Small-scale surf zone geometric roughness
Hansen, Ami S.
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Measurements of small-scale (O(mm)) geometric roughness (kϝ) associated with breaking wave foam were obtained within the surf zone on a sandy beach near Monterey, California. The kϝ is described by the vertical standard deviation of the foamy sea surface elevation and was estimated using stereo imagery techniques. A waterproof two-camera system with self-logging and internal power was developed using commercial-off-the-shelf components and commercial software for operations 1m above the sea surface within the surf zone. The kϝ of surf zone foam ranged from 1.7mm to 6.3mm with a mean of 3.2mm and confidence interval of 0.4mm for 57 stereo images; this is based on consistent area of 9cm² (3cmx3cm). The tested stereo vertical error is 0.43mm consistent with published errors for stereo cameras. kϝ is biased by the spatial area of estimate, with increasing geometric roughness occurring with increasing spatial area. This is associated with removal of a two-dimensional, two-element polynomial plane. The measured foam kϝ estimates are larger than the suggested bubble roughness of 2mm. Data fitting between the measured surf zone roughness and aerodynamic roughness suggests that the scaling parameter (β) between aerodynamic and geometric roughness is the same order of magnitude and very similar to land-based β estimates.
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