breaker, and reflection of the incident wave from the movable bed, partic- 

 ularly where the depth over the movable bed changes significantly. Depth 

 changes are significant if the depth difference is an appreciable fraction 

 of the average depth over a horizontal distance less than a wavelength. 

 For conditions of these experiments, the wavelength is 14.3 feet (4.36 

 meters) in the section seaward of the movable bed, and approximately 9 feet 

 (2.74 meters) over the inshore zone. 



a. Reflection from the Foreshore . The foreshore zone developed a 

 relatively stable slope within the first hour of testing, well before the 

 other elements of the movable-bed profile had become prominent. The devel- 

 oped foreshore had an average slope of 0.19 in experiment 70X-06 and 0.20 

 in experiment 70X-10 which is considerably steeper than the initial 0.10 

 slope of the movable bed. The initial high values of K^ are probably 

 the result of reflection from the foreshore of waves which dissipated 

 relatively little energy until almost at the foreshore. Reflection from 

 the foreshore is a function of the height of the wave reaching the fore- 

 shore, and this height would diminish as the inshore and offshore segments 

 of the profile (Fig. 9) became prominent. 



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 spilling 

 breaker. The concrete slab had the same slope (0.10) as the initial slope 

 of the movable bed. Because the total reflection was significantly less 



on the concrete slab (K^ = 0.05) where the plunger is assumed to contribute 

 relatively more to the total reflection, it is likely that reflection from 

 the movable bed by breaking was never very important, and became less impor- 

 tant, as the breaker type changed to spilling. 



c. Effect of Inshore and Offshore . As the experiment 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. 11 and 15 with the appropriate reflec- 

 tion curve 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 waves (one from the offshore, the other from the 

 foreshore). 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 off- 

 shore built seaward. 



Chesnutt and Galvin (1974) pointed out an apparent correlation between 

 the movement of the -0.7-foot contour and the variability of the reflection 



77 



