zone is found to be unsaturated at incident wave frequenc ies, with a significant proportion of the incident wave energy remaini ng at the shoreline in the form of bores. Wave heights in both the out er and inner surf zones are best described by a full Rayleigh distribu tion [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 ra ndom waves. Proc. 16th Int. Conf. Coastal Eng. ASCE, New York, pp. 569 -588]. A new parametric wave transformation model is outlined which pr ovides explicit expressions for the fraction of broken waves and the e nergy dissipation rate within the surf zone. On steep beaches, the mod el appears to offer improved predictive capabilities over the original Battjes and Janssen model. Cross-shore variations in the velocity var iance and velocity moments are best described using Linear Gaussian wa ve theory, with less than 20% of the velocity variance in the inner su rf zone due to low frequency energy. (C) 1998 Elsevier Science B.V. Al l rights reserved.