Megaripple migration in a natural surf zone

Migrating megaripples are bedforms that appear in the surf zone of sandy coasts(1). With heights of 0.1 -0.5 m and wavelengths of 1-5m, they are similar in size and shape to small dunes, large ripples, or sand waves. Such sedimentary bedforms have been studied in subaerial(2), steady-flow(3) and intertidal(4) environments, as well as in laboratory flume experiments(5). They affect overlying currents by introducing hydraulic roughness(4,6), and may provide a mechanism for sediment transport(7,8) as well as forming sedimentary structures in preserved facies(9,10). The formation, orientation and migration of such bedforms is not understood well(11,12). Dunes, for example, can be aligned with their crests perpendicular to steady unidirectional winds(13), but in more complex wind fields their orientation becomes difficult to predict(14-17). 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 maximized(17). This may need to be taken into account in modelling morphology change and interpreting existing and fossil morphologic patterns.