Cross-shore migration of Lunate megaripples and bedload sediment transport models

Abstract

Rotary fan-beam and pencil-beam sonar images collected during the DUCK94 field experiment are used to determine the geometrical scales, orientations, and cross-shore migration velocities of cross-shore oriented lunate megaripples. The measurements show that these megaripples ranged from 0.05 to 0.5 m in height, 0.5 to 4.0 m in separation-length and 0.25 to 2.75 m in both span and spacing. There is a good correlation between both megaripple heights and separation-lengths, and megaripple span and spacings. The ratio of height to separation-length ranged from 0.12 to 0.13. Megaripple scales are somewhat correlated with both the flow energy and wave-orbital excursion. Megaripples in general migrated onshore at speeds of 10-40 cm/h. This onshore migration was opposite to the offshore-directed mean cross-shore velocity, U, for |U| < 20 cm/s. Megaripple migration stalled as U approached 20 cm/s in the offshore direction. Offshore migration appeared to occur for higher speed offshore mean flows. Wave-orbital velocity skewness and mean velocity are weakly correlated to migration velocity. The measured lunate megaripple "lee" face slopes are consistent with the angle of repose. -- The observed migration velocities are reproduced reasonably well by stress-based bedload sediment transport models using the measured local waves and currents and best-fit values of the wave/current friction factor ratio. The model/data comparisons are not very sensitive to the stress exponent in the transport models. The values of the current friction factor fc, and the wave/current friction factor ratio fw/fc, giving the best fit to the migration velocities ranged from 4.3×10⁻³ to 8.0×10⁻³, and 2 to 15, respectively. Infragravity waves are found to contribute mainly to the offshore transport, and treating infragravity velocities as waves gives about 10% better comparison compared to treating these motions as slowly varying currents. The megaripple orientations are better correlated with the direction of net sediment transport than that of the gross bedform-normal sediment transport.