(Beach and Sternberg 1987). On this beach, infragravity motions accounted for 

 85 percent of the total variance in the inner surf zone with cross-shore 

 currents as high as 240 cm/sec. Sediment suspension events associated with 

 infragravity motions reached peak concentrations of 20 to 40 g/ft at 26 cm 

 above the bed and persisted for periods of 30 to 45 sec. The mean suspended 

 sediment load was three to four times larger than that associated with inci- 

 dent waves. Of course, this beach is unusual in the strength of infragravity 

 motions. However, it does lend insight into the importance of infragravity 

 motions in sediment suspension and transport during storm conditions on more 

 typical beaches. 



25. The offshore and longshore length scales of infragravity long waves 

 suggest a dynamic relationship with common morphological features such as the 

 linear and crescentic sandbars (Figure 6). Linear bars have been proposed to 

 form under the cross-shore nodes or antinodes of long waves (Carter, Liu, and 

 Mei 1973; Lau and Travis 1973; Short 1975; Bowen 1980; Sallenger, Holman, and 

 Birkemeier 1985). Models for the generation of crescentic and welded bars 

 (Bowen and Inman 1971, and Holman and Bowen 1982, respectively) have been 

 based on the interaction of two phase-locked edge waves. The mechanism behind 

 these generation models is the drift velocity of the long wave, where the 

 local beach slope will alter so as to balance the "push" exerted by the drift 

 velocity . 



Figure 6. Oblique aerial view of crescentic sandbars at 

 Cape Cod, MA. Infragravity edge waves offer an explana- 

 tion for these complex morphologies 



17 



