3 Infragravity Waves, 



Nearshore Morphology, and 

 Sediment Transport 



Nearshore bars are a common feature on natural beaches and one of the most 

 dynamic morphologic features, migrating offshore during storms and onshore 

 during low-energy periods. They assume a wide variety of forms, with single or 

 multiple bars, and different shape combinations of linear longshore (two- 

 dimensional), oblique, and periodic or nonperiodic crescentic (three- 

 dimensional) crests. As mentioned earlier, a number of authors have attributed 

 the formation of linear bars to the location of nodes and antinodes of cross-shore 

 standing waves from reflected incident waves (Carter, Liu, and Mei 1973; Lau 

 and Travis 1973) and from reflected low- frequency waves (Short 1975, Bowen 

 1980, Symonds and Bowen 1984). Bar spacings predicted from reflected free 

 infragravity waves are generally more consistent with field observations than 

 spacings predicted from reflected incident wave models. 



Other models of bar formation have considered cross-shore flow mechanisms 

 induced by the incident wave field, based primarily on sediment suspension from 

 breaker- induced turbulence and momentum decay-induced undertow (mean 

 offshore directed current). Dally (1987) performed a study in a wave flume 

 specifically designed to test bar formation from surf beat, but he determined that 

 bar generation was primarily produced by breakpoint/undertow mechanisms. A 

 similar laboratory study by Roelvink and Stive (1989) considered an additional 

 flow mechanism, a wave-induced asymmetric oscillatory flow. They determined 

 all flow mechanisms (incident wave turbulence, wave asymmetry, undertow, and 

 wave-group- induced long waves) contributed to the observed total flow 

 moments, to the same order of magnitude, with undertow locally dominant in 

 areas of strong dissipation. These observations support the field observations of 

 linear bar formation during storm events. However, as noted by Roelvink and 

 Stive (1989), an improved understanding of bar formation will need to address 

 the role of infragravity waves, specifically the interactions with incident waves 

 and the longshore variations in water motions and morphology. 



26 Chapter 3 Infragravity Waves, Nearshore Morphology, and Sediment Transport 



