Kinematics and Directionality of Waves in the Surf Zone

Kinematics and Directionality of Waves in the Surf Zone

Van Heteren, J.
Stive, M.J.F.

Rijkswaterstaat

1985-05-01

Realistic modelling of the internal kinematics of wind waves breaking on a beach is of great practical importance, e.g. for the modelling of coastal processes and for the design of coastal structures. A fundarnental aspect in this modelling concerns. the relation between the surface elevation and the internal kinematics. In this respect it appears that in water of interrnediate relative depth the linear Gaussian model of the wave motion performs satisfactorily (see Battjes and Van Heteren, 1983, 1984). Although the linearity assumption is expected to be violated near and in the surf zone, it is practically useful to check the quantitative performance of the linear Gaussian model in this case. An important part of the present study is devoted to the applicability of this theory, making use of field measurements which were collected in the spring of 1981 on the beach near Egmond. The quantitative performance of linear theory in predicting the wave kinematics from the surface elevation is investigated on the basis of spectral transfer functions between surface elevation and velocity and of measured and theoretical r.m.s. fluctuation of the velocity. As far as the spectral transfer functions are concerned the squared coherence, the gain and the phase were investigated between surf ace elevation and both horizontal and vertical velocity. Relevant earlier studies on the performance of linear theory in the surf zone are those of Thorn ton et al. (1976), of Mitsuguchi et al. (1980) and of Guza and Thornton (1980). The general findings are that linear theory generally overestimates wave induced horizontal veloci ties by 10% to 30%. Mitsuguchi et al. merely state their conclusion without analyzing apparent trends in their data with e.g. increasing frequency. They only consider alirnited set of horizontal velocity data. Guza and Thornton obtain their overall conclusion merely on the basis of the r.m.s. fluctuation of the horizontal velocity as measured and as predicted with the linear theory from the surface elevation. The present study confirrns the conclusions of the above investigations by and large, but extends the analysis both in width by including the vertical velocities and in depth by investigating the role of turbulent kinetic energy generated by breaking. Also special attention is given to the phase relationship between the vertical velocity and the horizontal velocity, and between the vertical velocity and the surface elevation.

interal kinemates
wind waves
wave breaking
surf zone
wave modelling
turbulent kinetic energy

TLJ400200
TLJ800400
TLJ100700

http://resolver.tudelft.nl/uuid:7840e62c-948c-4c50-b2bb-7de46ad9f020

Rijkswaterstaat, RIKZ

Nota WWKZ-84.S013

Hydraulic Engineering Reports

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(c) 1985 Rijkswaterstaat