Observations of Surf Beat Propagation and Energetics

A. J Bowen; Steve Elgar; Stephen M Henderson

doi:10.1061/40549(276)109

Near-bottom pressure and velocity, measured along a cross-shore transect extending from the shore to about 8-m water depth on a barred beach, are used to study the cross-shore propagation of surf beat and the associated cross-shore flux of surf beat energy. The dominant Empirical Orthogonal Functions (EOF's) of pressure fluctuations measured in less than 5 m depth at relatively low beat frequencies (0.005–0.025 Hz) had well developed nodes and antinodes, with ±π phase jumps at the nodes, suggestive of standing wave motions. The dominant EOF's of relatively high frequency surf beat (0.03–0.05 Hz) showed some nodal structure, but their phase increased approximately linearly with distance from the shore indicating net shoreward propagation. EOF's of combined pressure and cross-shore velocity fluctuations at beat frequencies showed a similar mixture of standing and progressive waves. Furthermore, phases between pressure and velocity components were close to ±π/2 at relatively low beat frequencies (< 0.25 Hz), as predicted by standing waves theories, but at higher beat frequencies (0.3–0.05 Hz) phases usually had magnitudes less than π/2, but significantly greater than zero, suggestive of mixed standing and shoreward progressive waves. Shoreward propagating surf beat carried a shoreward energy flux. During a storm, this shoreward flux was about half the flux that could have been carried if all surf beat propagated directly shorewards. The energy carried shoreward by propagating surf beat was lost either at the shoreface (for moderate waves) or on the offshore side of the bar (during a storm). On one occasion surf beat carried energy seawards outside the surf zone and shorewards inside the surf zone, consistent with theoretical models that predict surf beat forcing at the outer edge of the surf zone.

Observations of Surf Beat Propagation and Energetics

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