Critical to many theories of bar formation and migration attributed to 

 infragravity waves is that these waves are standing in the cross-shore (Short 

 1975, Wright et al. 1986, Sallenger and Holman 1987, Aagaard 1990, among 

 others). Symonds and Bowen (1984) modeled the resonant amplification of low- 

 frequency waves on a linear bar trough beach. A half wave resonance was 

 predicted with a surface elevation antinode (horizontal current node) at the bar 

 crest. Sallenger and Holman (1987) measured cross-shore flows across the surf 

 zone during a storm while the nearshore bar became better developed and 

 migrated offshore. Early in the storm they observed a dominant low-frequency 

 node in velocity that was clearly related to bar position, consistent with the 

 Symonds and Bowen (1984) model. This mechanism is expected to be more 

 significant when infragravity wave forcing occurs at discrete frequencies near 

 the resonant mode, with a velocity node at the bar crest. 



Other studies (e.g., Wright et al. 1986, Aagaard 1988, 1990) have also 

 observed preferential selection of infragravity modes that matched resonant 

 conditions on a beach with bar-trough topography. Wright et al. (1986) 

 concluded that frequencies of standing wave modes prevailed which had 

 amplitude nodes in the trough and antinodes on bar crest. Incident waves were 

 believed to have dominated in the surf zone and caused most of the sediment 

 suspension. However, infragravity waves, although secondary in energy, would 

 be fundamental at influencing surf zone morphology by altering the net drift 

 currents, thus controlling the net drift patterns of sediment. In contrast, many 

 studies have shown a near white infragravity spectrum during storms (e.g., 

 Holman 1981, Oltman-Shay and Guza 1987) which opposes the idea of selective 

 wave modes interacting to reshape the nearshore morphology. When no 

 dominant frequencies or modes occur, then the surf zone will have nodes and 

 antinodes throughout, and bar formation or sediment movement by infragravity 

 motions would not likely occur. 



In a field study on an extremely dissipative beach, Beach and Sternberg 

 (1988) observed suspended sediment concentrations associated with low- 

 frequency motions. These infragravity induced suspensions had mean loads 

 approximately three to four times greater than suspensions from incident waves. 

 Suspension events occurring at infragravity frequencies lasted 30-45 sec, and 

 only a few seconds when incident waves were dominant. Hanes (1991) analysis 

 of a limited data set (6 hr) observed sediment suspension at both incident and 

 wave group frequencies, and that concentrations were enhanced at the group 

 frequencies. Since most sediment transport models omit infragravity motions, 

 and these motions can be comparable or dominate over incident wave energy, 

 that could account for some of the difficulties in modeling sediment transport. 



Infragravity waves have also been associated with abrupt berm erosion 

 (Katoh and Yanagishima 1993). Their observations indicated that infragravity 

 waves were the dominant force in berm erosion during storms, and that low 



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



