16. For instance, one could imagine a directional distribution wherein 

 the main (or peak) wave energy was shore normal approaching a uniform, plane 

 beach, while a lesser amount of energy was at some angle other than normal. 

 If the peak direction were used in a unidirectional model of shoreline re- 

 sponse, the result would be entirely in the form of cross -shore sediment 

 transport. However, if the complete wave field were considered, the smaller 

 waves at an angle to the beach would induce a small, persistent longshore 

 current. The sediment being lifted by the stronger, shore -normal waves would 

 then be transported longshore by the currents induced by the smaller waves. 

 Over time, the net longshore transport could be substantial; that is, unidir- 

 ectional guidance based on the direction of peak wave energy would give an 

 answer that is incomplete in a potentially important way. 



Laboratory studies 



17. A well-described quantification of effects of wave directionality 

 at field sites has not been done. Lacking this, perhaps the most profound 

 indications of wave directional effects are the results of two laboratory 

 experiments, one reported by Vincent and Briggs (1989), and the other by 

 Kaihatu and Briggs (in preparation). In the first experiment, wave fields 

 with different distributions of frequency and direction were propagated over a 

 submerged mound in the directional wave basin at CERC . Effects of wave 

 directionality were observed by superposition of many wave trains traveling in 

 a range of directions (a measure of which is called the directional spread) 

 which varied from run to run. The ratio of total wave energy in the lee of 

 the mound to that in front of the mound was used as a measure of system 

 response . In all cases , it was observed that the greater the directional 

 spread of the attacking waves, the lower the local relative energy was in the 

 lee of the mound. For wave fields broadening from unidirectional to a 60-deg 

 spread, the relative energy dropped by a factor of about four, a very substan- 

 tial difference. The results suggest that if this amount of spread is a 

 characteristic of real wave fields, designs based on unidirectional studies 

 could overcorrect for wave attack. While the results are specific to the case 

 tested, it shows the degree of variability of system response in directionally 

 distributed seas. If thought of as an example of wave focusing by a mound of 

 placed, dredged material, these results could influence decisions concerning 

 mound shape, distance offshore of mound placement, and the risk of mitigating 

 beach erosion induced by mound- focused waves. 



