Megaripple migration in a natural surf zone

Migrating megaripples are bedforms that appear in the surf zone of sandy coasts1. With heights of 0.1–0.5 m and wavelengths of 1–5 m, they are similar in size and shape to small dunes, large ripples, or sand waves. Such sedimentary bedforms have been studied in subaerial2, steady-flow3 and intertidal4 environments, as well as in laboratory flume experiments5. They affect overlying currents by introducing hydraulic roughness4,6, and may provide a mechanism for sediment transport7,8 as well as forming sedimentary structures in preserved facies9,10. The formation, orientation and migration of such bedforms is not understood well11,12. Dunes, for example, can be aligned with their crests perpendicular to steady unidirectional winds13, but in more complex wind fields their orientation becomes difficult to predict14,15,16,17. Similarly, it is not known how sea-floor megaripples become aligned and migrate in the complex flows of the surf zone. Here we present observations in the surf zone of a natural beach which indicate that megaripples do not migrate in the direction of the vector sum of the currents, but are aligned so that the sediment transport normal to the bedform crest is maximized17. This may need to be taken into account in modelling morphology change and interpreting existing and fossil morphologic patterns.