field spacing measurements. In fact, much of the knowledge in this area 

 must still come from empirical correlations of the available data. 



One such effort was made by Sasaki (1977) (see Sasaki and Horikawa, 

 1975a, 1975b). Field data from the United States, South Africa, 

 and Japan were analyzed in terms of the rip current spacing Yj. (Lj. in 

 eq. Ill), surf zone width X^, beach slope tan 3> and wave steepness. The 

 latter two variables can be combined into the surf similarity parameter 

 E, (Battjes, 1974a, 1975, eq. 93) or here defined as the Irribaren Number, 

 Ij.. Note that the deepwater wave height H^ is used in contrast to Battjes' 

 similarity parameter E, which employed H, the height at the "toe" of the 

 slope. Gourlay defined Ij.^^ if H^, (at breaker) is involved. 



tan 3 



o o 



(166) 



From the data sources shown in Figure 77 for the correlation between Y 

 and X|j, Sasaki plotted the normalized rip current spacing Y /X. against 

 I_. Considerable scatter was found to exist, as shown in Figure 78, where 

 rip spacing ranged from about one to eight times the surf zone width. From 

 an extensive analysis of the data in which breaker type, beach slope, reflec- 

 tion coefficient, and other factors were considered, Sasaki (1977) hypothe- 

 sized the three domains for rip current generation mechanisms shown in 

 Figure 78. These were an edge wave, instability, and infragravity domain. 

 A detailed description of each domain for nearshore circulation systems 

 is given in Table 11 (after Sasaki, 1977). 



The demarcation between each domain was given as 1 = . 23 and Ij. = 1 

 as shown in Table 11. The reasons for this choice were never explicitly 

 stated by Sasaki (1977). The infragravity domain (Ij- < 0.23) was hypothe- 

 sized by Sasaki (1977) to explain why flat beaches with wide surf zones 

 (spilling -type breakers) could reflect relatively low waves (H^j < 0.3 m) 

 with longer periods of the order 30 to 120 seconds. These waves are known 

 as infragravity waves and are produced by irregular wind waves and also 

 called surf beat. The existence of these low mode edge waves within the 

 surf zone on flat beaches has subsequently been measured by Huntley (1976a) 

 and Sasaki, Horikawa, and Hotta (1976)'^ in the field and Bowen and Guza 

 (1978) in the laboratory. 



The data of Figure 78 for each domain give the ratio Yj-ZX^ as 



(1) infragravity domain (Ij. < 0.23) 



Y^/X^ = 157 1^2 (167) 



SASAKI, T. , HORIKAWA, K. , and HOTTA, S., "Nearshore Current on a Gently 

 Sloping Beach," Proceedings^ iSth Coastal Engineering Conferenaef 1976, 

 pp. 626-644 (not in bibliography). 



199 



