Wave Dissipation on a Barred Beach: A Method for Determining Sand Bar Morphology.

摘要

A technique is presented to remotely measure the scales and morphology of natural sand bars based on the preferential dissipation of wind waves and swell over the shallows of the bar. Images are recorded photographically or with video and statistical uncertainties associated with incident wave height modulations removed by averaging (time exposures). Ground-truth testing of the technique was carried out as part of the SUPERDUCK experiment in October 1986. The time exposures generally provided a good mapping of underlying morphology, allowing detection of the bar and determination of cross-shore and longshore length scales. However, during high wave, residual foam obscures the relationship of image intensity to local dissipation (modeled theoretically by dissipation of a random wave field), and an enhancement technique of image differencing must be done to remove the bias of persistent form. Errors in bar location should be less than 35 percent, but this wave depends on the geometry of the particular bar. Logistic simplicity and quantitative capabilities should make this technique very attractive. Keywords: Image time exposure; Photogrammetry; Sand bar morphology and scale; Superduck experiment; Video differencing; Wave breaking modeling; Wave shoaling. (kt)