Abstracts Earth and Environmental Sciences

Megaripple migration in the nearshore

by Angela Holcombe. Walker

Seafloor morphology in the surf zone of a sandy ocean beach was measured nearly continuously for 2 months with a 1.4 X 1.4 m coherent array of 7 sonar altimeters. Migrating megaripples, seafloor bedforms with amplitudes of O(10-50 cm) and lengths of O(1-5 m), were observed in about 2 m water depth in the trough between a sand bar and the shoreline for a wide range of wave and current conditions. Megaripple migration speed and direction are estimated from the array data using cross-correlations between seafloor elevation time series observed along the cross- and alongshore array legs. Megaripples were shown to be aligned in a direction that maximized the gross sediment transport normal to the bedform crest (Rubin and Hunter, 1987; Gallagher, et al. 1998). It is hypothesized that megaripple migration rate is related to the net transport in the direction of bedform alignment. The speed of megaripple migration is compared with the magnitude of the velocity field normal to the bedform crest in the direction of the mean, wave orbital, and resultant velocities.