unimodal (but narrow) sediment-size distribution. The figures are arranged 

 in ascending order of wave steepness. The smallest steepness tested 

 (Hq/Lq = 0.014) shows a tendency for a bar formation in tests with bimodal 

 and unimodal distributions (Fig. 9) ; however, in tests with a bimodal 

 distribution, the bar is formed farther offshore. The slope of the beach 

 face is also different. For a steeper wave with the same period. Figure 10 

 shows the bar formations even farther apart than those shown in Figure 9. 

 However, Figure 11 shows similar profiles for a wave steepness (H /L = 

 0.029) lying between that of Figure 10 (Hq/Lq = 0.021) and Figure 12 

 (Hq/Lq = 0.031). This apparent contradiction can only be explained by the 

 difference in wave height (4 centimeters versus 7 or 8 centimeters) . For 

 waves of very large steepness (Hq/Lq > 0.05), the differences between 

 bimodal and unimodal distributions diminish (Fig. 13). Finally, it was 

 found that the trimodal grain-size distributions tested always produced 

 profiles similar to those of unimodal distributions (Fig. 14). 



An apparent consistent difference between a uniform-sized sand and 

 a sand with broader size distribution is that the uniform sand tends 

 to form more ripples. Another phenomenon was noticed for some wave 

 conditions, where the uniform sand size produced an unstable equilibrium 

 profile which oscillated between two extreme profiles: one with a bar 

 and one without. This phenomenon appeared again in tests which compare 

 the effect of grain shape. 



2 . Effects of Grain Shape . 



Pairs of tests performed to determine the effects of grain shape are 

 summarized in Table 5. Examples of beach profiles using selected pairs 

 of runs are shown in Figures 15 to 18 (profiles have been shifted hori- 

 zontally to superpose the SWL intercepts; see Appendix B for actual 

 positions) . In most tests the effect of shape was represented more by 

 Figures 16 and 18 than Figures 15 and 17. The different PVC shapes, the 

 smallest glass sizes (D50 = 0.3 millimeter), and most of the graded mixes 

 seemed to produce similar profiles independent of grain shape. However, 

 some exceptions occurred as shown in Figures 15 and 17. Other pairs 

 (not shown in the figures) which illustrated similar phenomena only 

 occurred with the larger glass sizes (D50 =0.6 millimeter) and mixtures 

 having larger glass sizes, and then only for relatively small-wave 

 steepnesses. The profile differences appear to be related to some 

 instability associated with the formation of a bar. This phenomenon, 

 shown in Figure 9, has also been observed for sand having a very uniform 

 grain size. 



The phenomenon appears to follow the sequence: 



(a) Waves breaking on the beach slope (Fig. 19, a) causes scour just 

 inshore of the breaker and the scoured material is deposited on 

 the beach face and the bar (Fig. 19, b) ; 



(b) the bar continues to build (Fig. 19, c) until of sufficient size 

 to move the breaker point offshore of the bar (4 to 5 hours in 

 this particular test) ; 



39 



