0.08. Between 20 and 130 hours the K^ was higher and highly variable, 

 varying between 0.08 and 0.27. For the remainder of the experiment, K^ 

 was lower and less variable, varying between 0.07 and 0.10. 



The lower values during the first 15 hours occurred while the fore- 

 shore developed (slower than in the other four experiments) and began 

 retreating, the longshore bar developed and then eroded, and the breaker 

 type was strictly plunging (Table 8) . Between 20 and 130 hours, when the 

 K^ was high and varied greatly, the foreshore was retreating (except for 

 advancing between 125 and 130 hours), the breaker was mixed between plung- 

 ing and spilling (indicating minimal reflection), the inshore was becoming 

 longer and flatter, and the offshore was steepening, particularly after 

 95 hours. Between 135 and 180 hours, when K^ was smaller and less 

 variable, the foreshore was stationary, the offshore zone continued to 

 prograde seaward, and the inshore zone changed from an almost flat shelf 

 with an average elevation of -0.7 foot to a flat region at the seaward 

 end of the inshore (elevation -0.8 foot) and a trough at the shoreward 

 end of the inshore (elevation -1.3 feet). 



Some K^ variations after 100 hours, when the offshore slope was a 

 significant reflector, correlate well with the movement of the -0.7-foot 

 contour (Fig. 7). When the -0.7- foot contour was at a more seaward posi- 

 tion, K„ was high; when the contour moved shoreward, K„ was low. The 

 Kfl reached higher values quicker than in the first four experiments, 

 even though the initial slope was flatter. This earlier high in K^> 

 may have been caused by the earlier seaward movement of the -0.7-foot 

 contour in this experiment. 



The Kn was measured over the inshore shelf several times between 

 100 and 155 hours and varied between 0.06 and 0.12 (see Vol. IV). This 

 measurement included reflection both from the foreshore zone and from 

 the plunging breaker near the toe of the foreshore. The distance be- 

 tween the 0- and -1.5-foot contours, after the shelf developed, increased 

 12.4 feet (3.8 meters), enough for more than two 360° phase-difference 

 changes. 



(6) Summary of the Five Experiments . The average K^> in each 

 of the 1.90-second experiments with the 0.10 slope (70X-06, 70X-10, 

 71Y-06, and 71Y-10) varied from 0.07 to 0.09 (Table 2). However, in 

 experiment 72D-06 with the flatter initial slope, the average K^> was 

 0.12, much higher than the tests with the steeper initial slope, con- 

 trary to the hypothesis that as the ratio of the wave steepness to the 

 slope steepness increases, the Kn decreases. The close correlation 

 between the -0.7- foot contour and Kr> variations in experiments 71Y-06 

 and 72D-06 suggests that the elevation of the top of a steep, submerged 

 slope can be as important as the steepness of the slope in determining 



the K 



•i?- 



d. Reflection of the 2.35-Second Wave . 



(1) Experiment 72B-06 . The K^ versus time for experiment 

 72B-06 is shown in Figure 8(a). During the first 10 hours, K„ vari 



33 



