were based on experiments in Lake Michigan where local storm fronts 

 control the wave heights and windspeeds. This work made no effort to 

 sei>arate the wave- or wind-generated currents. 



Fundamental studies on boundary layer profiles and wind stress 

 in surf zones (e.g., Hsu, 1972) and wind- induced drift currents 

 (e.g.. Tang, et al., 1978) are available. Because of the significance of 

 winds as a fprcing function, it is surprising that few specific investiga- 

 tions have been conducted. 



d. Planform and Bathymetry . The shape of the coastline, the slope 

 of the beach face, the nearshore profile, bar formation, and offshore 

 bathymetry all influence longshore currents. These geometric features 

 primarily dictate the variation along the coast of the type, height, and 

 location of breaking waves. These are the primary mechanisms for near- 

 shore circulation and rip currents formation, as described in further 

 detail in the section on rip currents. 



Beach-face slope and nearshore profile influence variations normal 

 to the coast and can be used to distinguish two broad extremes of beach 

 conditions. A reflective beach system is characterized by the typical 

 profile shown in Figure 13(a) (after Wright, et al. , 1979). Much of the 

 incident wave energy is reflected from the beach face. Other distin- 

 guishing features summarized by Wright, et al. include: (1) surging 

 breakers with little setup, (2) well-developed beach cusps, and (3) the 

 rare appearance of inshore circulation cells and rip currents. Shore- 

 normal current spectra also have dominant peaks at incident wave periods 

 (T) or subharmonics (2T) . Increasing breaker heights are accompanied by 

 an increase in the strength of seaward flows which pulse at the subhar- 

 monic period. This subharmonic resonance will also dominate longshore 

 current oscillations. 



At the other extreme, a dissipative beach system has a wide surf 

 zone with complex and varied topography (Fig. 13, b) . One or more bars, 

 three-dimensional features, and different scales of circulation cells and 

 rips are frequently present. Subregions with contrasting turbulent 

 mixing intensities can be present. Wright, et al. (1979) classified 

 dissipative beaches into six basic subtypes. Each is dominated by a 

 different combination of surf zone processes and by different scales and 

 frequencies of resonant phenomena. 



The complexity of the nearshore planform and the bathymetry asso- 

 ciated with dissipative beach systems is partly the reason for the wide 

 number of hypotheses advanced as mechanisms for triggering rip currents 

 and circulation cells. 



2. Mechanisms Causing Nearshore Circulations and Rip Currents . 



These flows are treated together simply because they are physically 

 separate parts of circulation cells that occur in the nearshore zone. 



^TANG, F.L.W., et al., "Wind-induced Water Surface Set-up and Drift 

 Currents," Proceedings, 16th Coastal Engi-neering Confevenoe, Vol. I, . 

 Hamburg, 1978 (not in bibliography) . 



45 



