(2) instability domain (0.23 < I < 1) 



'r'\ ' 



and (3) edge wave domain (I > 1) 

 r ■ n = 1 



'.'\ 



2tt „ 

 Y~ n = 2 



(168) 



(169) 



Equation (16 7) was obtained empirically by Sasaki (1977) from the data 

 shown whereas the other relations are theoretical. The edge wave theory 

 of Bowen and Inman (1969) with modal number n appears to agree well with 

 data from the steep beach (tan 3 ^ 0.15) at El Moreno, Mexico (eq. 169). 



In the instability regime, the theory of Hino (1974) falls within 

 the variation of data shown. The theory of Dalrymple and Lozano (1978), 

 given by equation (112), and the theory by Miller and Barcilon (1978), 

 given by equation (114), result in Yj./X|j i 2 and Yj-/X^ i 10, respectively, 

 for typical values of tan 3, y> and C^. Thus, their more sophisticated 

 eigenvalue analyses appear to give results farther outside the range of 

 observed rip current spacings in Figure 78. More data are needed to sub- 

 stantiate the various theories. For example, use of the LEO data gave 

 Bruno and Dalrymple (1978)^-'^ encouragement for the applicability of equa- 

 tion (112) in the instability regime. 



Rip current generation mechanisms can also be categorized as either 

 the free type (instability, eigenvalue theory) or the forced typed (edge 

 wave, structural interaction, etc.). Mizuguchi and Horikawa (1976), based 

 on their experiments, argue that observed rips in the field should all 

 be interpreted as the forced type where irregular bottom bathymetry controls 

 the motion. 



Much research remains to be done on rip currents and the development 

 of two-dimensional numerical models such as by Vreugdenhil (1980) to study 

 migrating rip currents will provide needed insight in the future. No theory 

 exists for rip current velocity, width or size, and little field data are 

 available to verify the numerical simulations in this regard. Sasaki (1977) 

 does provide some data and correlation for the offshore extent of rip 

 currents beyond the breaker line (X in Fig. 14). 



''^BRUNO, R.O., and DALRYMPLE, R.A. , "Held Observations of Rip Currents," 

 University of Delaware Report, 1978 (not in bibliography). 



202 



