Principal bed states during SandyDuck97: Occurrence, spectral anisotropy, and the bed state storm cycle

[1] Results are presented from 70+ days of nearly continuous in situ acoustic imagery of the nearshore sandy seabed in ∼3-m mean water depth, at two locations separated by 40-m cross-shore distance. The bottom sediments were 150 μm median diameter sand, with nearly identical size distributions at the two locations. Five principal bed states were observed: irregular ripples, cross ripples, linear transition ripples, lunate megaripples, and flat bed. The linear transition and flat bed states were the most frequent, together accounting for 68% of the total time. Bed state occurrence was a strong function of incident wave energy, each bed state occurring within a relatively narrow range of sea-and-swell energies. During the 12 major storm events spanned by the record, the bed response was characterized by a repeatable bed state storm cycle, involving four of the five principal states (lunate megaripples did not appear repeatedly, and thus may be a special case), with no obvious dependence of bed state occurrence on prior bed state, or on third-moment measures of wave nonlinearity. Radial spectra from the rotary acoustic images indicate pronounced differences in the anisotropy of spatial scales for the different bed states, and exhibit onshore-offshore differences which are likely related to ripple migration.