Edge waves on complex beach profiles
In order to interpret field data in terms of edge wave modes, investigators usually assume that the beach profile is linear, allowing the use of simple analytical solutions for edge wave structure and wavelength. The validity of this assumption is checked by using a numerical model to find the edge wave modes on a typical concave beach. Results show that estimates of edge wave wavelength using the plane beach assumption can be wrong by +- 100% at a fixed mean sea level with a further error of +- 50% introduced by the tides. Greatly improved estimates can be made if the value of beach slope is chosen to reflect suitably the offshore extent of the edge wave. A rule of thumb for calculating the effective beach slope ..beta../sub e/ is ..beta../sub e/=h (x')/x',x'=1/11(2n+1) L, where h is the water depth at distance x' from shore, L is the edge wave wavelength, and n is the edge wave mode number. The motion of the offshore profile of the edge wave due to a change of tidal elevation is dramatically different on a concave beach than on a plane beach. The tendency for the profile to move offshore for a drop in tide is compensated by the decrease in mean beach slope and hence reduction in wavelength and offshore extent of the wave. For a fixed instrument a drop in tide may lead to a profile shift onshore or offshore or no shift at all. Edge wave damping on a concave beach is shown to change as a function of tide. At low tide the surf zone width is increased while edge wave wavelength is decreased, both factors increasing damping. Thus edge wave spectra taken at low tide may be less energetic than those at high tide, particularly at high frequencies.
- Research Organization:
- Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1
- OSTI ID:
- 5874846
- Journal Information:
- J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 84:C10; ISSN JGREA
- Country of Publication:
- United States
- Language:
- English
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