9 



test durations adopted for the larger tank on this basis were 4 hours 

 for the slope 1 on 30, 8 hours for the slope 1 on 50, and 12 hours for 

 the slope 1 on 70. 



The configuration of the bar shown in figui'e 4 is indicated by a 

 smooth curve. This is an idealization in that the bar surface and the 

 adjacent bottom actually were covered with ripple-marks. The 

 curve shown was derived from master traces of the actual surface; 

 it and similar data are the average of two or more runs. 



It is desirable to define the waves reaching the experimental beach 

 in terms of their deep-water characteristics but the waves generated 

 in both the small and large tanks were shallow water waves. Since 

 the period is constant, the wave length, X, and the wave height, a, 

 may be related to the corresponding deep water characteristics Xo and 



X 2TrH 



tto by the following approximate expressions: r~~tanh -^ — 



Ao A 



— =(— j- — \-sinh— Y- cosh ^—j cosh— T— (10) 



where X and a are measm-ed at a point having the undisturbed water 

 depth H (reference 7). The elimination of H between equations 1 

 and 2 gives an expression for the dependence of ag/a upon X/Xo. This 

 expression is shown graphically in figure 5 and is the basis of the 

 desired reductions. Taking the observed value of the wave period, 

 T, the deep water wave length X,, may be computed from 



\o=gT'/2T (11) 



The value ag/a corresponding to the ratio \/\o where X is the measured 

 wave length in front of the wave generator, may be read from the 

 curve in figure 5. KJaowing the value of the ratio «<,/«, the deep water 

 wave height ««, is readily obtained since the wave height a measured 

 in front of the wave generator is known.. These computations were 

 made for all the tests. 



Section IV. RESULTS OF LABORATORY EXPERIMENTS 



The initial changes in the form and movement of the bar are il- 

 lustrated in figure 6 which shows a typical bar development. The 

 form and movement changes do not progress uniformly with time, 

 but oscillate about mean values. The inset on figure 6 is a smooth 

 curve representing the average position of the bar with time. The 

 initial assumption as set forth in section Ila, to the effect that the 

 motion of the bar is decreased to an imperceptible value is thereby 

 shown to be substantiated by the laboratory observations. 



806229—48 3 



