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Geometric Roughness Estimates of Surf-Zone Wave-Breaking Foam

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Measurements of small-scale [O (mm)] geometric roughness length (\( k_{\text{f}} \)) associated with surface foam generated by depth-limited breaking waves were obtained within the surf zone on a sandy beach. The parameter \( k_{\text{f}} \) is described using the vertical standard deviation of the sea surface elevation for a foamy area as estimated from stereo imagery. A waterproof two-camera system with self-logging and internal power was developed for collecting stereo images using commercial off-the-shelf components and commercial software for operations 1 m above the sea surface within the middle of the surf zone. The surf-zone foam had a mean \( k_{\text{f}} \) value of 3.2 mm, ranging from 1.7 to 6.3 mm. Using an empirical land-based relationship results in a mean aerodynamic roughness length for surf-zone foam of 0.82 mm, with a range from 0.4 to 1.6 mm, which provides a reasonable estimate of the surf-zone drag coefficient compared with field measurements.

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This work is a continuation of the ONR funded Coastal Land Air Sea Interaction experiment (N0001417WX00612, N0001417WX01138). We recognize Mara Orescanin for her support in the effort. The Royal Australian Navy supported Ami Hansen for her M.S. at the Naval Postgraduate School, which was the basis of the present study. We thank Ed. Thornton and the three anonymous reviewers for improving the manuscript clarity.

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Correspondence to Jamie MacMahan.

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Hansen, A., MacMahan, J. Geometric Roughness Estimates of Surf-Zone Wave-Breaking Foam. Boundary-Layer Meteorol 170, 183–190 (2019).

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