b. Reflection as a Result of Wave Breaking . On the concrete slab 

 the wave broke as a plunging breaker and on the movable-bed profile, the 

 wave was initially a less well-developed plunger and evolved to a spill- 

 ing breaker. The reflection from the concrete slab was an average 0.12 

 for both experiments, where the plionger is assumed to contribute more 

 significantly to the total reflection. The lowest values of reflection 

 in the movable-bed tanks were slightly lower than the K„ for the fixed 

 bed and occurred during the period when the wave broke by plunging. The 

 reflection from the spilling breaker later in the experiments is assumed 

 to be negligible. 



c. Effect of Inshore and Offshore . As the experiments proceeded, 

 the inshore widened and flattened and the offshore steepened. At first, 

 the widening of the inshore dominated; the lowering of the reflection 

 after the high initial values (Figs. 2 and 3) is attributed to the 

 greater energy dissipation in the inshore. The later steepening of the 

 offshore correlates well with the trend toward higher K^ later in the 

 experiments (compare the offshore contour positions in Figs. 8 and 12 

 with the appropriate reflection values in Figs. 2 and 3). 



With the development of the two reflecting zones (foreshore and off- 

 shore) separated by a relatively flat inshore zone, the measured reflected 

 wave was composed of two reflected waves. A change in phase or amplitude 

 of either reflected wave would change the phase and amplitude of the 

 measured wave. Part of the long-term K^ variability can be attributed 

 to the change in phase difference between these two reflected waves as 

 the foreshore retreated landward and the offshore built seaward. 



Chesnutt and Galvin (1974) examined results from experiment 71Y-06 

 and pointed out an apparent correlation between the movement of the -0.7- 

 foot contour and the variability of the reflection coefficient, and sug- 

 gested that the reflection is very sensitive to small changes in the depth 

 near the seaward edge of the inshore zone. These depth changes would 

 cause variability in the reflection of the incident wave from the offshore 

 slope and variability in the amount of energy trapped on the inshore shelf. 



The position of the -0.7- foot contour and the reflection coefficient 

 versus time for the two experiments are compared in Figure 45. The sea- 

 ward (downward) movement of the -0.7- foot contour in the figure is an 

 indication of the development of the steeper offshore slope. Both ex- 

 periments show a general increase in the reflection coefficient as the 

 -0.7- foot contour moved seaward (and the offshore slope increased). 



In experiment 71Y-06, the K^ values are highest at 320, 360, and 

 375 hours when the -0.7- foot contour is at the seawardmost position; the 

 K^ values are low at 225 and 335 hours when 4:he -0.7- foot contour is at 

 the landwardmost position. The same relationship exists at other times 

 (275, 290, and 300 hours), but the variation is not as great. A scatter 

 plot (Fig. 46) of K^ versus position of the seawardmost -0.7- foot con- 

 tour for all times after 220 hours indicate the correlation. 



94 



