Time‐sequence observations of wave‐formed sand ripples on an ocean shoreface

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Abstract
Analysis of an 18‐day time‐lapse film record of shoreface ripple development, with concurrent measurements of near‐bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low‐energy summer wave conditions. Significant wave‐height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short‐wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short‐crested ripples; (5) long‐wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on‐ and off‐shore (with and against the wave advance direction) at rates up to ±0–1 m h‐1, associated with net flows other than wave‐induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near‐bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight‐crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation and crest termination in types 2,3,4 and 6.

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