Field studies of sediment motion under waves can significantly 

 improve understanding. In particular, repetitive observations made 

 consistently at one site (e.g., Short, 1978) can provide insight into 

 the important processes which the engineer will actually meet, free of 

 possible scale effects. 



To summarize this section on sediment transport, it is evident that 

 wave-current interaction is a fundamental aspect of most littoral 

 sediment transport. Yet it is equally evident that a physically-based 

 prediction of this transport is lacking. Experimental results are 

 contradictory, and may even appear inconsistent with physical insight. 

 Model effects and experimental error are possible explanations. Some 

 confusion may be due to lack of common definition for the phenomena 

 under investigation, and it is always possible that appropriate 

 phenomena to be measured are yet undiscovered. Differences between 

 sediment transport in steady, uniform flow (the standard textbook 

 condition) and in quasi-periodic, unsteady flow under waves are 

 incompletely appreciated up to now. More well-defined data from field 

 and laboratory are needed. 



4. Field Data . 



Observations of current and wave properties suitable for comparison 

 with theoretical predictions of wave-current interaction are desirable. 

 Some observations have been reviewed in Section II, 9. The major 

 limitation in available data is the lack of detail for describing the 

 current field over which waves have propagated. Indeed, unless new 

 remote-sensing techniques are developed to give such information, the 

 best available data will be from point measurements of current. How- 

 ever, for many applications a computed current field is likely to be 

 more satisfactory, especially if it can be verified by comparison with 

 observed currents. 



Numerous current and wave observations have been made in coastal 

 waters. Many are from areas with strong, predictable tidal currents and 

 hence are suitable for comparison with theory. The Marine Information 

 Advisory Service, at the Institute of Oceanographic Sciences, Wormley, 

 England, maintains a worldwide register of such data. Data such as 

 these should be adequate for a large-scale study (i.e., where wave 

 propagation over distances of about 100 kilometers are considered) . A 

 study on a scale of 10 kilometers or less would require special 

 measurements . 



The problems of comparison between field data and theory are partly 

 revealed by Cardone, et al. (1981) who analyzed ocean waves measured in 

 the North Atlantic in connection with the GATE program. In that case 

 waves arrive from different directions and sources, and not all can be 

 identified. Comparisons involving current fields are made more 

 difficult by the directional dependence of the dispersion equation (Sec. 

 11, 2). This makes it particularly important for some sort of 

 directional measurements to be made. 



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