that a purely hydro dynamic instability, i.e., an initial disturbance (or 

 perturbation) of the proper wavelength, can extract potential energy from 

 the setup regime and convert it into horizontal circulation patterns and 

 rip currents. Thus, perhaps a better title for Dalrymple's mechanism 

 (Table 1) is hydrodynamic instability theory. 



Hino (1974) was the first to postulate and use this mechanism to dev- 

 elop a theory of rip current spacing. Details of the theory are reviewed 

 in Chapter 3. When the instability grew out of an allowed, movable bottom 

 boundary, Hino found rip currents theoretically formed with an alongshore 

 spacing about four surf zone widths apart. This was close to those obser- 

 ved in nature. However, when instabilities were purely hydrodimamic , the 

 spacings were far too low and the analysis untenable. LeBlond and Tang 

 (1974) using similar analysis procedures investigated the possibility that 

 a wave-current feedback mechanism may cause a preferential spacing for rip 

 currents (see also Iwata, 1976, 1978). But these models did not predict 

 rip current spacings (Dalrymple and Lozano, 1978) unless an extra condition 

 was introduced. LeBlond and Tang (1974) invoked the condition that rip 

 currents will be found where the relative rate of energy dissipation is a 

 minimum, i.e., a path of least resistance approach (Miller, 1977). Theo- 

 retical rip spacings were still far too small. Mizaguchi (1976) intro- 

 duced (rather arbitrarily) a longshore variation in bottom friction as the 

 extra condition. Dalrymple and Lozano (1978) showed that if the extra 

 effect from currant refraction of incident waves by the outgoing rip 

 currents is included, reasonable rip current spacings are predicted. 

 Finally, Miller (1977) and Miller and Barcilon (1978) postulated that the 

 dominant rip current instability occurs at those wave numbers for which a 

 balance can be steadily maintained between the kinetic energy dissipated 

 by friction, wave breaking, and the potential energy released. Their model 

 will be discussed further in Chapter 3 as will the key, empirical assump- 

 tions about the surf zone. 



In retropsect, many of these arguments and theories about mechanisms 

 are of the "chicken or egg" variety and somewhat academic. As summarized 

 by Komar (1976), the rip currents probably come first as generated by 

 some mechanism and cause sediment transport to produce the local bathy- 

 metry. (The analogy with popular "mechanisms" for winds to begin to 

 generate surface water waves is appropriate.) These bottom irregularities 

 probably maintain hydraulic control over the near shore circulation and 

 rip currents observed in today's field experiments. It is undoubtedly 

 true that two or more factors could be present together and the unsteadi- 

 ness of nature may prevent being absolutely sure. However, there is the 

 need to sort out and understand the forced- type mechanisms (structural 

 and wave intersection types) in relation to the free, intrinsic insta- 

 bility type for purposes of correct numerical modeling of nearshore 

 systems. 



(4) Other Factors . Fluid properties and sediment character- 

 istics also play a role in wave interaction models. Water temperature 

 changes its viscosity so that viscous shear effects are different on 

 Alaskan versus Gulf of Mexico beaches. Density differences can create 

 density currents. It is usually assumed homogenous fluids are present 



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