The Idea of stopping waves by an adverse current has been employed 

 in pneumatic and hydraulic breakwaters. Submerged buoyant jets of 

 water or air cause an outflowing surface current which can stop waves 

 up to some limiting wave period. They are ineffective for shallow-water 

 waves. Extensive experiments were conducted by Bulson (1963, 1968). 

 See also Evans (1955) for experiments, Green (1961) for an application, 

 and Taylor (1955) and Brevik (1976) for theory applied to deep and 

 finite-depth water, respectively. In most practical cases, the velocity 

 variation with depth must be accounted for. 



Coastal engineering implications of interactions in a uniform 

 current are significant. In general, the result of the interaction is 

 not a simple superposition of the effects of currents and waves, each 

 considered separately. Section III of this report deals with 

 engineering applications, particularly forces on structures (Section 

 III, 2) and sediment transport (Section III, 3). To emphasize the 

 importance of this interaction, a different, and usually neglected, 

 application is considered in the following paragraphs; namely, the 

 effect of the interaction on the basic characteristics of measured 

 waves . 



As shown by Figure 2, the wave number k (the horizontal axis) is 

 quite sensitive to the addition of a uniform current (the slope of 

 ui-ku) . Ignoring the effect of a current can introduce significant error 

 in coastal engineering analyses, particularly in the use of bottom- 

 mounted pressure gages to measure waves. Reduction of data from such 

 gages requires the transfer of wave properties from bottom to surface. 

 The wave number is used to evaluate terms in the transfer like cosh kd 

 or e- . When kd has value near 1 or greater, errors in k due to 

 neglecting currents are much amplified. This is clearly shown by 

 comparing the results with and without a current, as was first done by 

 Jonsson, Skougaard, and Wang (1970). The point is illustrated 

 graphically in Figure 3, and specific examples are given in the Table, 

 both from Peregrine (p. 25, 1976). 



Table. Minimum period of waves for which a current of 0.5 meter per 



second may be ignored in calculating surface amplitudes from 

 bottom pressure measurements if errors are to be less than 5 

 and 20 percent (from Peregrine, 1976). 



Depth (m) 1 2 5 10 100 



Period (s) (with 5 percent error) 4.5 5.4 6.9 8.0 14 

 Period (s) (with 20 percent error) 2.7 3.2 4.3 5.3 11 



The individual components of a wave spectrum are affected in the 

 same way as an individual wave train. Several workers have formally 

 transformed spectra by using the Doppler relation and refraction theory 



20 



