Observations of seafloor ripples and a time‐dependent ripple geometry model

Abstract

Recent seafloor acoustic scattering experiments and theoretical developments have shown that the presence of ripples on the seafloor can increase the amount of subcritical angle acoustic penetration into the seafloor. A ripple observation and acoustic scattering experiment (SAX04) was recently conducted near Fort Walton Beach, Florida. During this experiment a sidescan sonar on an REMUS AUV was used to observe ripple geometry from the nearshore (5‐m depth) to the shelf break (50‐m depth). The ripples were found to increase in wavelength from 20 cm in the nearshore to 1.2 m at the shelf break. The offshore ripples were a product of Hurricane Ivan, which had passed the site 2 weeks prior. Time series of ripple geometry, measured with a tripod‐mounted rotary sidescan sonar and a two‐axis rotary pencil beam sonar, from a coarse sand site at the Martha’s Vineyard Coastal Observatory, also revealed long‐wavelength (0.6 to 1.2 m) relict ripples after wave events. Ripple geometry models that assume the ripples are in equilibrium with hydrodynamic forcing underpredict the wavelength of the relict ripples at both sites; thus a new model that allows for a time delay between ripple adjustment and forcing was developed. [Work supported by ONR.]