SET-UP DRIVEN UNDERTOWS ON A BARRED BEACH

Philip D. Osborne, Brian Greenwood

Abstract


Field measurements of cross-shore velocity and water surface elevation from a natural barred surf zone during two storm events confirm the presence of undertow. The undertow is characterized by: i) mean cross-shore velocities reaching 0.20 m s"1 and directed offshore; ii) mean cross-shore velocity decreasing towards the surface with occasional reversals in the mean flow to a landward direction in the upper water column; iii) a strongly oscillatory velocity field with speeds increasing towards the surface and a distinct landward skewness superimposed. The undertow is strongly correlated with set-up of the mean water surface. The set-up is characterized by: i) increasing values shoreward with perturbations closely following topography indicative of a primary forcing by waves; ii) maxima (0.35 m in the inner system) occurring over the bar troughs in association with decreasing wave height over the preceding bar crest as predicted by theory and laboratory experiments.

Keywords


barred beach; set up; undertow

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