Shoaling waves: A discussion |
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Authors: | Kern E Kenyon |
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Institution: | (1) 4632 North Lane, Del Mar, CA 92014-4134, USA |
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Abstract: | Surface gravity waves are commonly observed to slow down and to stop at a beach without any noticeable reflection taking place.
We assume that as a consequence the waves are continuously giving up their linear and angular momenta, which they carry with
them, along with energy, as they propagate into gradually decreasing mean depths of water. It takes a force to cause a time
rate of decrease in the linear momentum and a torque to produce a time rate of decrease in the angular momentum. Both a force
and a torque operate on the shoaling waves, due to the presence of the sloping bottom, to cause the diminution of their linear
and angular momenta. By Newton’s third law, action equals reaction, an equal but opposite force and torque are exerted on
the bottom. No other mechanisms for transferring linear and angular momenta are included in the model. Since the force on
the waves acts over a horizontal distance during shoaling, work is done on the waves and energy flux is not conserved. Bottom
friction, wave interaction with a mean flow, scattering from small-scale bottom irregularities and set-up are neglected. Mass
flux is conserved, which leads to a shoreward monotonic decrease in amplitude consistent with available swell data. The formula
for the time-independent force on the bottom agrees qualitatively with observations in seven different ways: four for swell
attenuation and three for sediment transport on beaches. Ardhuin (2006) argues against a mean force on the bottom that is
not hydrostatic, mainly by using conservation of energy flux. He also applies the action balance equation to shoaling waves.
Action is a difficult concept to grasp for motion in a continuum; it cannot be easily visualized, and it is not really necessary
for solving the shoaling wave problem. We prefer angular momentum because it is clearly related to the observed orbital motion
of the fluid particles in progressive surface waves. The physical significance of wave action for surface waves has been described
recently by showing that in deep water action is equivalent to the magnitude of the wave’s orbital angular momentum (Kenyon
and Sheres, 1996). Finally, Ardhuin requires that there be a significant exchange of linear momentum between shoaling waves
and an unspecified mean flow, although the magnitude and direction of the exchange are not predicted. No mention is made of
what happens to the orbital angular momentum during shoaling. Mass flux conservation is not stated. |
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Keywords: | Shoaling waves bottom force linear momentum balance bottom torque angular momentum balance |
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