In experiment 71Y-10 the -Oo 7-foot contour varied in position, but not 

 uniformly across the tank. The K^ in this experiment did not fluctuate 

 in response to any particular contour movements; however, with the com- 

 plex profile development, a lack of correlation is expected. These lateral 

 variations in profile development certainly contributed to a complex wave 

 reflection pattern, which appeared simple and less variable when only re- 

 corded along the center of the tank. 



IV. DISCUSSION OF RESULTS 



1. Wave Height Variability . 



Three probable causes of wave height variability in the two experi- 

 ments are: (a) Wave reflection from the changing profile, (b) re- 

 reflection from the wave generator, and (c) secondary waves. These 

 experiments were designed primarily to quantify the amount of variability 

 due to reflection. 



a. Wave Reflection From the Profile . The Kj? in the fixed-bed tanks 

 increased during the early hours of the experiments and decreased in the 

 later hours. In the narrower fixed-bed tank, the K^ was always higher. 

 The K/? in the movable-bed tanks varied from 0.08 to 0.30 in experiment 

 71Y-06 and from 0.03 to 0.18 in experiment 71Y-10 (Figs. 2 and 3). K^ 

 values during the development of the foreshore were relatively high, then 

 decreased as the remainder of the profile began to adjust. Later, after 

 the profile had developed a relatively steep offshore slope, the K^ in- 

 creased and the variation in Kj^ increased. The variations appear to 

 have been caused by small changes in depth near the seaward edge of the 

 inshore zone (the top of the offshore reflecting surface) and by the 

 gradual separation of the two reflecting surfaces as the offshore slope 

 prograded seaward (see Fig. 45, and Chesnutt and Galvin, 1974). 



b. Re-Reflection From the Generator . The reflected wave advanced to 

 the generator and was reflected. As the height of the reflected wave 

 varied, the height of the re-reflected wave varied; as the phase differ- 

 ence between the reflected wave and the generator motion varied with 

 changes in the profile, the height and phase of the re-reflected wave 

 varied. The height of the wave incident to the profile, which was the 

 average of wave heights along the full tank length and was composed of 

 the generated wave and the re-reflected wave, varied from 0.32 to 0.41 

 foot (9.8 to 12.5 centimeters) in experiments 71Y-06 and 71Y-10 (Tables 



6 and 7). Part of that variation (0.04 foot in experiment 71Y-06 and 

 0.03 foot in experiment 71Y-10) could be attributed to measurement errors, 

 variations in the generated wave, and all other errors not caused by a 

 changing profile. The remainder of the variation (0.05 and 0.06 foot) is 

 likely due to varying re-reflection. 



c. Secondary Waves . Along the length of the tank, between the gener- 

 ator and the toe of the profile, wave heights on a given recording varied 

 as the result of secondary waves. Galvin (1972) and Hulsbergen (1974) 



97 



