in regular use, it would be desirable to see if inclusion of current 

 refraction improves accuracy. 



Many observations are anecdotal, e.g., Isaacs (1948). One large 

 class of such events concerns "giant" or "freak" waves when waves and 

 currents are in opposing directions. One area where severe wave damage 

 to large ships has been reported is in the Agulhas Current. Mallory 

 (1974) has studied some incidents in detail and reports cases of waves 

 subject to strong winds over a fetch of 2000 kilometers before meeting 

 the adverse current. Mallory suggests that interference with locally 

 generated shorter waves may also contribute to particularly steep- 

 fronted waves. See also the discussion and anecdotes of Draper (1975). 



Several authors have suggested that wave caustics may enhance wave 

 magnitude. It is commonly observed that over the Continental Shelf, 

 inside the Agulhas Current, wave conditions are significantly less 

 severe; Schumann (1976) provides wave observations from this area. 

 Sugimori (1973) also gives wave observations on a strong current, the 

 Kuroshlo, which show different wave conditions on and off the current. 

 Such strong changes in wave conditions are best interpreted as wave 

 caustics, and it is likely that in some places the pattern of caustics 

 forms a cusp with a focus of wave energy that increases the chance of a 

 "freak" wave. 



Remote sensing in the form of satellite photography (e.g.. Perry and 

 Shimke, 1965; Strong and DeRyke, 1973; Osborne and Burch, 1980), short 

 wave radar scanning, and high-frequency radio scattering can all provide 



wave information on large sea areas. Some verification of wave 

 refraction is possible as suggested by work of Mattie, Lichy, and Beal 

 (1980). The traces of internal waves show up particularly well in 

 aerial photography. It is very likely that the bands of rough water 

 which occur at their crests may be the same as the "tide rips" described 

 in Maury (Sees. 751 to 755, 1861). 



A number of experiments have been performed with capillary waves on 

 currents (e.g., Hughes and Stewart, 1961; Wu, 1979; Plant and Wright, 

 1980). These may appear to be of little interest to engineers. 

 However, they are important in that surface roughness (gravity-capillary 

 waves) is measured by some remote techniques, such as short wave radar. 

 An appreciable fraction of the wind stress can be supported, and 

 momentum transferred, by such surface roughness in moderate winds. 

 These short waves are strongly affected by the currents of the larger 

 scale wave motion. 



10. Nonlinear Effects in Refraction . 



For refraction calculations with finite-amplitude waves, it is 

 necessary to allow the currents and depth of water to be determined as 

 part of the solution. That means appropriate boundary conditions must 



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