interference in the steeper region near C (in Fig. 20), the uprush 

 between A and B is greater, erodes farther into the scarp, and carries 

 more sediment into the gully, further generating the three-dimensional 

 system of flow and erosion. 



After this erosion process has progressed a while, the uprush no 

 longer has sufficient velocity to erode material between points A and B 

 (Fig. 20), the sediment moving into the gully decreases, the interference 

 between the backwash in the gully and the incident wave decreases, and 

 the uprush increases in the region of the gully. Then, the uprush begins 

 to erode material from other areas of the scarp, and changes the positions 

 of the gully and the steep region. 



Even though the foreshore shape varied laterally and the slope of the 

 foreshore along any range varied with time, the average slope of the fore- 

 shore did not vary with time. 



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

 two experiments is compared in Figure 21. The slope of the contour 

 line indicates the shoreline recession rate. Because the slope of the 

 backshore was 0.10 (and not flat), the volume rate of erosion was propor- 

 tional to the square of the shoreline recession rate. 



In experiment 70X-10, the shoreline recession rate between 12 and 62 

 hours averaged 0.08 foot per hour (2.4 centimeters per hour). However, in 

 experiment 70X-06, a significant increase occurred in the retreat rate 

 during the first 50 hours. The rate was 0.06 foot per hour (1.8 centimeters 

 per hour) before 22 hours and 0.14 foot per hour (4.2 centimeters per hour) 

 after 22 hours. The calculation of these rates is shown in Table 9. 



After the beach had eroded to the back of the tank and the backshore 

 nourishment began, the position of the foreshore was stabilized (the 

 Appendix discusses the procedures used in nourishment). Table 10 gives 

 data on weight of sand added to the backshore in each experiment in 10- 

 hour increments and the average weight for a 10-hour increment. In experi- 

 ment 70X-10, the greatest backshore erosion occurred between 130 and 150 

 hours and between 170 and 180 hours; the least erosion occurred between 

 62 and 70 hours and between 160 and 170 hours. In experiment 70X-06, the 

 greatest erosion was between 90 and 100 hours with minimal erosion between 

 60 and 70 hours and between 170 and 175 hours. 



(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, the area between -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 experiments. Changes 

 in the inshore zone for each experiment are divided into an inner region 

 (between -0.2 and -0.5 foot) and an outer region (between -0.5 and -0.8 

 foot) . 



(a) Inner Region (Experiment 70X-06) . The movement of all 

 contour intercepts in the inshore zone along the three ranges for experiment 



46 



