Geometry, migration, and evolution of wave orbital ripples at LEO-15

Observations of the temporal evolution of the geometric properties and migr ation of wave- formed ripples are analyzed in terms of measured suspended s and profiles and water velocity measurements. Six weeks of bedform observat ions were taken at the sandy (medium to coarse sized sand) LEO-15 site loca ted on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wave lengths of up to 100 cm and with 15 cm amplitudes were observed. The predom inant ripples were found to be wave orbital scale ripples with ripple wavel engths equal to 3/4 of the wave orbital diameter. Although orbital diameter s become larger than 130 cm during the maximum wave event, it is unclear if a transition to non-orbital scaling is occurring. Ripple migration is foun d to be directed primarily onshore at rates of up to 80 cm/day. Suspended t ransport due to wave motions, calculated by multiplying acoustic backscatte r measurements of suspended sand concentrations by flow velocity measuremen ts, are unable to account for a sufficient amount of sand transport to forc e ripple migration and are in the opposite direction to ripple migration. T hus it is hypothesized that the onshore ripple migration is due to unobserv ed bedload transport or near-bottom suspended transport. Bedload model calc ulations forced with measured wave velocities are able to predict the magni tude and direction of transport consistent with observed ripple migration r ates. Sequences of ripple pattern temporal evolution are examined showing m echanisms for ripple directional change in response to changing wave direct ion, as well as ripple wavelength adjustment and erosion due to changing wa ve orbital diameter and relative wave-to-current velocities.