Observations of megaripples in the surf zone

Bedform measurements were obtained under shoaling and breaking wave conditions using a pencil-beam sonar in the intertidal zone at Skallingen, Denmark. Measurements were made during four individual tidal cycles starting from flat bed conditions at low tide and as the tide rose and the instruments entered the surf zone, megaripples appeared when the mobility number exceeded 240, which equates to the classic flat-bed criterion proposed by Dingler and Inman (1976). The megaripples were oriented cross-shore and their wavelength scaled with the wave orbital diameter. Wave ripples were observed when the mobility number was smaller which occurred mainly under shoaling waves. Since all bedforms were erased as the seabed became dry at low tide and perpendicular longshore currents were occasionally strong, the megaripples were of small amplitude but their dimensions were consistent with predictions from the literature. Their migratory behaviour in the first few hours of existence appeared more strongly related to self-organisation rather than to ambient hydrodynamics. A clear gap in bedform spacing at λ = 120 cm was identified between wave ripples and megaripples.