ranges 1, 3, and 5 in experiment 70X-06; Figures 13 to 17 are for ranges 

 1, 3, 5, 7, and 9 in experiment 70X-10. The heavier lines for the -0.2- 

 and -0.8-foot contours distinguish the three profile zones in the figures. 

 In the foreshore and offshore zones the contour lines are close together 

 indicating steeper slopes; in the inshore zone the lines are spaced farther 

 apart indicating flatter slopes. 



(1) Foreshore Zone . Within the first hour of each experiment, 

 the foreshore developed the basic shape which it maintained throughout 

 the experiments, as shown in the contour movement plots of the foreshore 

 zone for the first 10 hours of experiments 70X-06 (Fig. 18) and 70X-10 

 (Fig. 19). The foreshore slope and length remained fairly constant, but 

 not the foreshore position (see Figs. 10 to 17). The foreshore retreated 

 as material was eroded from the foreshore and the backshore (upward- sloping 

 lines in the figures). The foreshore position was stabilized (horizontal 

 contour lines in Figs. 10 to 17) by the backshore nourishment beginning at 

 54 hours in experiment 70X-06 and 62 hours in experiment 70X-10. 



Although the contour lines of the foreshore zone moved together, the 

 lines were not always parallel, indicating a variation in foreshore slope 

 with time at each range (Figs. 10 to 17). Table 8 gives slope values at 

 the SWL intercept for the regularly surveyed profiles in experiments 

 70X-06 and 70X-10. The steepest slope was about 0.60, but at any one time 

 the slope along any range may have been much flatter. The average slope 

 was 0.19 in experiment 7QX-06 and 0.20 in experiment 70X-10. 



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

 result of concentrations of backwash, which created gullies, or flatter 

 slopes (Fig. 20). The shape of the scarp formation and the schematic 

 diagram of flow in this figure demonstrate the process of erosion. A ridge 

 extends seaward from the point of the scarp (A in Fig. 20) to the intersec- 

 tion with the beach face at its steepest slope. Because the slope and the 

 elevation of the berm crest are greatest along this range, less water 

 washes over the crest and more water rushes directly back to form the 

 reflected wave. The backrush velocity is greatest in the vicinity of C in 

 Figure 20, thus maintaining the steepest slope; the overwash velocity is 

 least, thus ero,ding less from the scarp and maintaining the point at A, 



The part of the uprush that washes over the crest divides along the 

 ridge, proceeds laterally in either direction, and erodes material from the 

 scarp until the landward velocity component reaches zero (B in Fig. 20). 

 The lateral velocity component is still great enough to move sediment and 

 the backwash flows into the gully, carrying some of the sediment eroded 

 from the scarp out to the toe of the foreshore zone. When the backwash 

 in the gully (where the volume is greatest) meets the incident wave, the 

 seaward velocity of the backwash decreases to zero and deposits sediment 

 at the toe of the foreshore. This action causes a longer, flatter region 

 below the SWL intercept at this range. The interference of the backwash 

 with the incident wave decreases the uprush in the region of the gully and 

 prevents sediment from being deposited on the upper part of the beach face, 

 thus maintaining the flatter slope. Because the incident waves meet less 



40 



