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- Title
Time-sequence observations of wave-formed sand ripples on an ocean shoreface.
- Authors
Boyd, R.; Forbes, D. L.; Heffler, D. E.
- 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 behavior. 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 λ=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. Ripple were observed to migrate both on-and off-shore (with and against the wave advance direction) at rates up ±0.1 m h[SUP-1], associated with net flows other than wave-induced onshore asymmetry and mass transport. Migration (mainly of ripple 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 ripple 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.
- Subjects
RIPPLE-marks; SAND; OCEAN wave power
- Publication
Sedimentology, 1988, Vol 35, Issue 3, p449
- ISSN
0037-0746
- Publication type
Article
- DOI
10.1111/j.1365-3091.1988.tb00997.x