Cross-shore migration of lunate megaripples during Duck94

Abstract

Remote acoustic observations of lunate megaripple migration are presented from two storm events during the Duck94 nearshore dynamics experiment for time periods during which longshore currents were weak (<20 cm/s). During these periods, significant wave orbital velocities were 50-80 cm/s; megaripple heights were 0.05-0.5 m; megaripple horns were directed shoreward; the crest-to-trough slope of the shoreward advancing ripple face was close to the angle of repose; and migration speeds were 10-40 cm/h onshore. The observations also indicate that the megaripples stalled, and may have begun to migrate offshore, when the mean offshore flow exceeded 20 cm/s during the second storm. Stress-based bed load sediment transport models are moderately successful in predicting the observed dependence of migration velocity on measured fluid velocities separated into zero (mean current), inftragravity wave, and sea-and-swell wave frequency bands. Wave and mean current friction factors,fw, and fc, are obtained by best fit between the predicted and observed migration velocities, for two choices (3/2 and 5/2) for the stress exponent ξ in the bed load transport part of the model. Net transport is computed using wave velocity amplitudes determined from both run length statistics and wave by wave. Improved agreement with observations is obtained for the wave-by-wave net transport predictions and for the wave-like treatment of the infragravity band. The level of agreement is relatively insensitive to the value of ξ. The best fit current and wave friction factor values, for ξ = 5/2 and 3/2 are fc = 4.3 × 10-3 and 8.0 x 10-3 and fw = 1. 7 x 10-2 and 4.8 × 10-2 , respectively. Copyright 2004 by the American Geophysical Union.