hours. After 205 hours in experiment 71Y-10 the recession rate increased 

 to 0.025 foot per hour along the sides of the 10-foot tank while remain- 

 ing at 0.016 foot per hour in the center. Figure 44 compares the shore- 

 line movement along the center ranges of the two tanks and shows that the 

 erosion rate was slightly greater in the 6- foot tank. 



(2) Inshore and Offshore Zones . In both experiments along a 

 given range, the sequence of events in profile development was the same: 

 development of a longshore bar, deposition in the offshore zone, seaward 

 movement of the breaker, erosion of the bar, and development of the shelf. 

 In the narrower tank, this development occurred along all ranges almost 

 simultaneously; in the wider tank, it occurred first along range 9 and 

 then progressed slowly across the tank. This unusual development caused 

 significant lateral variations in breaker depth, breaker type, and lit- 

 toral currents. The slower development is further amplified by the fact 

 that the center range in the 6-foot tank (solid line in Fig. 44) was rep- 

 resentative of all three ranges; whereas, the dashline in Figure 44 was 

 the mean value of contour position in the 10-foot tank and this mean was 

 more representative of changes along range 1 where the development was 

 slower than the mean, 



2. Profile Reflectivity . 



The basic profile shapes which evolved during the profile development 

 are shown in Figure 6. Early profiles (solid line in Fig. 6) had a steep 

 foreshore, a short inshore with a longshore bar formed by the plunging 

 breaker, and a gently sloping offshore zone. Later profiles (dashline in 

 Fig. 6) also had a steep foreshore, but the inshore widened to a long, 

 flat shelf which terminated in a relatively steep offshore zone. 



Chesnutt and Galvin (1974) discussed the processes which reflect wave 

 energy from movable beds in these experiments. The processes include the 

 conversion of potential energy stored in runup on the foreshore into a 

 seaward-traveling wave, the seaward radiation of energy from a plunging 

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

 ticularly 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 approxi- 

 mately 9 feet (2.74 meters) over the inshore zone. 



a. Reflection From the Foreshore . The foreshore zone developed 

 within the first hour of testing, well before the other elements of the 

 movable-bed profile had become prominent. The developed foreshore had a 

 slope of about 0.20, considerably steeper than the original 0.10 slope. 

 The initial high values of K^ are probably the result of reflection 

 from the foreshore of waves which dissipated little energy until almost 

 at the foreshore. Reflection from the foreshore is a function of the 

 height of the wave reaching the foreshore, and this height would diminish 

 due to increased bottom friction as the inshore and offshore segments of 

 the profile (Fig. 6) became prominent. 



92 



