Wave-wave interaction feeds wave energy from the part of the wave 

 spectrum where it is received to both longer and shorter waves. Several 

 wave-wave interaction processes have been identified; all require the 

 preexistence of a range of wavelengths, and some depend on the three- 

 dimensional characteristics of the natural wave field. These mechanisms 

 become significant only at scaled fetch lengths unobtainable with wind- 

 generated laboratory waves when waves large enough for engineering studies 

 are required. Laboratory studies of wave-wave interaction, where a pro- 

 gramable wave generator is used to develop the desired range of wavelengths, 

 may be useful in further development of this concept. 



Wind-stress coefficients above a rigid boundary in the laboratory 

 decrease with fetch because the increase in boundary layer thickness leads 

 to a decrease in the intensity of the shear near the surface. This mech- 

 anism is effective only for short fetches, probably no more than a few 

 kilometers in the field. Wind-stress coefficients in the field also 

 appear to decrease with increasing fetch, but here the cause is varia- 

 tion in the stage of wave growth. This effect could be measured in a 

 well-designed laboratory experiment, but the decrease will not follow the 

 scaling laws expected to govern wave growth or wave forces. 



4. Summary . 



The growth of boundary layers on the sidewalls and ceiling, and above 

 the air-water interface, leads to a constriction of the airflow and a 

 pressure gradient in the direction of the airflow in wind tunnels of con- 

 stant cross section. This pressure gradient provides a contribution to 

 wave growth not present in nature. The importance of the pressure gradient 

 was not recognized before 1970, and has been neglected in the analyses of 

 most laboratory data dealing with the growth of waves and wind stress on 

 water. Boundary layer growth also leads to a reduction in the wind-stress 

 coefficient with fetch in laboratory experiments dealing with rigid bound- 

 aries. Laboratory studies of wind stress over water have generally con- 

 sidered only the mean stress between two designated positions in the wind 

 tunnel. Studies of wind-stress variability over natural water surfaces 

 also indicate a decrease in the wind-stress coefficient with increasing 

 fetch, but for different reasons than those applicable to laboratory 

 flows. 



Wave growth with fetch is rather slow in nature, and can be modeled 

 in the laboratory only for very low windspeeds or very short-scaled fetches. 

 Wave height obtained for very low windspeeds is too small for use in engi- 

 neering experiments. Large waves with natural characteristics can be 

 obtained only with the aid of programable mechanical wave generators. 



The microscale processes responsible for wave growth vary with fetch, 

 the wave spectrum, and the stage of wave growth. It seems unlikely that 

 all important processes can be modeled to scale in a single experiment. 



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