MEGARIPPLE MIGRATION IN A NATURAL SURF ZONE

Migrating megaripples are bedforms that appear in the surf zone of san dy 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), st eady-flow(3) and intertidal(4) environments, as well as in laboratory flume experiments(5). They affect overlying currents by introducing hy draulic roughness(4,6), and may provide a mechanism for sediment trans port(7,8) as well as forming sedimentary structures in preserved facie s(9,10). The formation, orientation and migration of such bedforms is not understood well(11,12). Dunes, for example, can be aligned with th eir crests perpendicular to steady unidirectional winds(13), but in mo re complex wind fields their orientation becomes difficult to predict( 14-17). Similarly, it is not known how sea-floor megaripples become al igned and migrate in the complex flows of the surf zone. Here we prese nt observations in the surf zone of a natural beach which indicate tha t megaripples do not migrate in the direction of the vector sum of the currents, but are aligned so that the sediment transport normal to th e bedform crest is maximized(17). This may need to be taken into accou nt in modelling morphology change and interpreting existing and fossil morphologic patterns.