hours, further erosion of the inner inshore would possibly have continued 

 across the tank forming the steep slope just below the foreshore, similar 

 to experiment 70X-06. After 160 hours the profiles varied considerably 

 across the tank. Along range 1 the inner region of the inshore was very 

 steep, the shelf at station +8 reached an elevation of -0.5 foot at times, 

 and the offshore region was very steep, with the toe of the offshore at 

 station 23. In contrast, along range 9, the inner region was fairly flat, 

 the shelf elevation at station +8 was only rarely above -0.8 foot, and 

 the offshore zone was flatter, with the toe of the offshore at station 25. 

 The profiles along ranges 3, 5, and 7 varied progressively between these 

 two shapes. 



The definite decrease in the median grain size in the seaward direction 

 indicates that material eroded from the upper part of the profile and depos- 

 ited offshore was generally finer, as would be expected. 



c. Comparison of the Two Experiments . The general shape of the pro- 

 files and the sequence of events during the development of the profiles 

 appeared to be similar in the two experiments. Also, neither profile 

 reached equilibrium. However, there were differences in the rate of devel- 

 opment and the profiles in the wider tank exhibited some significant lat- 

 eral variations in shape which did not occur in the narrower tank. 



(1) Shoreline Recession Rate and Foreshore Shape . The shoreline 

 in experiment 70X-10, after the initial development during the first 12 

 hours, retreated at an average rate of 0.08 foot per hour for the next 50 

 hours. In experiment 70X-06, the shoreline retreated at a rate of 0.06 

 foot per hour for the first 22 hours and then increased to an average rate 

 of 0.14 foot per hour for the next 28 hours. During the period when the 

 shoreline was stabilized by filling backshore, the erosion rate in experi- 

 ment 70X-06 was 3.53 pounds per hour per foot (5.25 kilograms per hour per 

 meter) of the beach and in experiment 70X-10 was 3.00 pounds per hour per 

 foot (4.46 kilograms per hour per meter] of the beach, or nearly the same. 

 There were no apparent differences in foreshore shape. 



(2) Inshore Zone . A bar developed in both tanks almost immedi- 

 ately at station 4 as a result of the plunging breaker and the position 

 of the bar remained fairly stable until the bar disappeared. The erosion 

 of the bar in the inner region created the wide, flat outer region of the 

 inshore. There are two significant differences between the two tanks in 

 this change. In the 6-foot tank this change (from the time the -0.4-foot 

 contour began moving shoreward until the -0.5-foot contour stopped moving 

 shoreward) occurred in about 20 hours with no lateral variation. In the 

 10-foot tank this change occurred over a 40-hour period, with considerable 

 lateral variation in time of change. The erosion of the bar began and 

 ended sooner along ranges 1, 3, and 5 than along ranges 7 and 9. 



The movement of the seawardmost -0.8-foot contour is indicative of 

 the deposition of material offshore and seaward development of the inshore 

 zone. In the 6-foot tank the -0.8-foot contour moved seaward at 22 hours 

 along all three ranges; in the 10-foot tank this change occurred at 16 



75 



