At 105 hours the breaker type became mixed between plunging and spill- 

 ing, indicating that the flatter slope in the offshore had begun to affect 

 the waves. At 175 hours, erosion at elevation -0.6 foot and deposition at 

 -0.7 and -0.8 foot caused the breaker at 180 hours to become consistently 

 spilling and to move seaward with the -0.7-foot contour. After 200 hours, 

 with the wave no longer breaking near the bar, the bar eroded and a shelf 

 developed (erosion at -0o5 and -0.6 foot, and deposition at -0o7 and -0.8 

 foot; see Fig. 37). Deposition mostly at elevations -0.9 to -1.3 feet in 

 the offshore zone steepened the offshore slope and caused a farther sea- 

 ward extension of the inshore zone (Figs. 7, 8, and 9). 



From 220 to 315 hours the wave broke a second time (in the inner in- 

 shore) , further eroding and steepening that region. Continuous erosion 

 of the foreshore and inner inshore, and deposition in the offshore caused 

 the flat shelf in the outer inshore to grow in both directions (Fig. 37). 



More finer material eroded from the foreshore and inner inshore zones 

 leaving the sediment-size distribution coarser in those areas and decreas- 

 ing the median grain size in the offshore zone where it was deposited 

 (Table 12). 



The daily mean water temperature with shoreline position is compared 

 in Figure 43. For the first 15 hours the shoreline recession rate was 

 0.113 foot per hour; after 15 hours the shoreline recession rate was an 

 average 0.025 foot per hour. Because the backshore slope was 0.10 and 

 not flat, the volume rate of erosion was continually increasing. The 

 water temperature was increasing for the first 25 hours and then fairly 

 high and constant until 200 hours. From 200 hours to 345 hours the 

 temperature gradually dropped; from 345 to 365 hours the temperature 

 dropped sharply. The drops in temperature, particularly the sharp drop, 

 were not accompanied by an increase in the shoreline recession rate. 



b. Experiment 71Y-10 . The major events of the profile development 

 in experiment 71Y-10 are summarized in Table 14. During the first hour 

 the foreshore developed a characteristic shape, and a longshore bar was 

 formed in the inner inshore by the plunging breaker. This material was 

 deposited at depths of 0.6 to 1.4 feet. As the shoreline retreated at 

 a rate of 0.133 foot (4.05 centimeters) per hour (for the first 15 hours), 

 the eroded material was deposited along all ranges at depths from 0.6 to 

 1.4 feet until 5 hours, and along all ranges at depths from 0.9 to 1.4 

 feet until 10 hours (Figs. 10 to 14)-. 



After 10 hours the lateral variations became significant. The ero- 

 sion rate dropped to 0.016 foot per hour after 15 hours. For an unknown 

 reason, all the material was deposited in the offshore zone along the 

 range 9 side of the tank, while the erosion from the foreshore and inner 

 inshore was uniform across the tank. This situation continued for 100 

 hours when the offshore along the center of the tank (range 5) began to 

 prograde at the same rate. However, by this time the offshore zone along 

 range 9 was already 2 feet farther offshore (Fig. 31). 



88 



