Chesnutt and Galvin (1974) examined the 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 depth 

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

 cause variability in the reflection of the incident wave from the off- 

 shore slope and variability in the amount of energy trapped on the in- 

 shore shelf. 



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

 versus time for experiment 72D-06 are compared in Figure 21. The initial 

 seaward (downward) movement of the -0.7-foot contour is an indication of 

 the development of the flat inshore shelf and steeper offshore slope. 

 The K/f increased as the shelf developed. The decrease in K/^ between 

 60 and 85 hours is possibly due to a phase difference change. The increase 

 of K^ to a maximum coincided with the large seaward movement of the 

 -0.7-foot contour, and the steepest offshore slope at 100 hours. The 

 decrease in K^ at 135 hours coincided with the shoreward movement of 

 the -0.7-foot contour and the decrease in the offshore slope steepness. 



IV. SUf-IMARY OF RESULTS 



1. Wave Height Variability . 



Three possiT^le causes of wave height variability in experiment 72D-06 



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



from the wave generator, and (c) secondary waves. This experiment was 



designed primarily to quantify the amount of variability due to reflec- 

 tion. 



a. Wave Reflection from the Profile . The K^ varied from 0.04 to 

 0.27 in this experiment. K^ values were low initially and increased as 

 the foreshore slope and inshore shelf developed. Later, as the offshore 

 reflecting surface became much steeper, the K^ increased in mean value 

 and variability. The large variations appear to have been caused by the 

 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 

 (Chesnutt and Galvin, 1974). 



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



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

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

 difference between the reflected wave and the generator motion varied 

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

 wave v^yied. 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 0.34 to 0.42 

 foot (10.4 to 12.8 centimeters) in the movable-bed tank. Part of that 

 variation (0.03 foot) can be attributed to measurement errors or to vari- 

 ations in the generated wave. The remainder of the variation (0.05 foot) 

 is likely due to re-reflection. 



49 



