15 



In examining the forms of the bars obtained in the laboratory on 

 the basis of equation 9 to determine -the importance of ad^o, it is 

 desirable to replace the bar base depth IIb with the depth of the initial 

 beach surface Hb measured at the point just below the crest. See 

 figure 1. This substitution is useful because H'b is more accurately 

 determined than Hb and is permissible because the ratio Hb/Hb 

 approximates a constant quantity, which is a function of the slope i. 



Figure 8 gives the results of data obtained in the small tank for a 

 slope of 1/15. The values of 'K/H'b as plotted in figure 8 were grouped 

 and averaged for certain intervals of the wave steepness ratio. No 

 systematic variation in the configuration of the bars with wave steep- 

 ness is discernible. Therefore a single curve passing through the points 

 may be considered as defining the form of the bar for this slope 

 (i=l/l5). The results indicate that the form of the bar is apparently 

 independent of the size of the generating waves. The bar base and 

 the initial beach surface also are shown in the same figure. It is seen 

 that here Hb/H'b^ 1.2. This ratio differs from unity due to the fact 

 that the slope of the initial beach surface is steep and the bar troughs 

 have descended relatively far below the initial surface. A similar 

 analysis of the bar form developed in tests made on beaches with a 1/30 

 slope leads to the same conclusion; i, e., the form of the bar is inde- 

 pendent of the wave steepness ratio. The ratio Hb/Hb in this case 

 has the value of 1.09, indicating that although the troughs of the bar 

 descend below the initial surface, the effect is not so pronounced as 

 that obtained with the steeper slope, ^=l/15. 



The results of the test with 1/70 beach slopes are shown in figure 

 10. In this case the effect of wave steepness is noticeable. Bars 

 formed by waves of small steepness ratio have wider crests and longer 

 bar bases. With an increasing steepness ratio the bars become slender 

 and pointed. 



An examination of figures 8 and 9 shows that in the bar environ- 

 ment of beaches having steep slopes the troughs descend markedly 

 below the level of the initial undisturbed beach. In the bar environ- 

 ment of beaches having flat slopes the troughs hardly descend below 

 the level of the undisturbed beaches. As a result of this behavior, 

 the trough depth below the undisturbed water surface manifests a 

 remarkably uniform relationship with the depth of water over the 

 bar crest. 



Let Ht denote the depth of the trough below still water level. The 

 dependence of Ht/Hc on ao/Xo is indicated in figm-e 10. To facilitate 

 the comparison the data for the different slopes are shown separately. 

 It is seen, making due allowance for errors of observation, that HJHe 

 is practically independent of wave steepness and of beach slope. The 

 ratio is found to have an average value of 1.69. 



