70X-10 and 71Y-10) the initial test length was the only variable; in the 

 other two pairs (experiments 72B-06 and 72B-10 and experiments 72A-06 and 

 72A-10) both initial test length and tank width varied, but the effects 

 of initial test length are distinguishable from the tank width effects. 



(1) Experiments 70X-06 and 71Y-06 (1.90-Second Wave) , In each 

 experiment the effect of re-reflection on the incident wave height was 

 the same, 0.03 foot (Table 13). However, the average incident wave 

 height was 0.34 foot in experiment 70X-06 and 0.37 foot in experiment 

 71Y-06, and the difference in incident height is likely due to the dif- 

 ference in the phase difference as a result of the 7-foot difference in 

 initial test length. 



The profiles in the two experiments developed similar shapes (Fig. 

 24), with the length of the inshore shelf the only difference, due pri- 

 marily to the 200-hour difference in the duration of the experiments. 

 However, the rate of shoreline recession was quite different (Fig. 34). 

 In experiment 70X-06 the shoreline recession rate was 0.06 foot per hour 

 between 1 and 22 hours, 0.14 foot (4.2 centimeters) per hour between 22 

 and 30 hours, 0.10 foot per hour between 30 and 44 hours, and there- 

 after. The backshore was artificially nourished after 54 hours, thus 

 maintaining the stable shoreline after that time. In experiment 71Y-06 

 the rate was 0.113 foot per hour between 1 and 15 hours and 0.025 foot 

 per hour thereafter (for 360 hours) . 



The differences in profile adjustment rates may have been caused by 

 the difference in initial test length; if so, the difference was not due 

 to re-reflection effects, since the higher recession rate was associated 

 with the lower incident wave height. It is unlikely that secondary waves 

 would have caused the difference in shoreline recession rates without 

 also affecting the profile shape and such profile shape differences were 

 not observed. 



(2) Experiments 70X-10 and 71Y-10 (1.90-Second Wave) . In each 

 of these experiments the effect of re-reflection on the incident wave 

 height was different, 0.02 foot in experiment 70X-10 and 0.06 foot in 

 experiment 71Y-10 (Table 13). However, the average incident wave height 

 was almost the same, 0.37 foot in experiment 70X-10 and 0.36 foot in 

 experiment 71Y-10, even though the initial test length had a difference 

 of 7 feet in the two experiments. 



The profiles in the two experiments developed similar shapes (Fig. 

 24), with the length of the inshore shelf the only difference, due pri- 

 marily to the 125-hour difference in the duration of the experiments. 

 However, the rate of shoreline recession was quite different (Fig. 34). 

 In experiment 70X-10 the shoreline recession rate was 0.08 foot per hour 

 between 12 and 62 hours, and thereafter because the backshore was re- 

 nourished to maintain a stable shoreline position. In experiment 71Y-10 

 the rate was 0.133 foot (4.05 centimeters) per hour (uniform laterally) 

 between 1 and 15 hours, 0.016 foot per hour (uniform laterally) between 



76 



