refraction was offered as one mechanism for producing the longshore varia- 

 tion in breaker height. 



(2) Coastal Boundaries . Natural headlands or manmade lateral 

 barriers (breakwaters, jetties, groins) can also cause nearshore circula- 

 tions. The wave fields are diffracted and reflected by these structures 

 (some wave refractions also) causing wave breaking variations in complex 

 patterns. Rubble-mound structures also absorb wave energy present. 

 Figure 8 from Gourlay's (1978) laboratory study is a good example of a 

 circulation pattern in the lee of a breakwater. 



(3) Barred Coastlines . From field observations and measurements 

 in an area with irregular bottom topography at Seagrove, Florida, Sonu 

 (1972) found that shoreward currents in a cell occurred over the shoals 

 while rips were observed over the troughs (Fig. 2). Paradoxically, 

 breaker heights were uniform down the coast so that the observed circula- 

 tion must be due to another mechanism. Over the shoals spilling breakers 

 were observed which continuously dissipated energy and created some setup 

 in the surf zone. Waves entering the rip current areas broke by plunging, 

 re-formed, and created little MWL change at the trough positions along the 

 beach. Thus, this difference in wave breaking type produced mean surface 

 gradients in the surf zone that were said to create the nearshore circula- 

 tions and rip currents present. Sonu (1972) took actual measurements and 

 used radiation stress theory to demonstrate theoretically that cell 

 circulations can be created by these conditions, i.e., a barred coastline 

 with seaward-directed troughs and no variation in breaker height. 



An additional forcing for the circulation is suggested to stem from 

 wave reflection off the submerged bar (Dalrymple, 1978). Investigations 

 listed in Table 1 and not mentioned here will be discussed in Chapter 3. 



b. Wave Interaction Models . The other major mechanism category in 

 Table 1 is more subtle. Circulation cells can exist on long, straight, 

 beaches with plain profiles as noted by Shepard and Inman (1950, 1951). 

 Three subcategories are presented by Dalrymple to explain how this may 

 occur. 



(1) Incident Edge Wave . This was the earliest mechanism pro- 

 posed (Harris, 1967; Bowen, 1969) and first required an understanding of 

 edge wave modes (Eckart, 1951) , standing edge waves, and proof that they 

 actually existed on natural beaches. Waves propagating along the shore- 

 line or standing waves along the shoreline are termed edge waves. Huntley 

 and Bowen (1974) measured edge waves on the southern coast of England with 

 a period of 10 seconds that was twice (subharmonic) the incident wave 

 period. The theory states that incident swell waves can generate standing 

 edge waves of the same period or the moving type (synchronous) of the same 

 period on the beach. The linear superposition of these wave fields pro- 

 duces alternately high and low breakers along the shoreline. A regular 



^°ECKART, C, "Surface Waves in Water of Variable Depth," The Theory 

 of Edge Waves, Report No. 100, SIO, Ref. 51-12 (not in bibliography) 



48 



