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

Abstract

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.