then remained essentially stable for the remainder of the experiment. 
The position of the shoreline varied across the tank (as indicated by 
the wider spacings at times), but there was never more than a 0.5-foot 
difference. 
During the first 100 hours of experiment 72B-10, considerable lateral 
variation occurred in shoreline position. The shoreline was skewed across 
the tank; the shoreline retreated along ranges 7 and 9 and advanced along 
ranges 1 and 3. There was a 1.2-foot difference across the 10-foot tank 
in the position of the shoreline. By 100 hours the 0 contours had coa- 
lesced, with a net recession of 0.4 foot. Between 100 and 150 hours the 
shoreline receded at a rate of 0.018 foot per hour (0.55 centimeter per 
hour). During the last 20 hours, considerable lateral variation again 
occurred in the position of the shoreline. Because the backshore slope 
was 0.10 (and not flat) the volume rate of erosion was not constant and 
increased at a rate proportional to the square of the shoreline recession 
rate. 
(2) Inshore Zone. 
(a) Experiment 72B-06. The movement of all contour inter- 
cepts in the inshore zone along the three ranges in experiment 72B-06 is 
shown in Figures 25, 26, and 27. In the first 10 minutes a longshore bar 
formed by the plunging breaker near station 4 with a crest elevation of 
-0.3 to -0.4 foot, as indicated by the multiple intercepts (Figs. 25, 26, 
and 27). The inshore zone remained stationary throughout the experiment, 
as indicated by the essentially horizontal lines for the -0.2-, -0.4-, 
-0.5-, and -0.6-foot contours. The absence of multiple intercepts for 
the -0.2- to -0.4-foot contours indicated that at times the bar became 
essentially a flat region. The movement of the -0.3-foot contour and 
appearance of the -0.4-foot multiple intercepts indicate that the bar 
crest elevation varied between -0.3 and -0.4 foot. 
The movements of the -0.3-, -0.4-, -0.5-, and -0.6-foot contours 
along the three ranges are compared in Figure 28. Little lateral varia- 
tion occurred at -0.6 foot and only small variations at -0.4 and -0.5 
foot. The considerable lateral variation in the -0.3-foot contour indi- 
cates that the bar crest reached the -0.3-foot elevation at different 
times along the different ranges. 
(b) Experiment 72B-10. The movement of all contours in 
the inshore zone along the five ranges in experiment 72B-10 is shown in 
Figures 29 to 33. In the first 10 minutes a longshore bar formed by the 
plunging breaker near station 2 between the -0.2- and -0.3-foot contours 
and a flat region also developed near station 5 between the -0.4- and 
-0.5-foot contours. The bar later became just a flat area and then 
eroded completely. The flat region between the -0.4- and -0.5-foot con- 
tours eventually developed into a bar. These changes occurred along all 
ranges, but at different times and to different extents. 
Along range 1 (Fig. 29) the bar near station 2 was eroded by 5 hours 
and the area between the -0.2- and -0.3-foot contours varied between 
47 
