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Low-energy bedload transport by combined wave and current flow on a southern mid-Atlantic bight shoreface
Green, Malcolm O.
Green, Malcolm O.
Abstract
The control of the mean flow, wind-wave energy and wind-wave asymmetry on low-energy bedload transport magnitude, direction and divergence on a wave-dominated shoreface was investigated. The objective was to develop a mechanistic basis for postulated fairweather replenishment of the beach sediment prism by sediments derived from offshore. Fairweather currents were measured at 17.5-m and 22-m depth. Periods of competent flow were identified using a combined-flow boundary-layer model. Madsen and Grant's (1976) combined-flow bedload-transport model was used to calculate sediment fluxes after modification of the expression for instantaneous transport rate to include a threshold criterion; this was necessary since sediment is not in motion over the whole wave period in low-energy conditions. The asymmetry thus introduced into the instantaneous transport rate resulted in partial control over transport direction by the wave. When the flow was competent, oscillatory motion contributed &>&80% of the total skin friction. Net transport direction was controlled by: relative orientation of the wave and mean flow; orbital velocity asymmetry; and the threshold criterion. Threshold-induced rotation of the net transport vector was significant on the lower shoreface where the flow was not intense and the wave-orbital velocity not greatly skewed. Bedload-flux divergence associated with each observed event was simulated using a one-dimensional numerical model. Two scenarios were identified: (1) offshore transport seaward of 10-m depth under the dominant influence of the mean flow, and onshore transport at the top of the shoreface under the dominant influence of shoreward-skewed wave-orbital velocities, and (2) onshore transport at every point on the profile. Direct observations of currents and upper-shoreface accretion in 8-m depth (a total of 6 cm in 4.5 days) were consistent with the model simulations, with the mean flow controlling net transport direction. Observations and model calculations were consistent with the concept of fairweather nourishment of the beach sediment prism by sediment transported from offshore, however the mean flow may play a more important role than previously recognized.
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1987
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8803969.pdf
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Virginia Institute of Marine Science
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https://dx.doi.org/doi:10.25773/v5-p3h6-q242