coefficient, and suggested 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 tra.p- 

 ped 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 Figures 38 and 39. 

 The seaward movement of the -0.7-foot contour is an indication of the 

 development of the steep offshore slope. Both figures show the general 

 increase in the reflection coefficient as the -0.7-foot contour moved sea- 

 ward. In Figure 38 (experiment 70X-06) , the same kind of fluctuations 

 (but not as great) in the contour position and reflection coefficient 

 appear as pointed out by Chesnutt and Galvin (1974) . The large fluctua- 

 tions in the reflection coefficient near the end of experiment 70X-10 

 (Fig. 39) are not matched by large movements in the -0.7-foot contour. 

 However, some lateral variation in the shape of the inshore and offshore 

 zones occurred in the 10-foot tank (Fig. 40) which would cause lateral 

 variations in reflection, and might have confused the reflection measure- 

 ments in the center of that tank. 



IV. DISCUSSION OF RESULTS 



1. Wave Height Variability . 



Three probable causes of wave height variability in the two experiments 

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

 from the wave generator, and (c) secondary waves. Preliminary experiments 

 had indicated that wave reflection from the movable-bed profile was the 

 major cause of wave height variability and these experiments were conducted 

 primarily to quantify the amount of variability due to reflection. 



a. Wave Reflection from the Profile . The K^ varied from 0.08 to 

 0.20 in experiment 70X-06 and from 0.04 to 0.19 in experiment 70X-10. The 

 Kj^ 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^ 

 increased and the variation in K^ increased. The variations appear to 

 have been caused by changes in the shape of the second reflecting surface 

 and by the gradual separation of the two reflecting surfaces as the off- 

 shore slope prograded seaward (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 difference 

 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 0.38 foot in experiment 

 70X-06 and from 0.34 to 0.39 foot in experiment 70X-10. Part of that 



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