The lateral variation in the slope of the foreshore developed as a 

 result of concentrations of backwash, which created gullies or flatter 

 slopes. The flow of the wave uprush and backrush for the same wave con- 

 ditions that shaped the foreshore is discussed in Volume II (Chesnutt 

 and Stafford, 1977) . 



Near the end of experiment 71Y-10, the changes in the foreshore zone 

 became more complex (Fig. 17) , Erosion of the backshore was greatest 

 along the outside ranges. A large concentration of backwash occurred 

 along the center of the tank and at various times was skewed toward 

 different sides of the tank. The steepest slopes were not perpendicular 

 to the wave approach. A greater lateral variation occurred in the fore- 

 shore zone of the 10-foot tank than in the 6-foot tank. 



The shoreline (0 contour) movement along the several ranges of the 

 two experiments is compared in Figure 18. The slope of the contour 

 indicates the shoreline recession rate. Because the slope of the back- 

 shore was 0.10 (and not flat), the volume rate of erosion was not con- 

 stant and increased at a rate proportional to the square of the shore- 

 line recession rate. The lateral variations discussed previously are 

 also shown in the top set of curves in Figure 18 for experiment 71Y-10. 

 The rate of shoreline recession increased along the sides of the tank, 

 as indicated by the widening of the family of curves, with range 5 on 

 the bottom and ranges 1, 7, and 9 on the top. 



During the first 15 hours the shoreline retreated 1.7 feet (0.52 

 meter) in experiment 71Y-06 and 2 feet (0.61 meter) in experiment 71Y- 

 10. The average erosion rate in experiment 71Y-06 between 15 and 375 

 hours was 0.025 foot (0.76 centimeter) per hour. The rate along range 

 5 in experiment 71Y-10 between 15 and 335 hours was 0.016 foot (0.49 

 centimeter) per hour. At 205 hours the erosion rate along the outside 

 ranges increased from 0.016 to 0.025 foot per hour. 



(2) Inshore Zone . Within the first hour of each experiment, a 

 longshore bar developed at the shoreward end of the inshore zone between 

 elevations -0.2 and -0.5 foot. Later, but at different times, the bar 

 disappeared, and the area between elevations -0.2 and -0.5 foot steepened, 

 and a long, flat shelf developed between elevations -0.5 and -0.8 foot. 

 The shelf continued to grow in length for the remainder of the experi- 

 ments. Changes in the inshore zone are divided into an inner region 

 (between elevations -0.2 and -0.5 foot) and an outer region (between 

 -0.5 and -0.8 foot) . 



(a) Inner Region (Experiment 71Y-06) . The movement of all 

 contour intercepts in the inshore zone along the three ranges for experi- 

 ment 71Y-06 is shown in Figures 19, 20, and 21; the movement of selected 

 individual contours along the three ranges is compared in Figure 22. 



During the first 10 minutes of testing a longshore bar formed at sta- 

 tion +4. For the first 200 hours the bar crest elevation varied between 

 -0.3 and -0.4 foot, and the bar moved in the shoreward direction at an 



47 



