indicates the steepening and increasing of the reflectivity of the off- 

 shore zone. The highest K^ values, at 235, 320, 360, and 375 hours, 

 occurred at times when the -0.7- foot contour was at its seawardmost 

 position; low K^ values at 195, 240, and 340 to 355 hours occurred 

 when the -0.7- foot contour was at more shoreward positions. An exception 

 to this occurred at 270 to 275 hours, when the -0.7- foot contour was in 

 a seaward position and the K^> was low. At other times the relation- 

 ship existed, but the variation was not as great. 



The continued separation of the foreshore and offshore zone would 

 have caused the phase difference between the two reflected waves to vary 

 and the measured reflected wave to have a long-period variation. After 

 the shelf developed, the distance between the 0- and the -1.5-foot con- 

 tour increased 8.6 feet (2.6 meters), enough for two cycles of phase- 

 difference change, which may have contributed to some of the long-term 

 Kft variation. 



(4) Experiment 71Y-10 . K^ versus time for experiment 71Y-10 is 

 shown in Figure 6. During the first 10 hours, K^> varied from 0.05 to 

 0.11. Then, for 195 hours the K^> remained relatively low, varying from 

 0.01 to 0.08. For the remainder of the experiment, K^, was generally 

 higher, varying from 0.05 to 0.13. 



The higher K^> values during the first 10 hours occurred while the 

 foreshore and longshore bar were developing, the breaker was plunging, 

 and the foreshore was eroding (Table 7). Between 10 and 205 hours, while 

 K ff was low, the foreshore retreated at a rate of 0.016 foot (0.5 centi- 

 meter) per hour, the bar was first stationary and then eroded, the breaker 

 type changed from plunging to plunging and spilling, the inshore developed 

 into a long, flat shelf, and the offshore zone gradually steepened. The 

 Kft was higher, after 205 hours, when the inshore zone had fully devel- 

 oped, the foreshore was eroding and the offshore prograding. The dis- 

 tance between the 0- and -1.5-foot contours increased 7 feet (2.1 meters), 

 enough for a 560° change in phase difference, after the shelf developed. 



Variations in K ff relate only generally to the movement of the -0.7- 

 foot contour (Fig. 6); i.e., the Kn increased about the time the -0.7- 

 foot contour began moving seaward with the prograding offshore zone. The 

 development of the profile in this experiment varied laterally, the devel- 

 opment of the shelf began first along one side and progressed across the 

 tank. This lateral variation in development obviously created a lateral 

 variation in the profile reflectivity. Although this variation could not 

 be measured by the one gage in the center of the tank, the variable pro- 

 file reflectivity certainly contributed to the variations measured along 

 the center of the tank. 



(5) Experiment 72D-06 . This experiment varied from the four 

 other experiments with a 1.90-second wave in having an initial slope of 

 0.05 rather than 0.10. The K ff versus time for experiment 72D-06 is 

 shown in Figure 7. During the first 15 hours, K^> varied from 0.04 to 



29 



