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.
References
Ardhuin, F. (2006): Momentum balance in shoaling gravity waves: Comment on ’shoaling surface gravity waves cause a force and a torque on the bottom’ by K. E. Kenyon. J. Oceanogr., 62, this issue, 917–922.
Ardhuin, F., W. C. O’Reilly, T. H. Herbers and P. F. Jensen (2003): Swell transformation across the continental shelf. Part I: Attenuation and directional broadening. J. Phys. Oceanogr., 33, 1921–1939.
Corben, H. C. and P. Stehle (1977): Classical Mechanics. 2nd ed., Dover, New York, 389 pp., Ch. 11.
Elgar, S., T. H. C. Herbers and R. T. Guza (1994): Reflection of ocean surface gravity waves from a natural beach. J. Phys. Oceanogr., 24, 1653–1658.
Hasselmann, K., T. P. Barnett, E. Bouws, H. Carlson, D. E. Cartwright, K. Enke, J. A. Ewing, H. Gienapp, D. E. Hasselmann, P. Kruseman, A. Meerburg, P. Muller, D. J. Olbers, K. Richter, W. Sell and H. Walden (1973): Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP). Deutsches Hydrographisches Institut, Hamburg.
Kenyon, K. E. (1971): Wave refraction in ocean currents. Deep-Sea Res., 18, 1023–1034.
Kenyon, K. E. (2004a): Shoaling surface gravity waves cause a force and a torque on the bottom. J. Oceanogr., 60, 1045–1052.
Kenyon, K. E. (2004b): Force and torque on a wall from reflected surface gravity waves. Phys. Essays, 17, 95–102.
Kenyon, K. E. and D. Sheres (1996): Angular momentum and action in surface gravity waves: application to wave-current interaction. J. Geophys. Res., 101, 1247–1252.
Kenyon, K. E. and D. Sheres (2006): Wave force on an ocean current. J. Phys. Oceanogr., 36, 212–231.
Munk, W. H., G. R. Miller, F. E. Snodgrass and N. F. Barber (1963): Directional recording of swell from distant storms. Proc. Roy. Soc., A, 255, 505–584.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Kenyon, K.E. Shoaling waves: A discussion. J Oceanogr 62, 923–927 (2006). https://doi.org/10.1007/s10872-006-0110-2
Received:
Revised:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10872-006-0110-2