Turbulence Scales in the Surf and Swash

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Abstract
Ocean surface gravity waves breaking on gently sloping beaches generate substantial turbulent velocity fluctuations, both from overturning at the surface bore and from shear stresses at the bottom. We have used measurements made with laboratory-style hotfilm anemometers in the surf and swash on a natural beach to determine the relevant length and velocity scales. Battjes (1975) has pointed out the importance of determining the turbulence scales in the surf zone. Modelers, such as Svendsen and Madsen (1984), for example, rely on length and velocity scale estimates to parameterize and solve the complicated equations that govern surf zone flows. We find that turbulence length scales depend essentially on the bore height, and therefore on the local depth, but may decrease sharply under the bore. We also determine that at least the horizontal velocities approach isotropy at frequencies of 2 to 3 Hz, which turn out to also correspond to length scales on the order of the local depth.

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