(Longuet-Higgins and Stewart 1964). Swash is defined as the continuous motion 

 of runup relative to the setup level. 



Munk (1949) and Tucker (1950) were the first to detect low-frequency waves 

 and found a linear relation between their amplitudes and the amplitudes of the 

 wind waves. These waves were measured with pressure recorders, 300 m 

 offshore in approximately 6-m water depth, and were shown to have small 

 amplitudes and periods of 2 to 3 minutes. Munk and Tucker observed that the 

 waves were correlated with, but lagged behind, the incident wave groups. They 

 concluded that the low- frequency motions were forced with the incident wave 

 group and released as a free wave when the incident waves broke. They 

 attributed the observed lag as the time needed for the forced motion to travel to 

 the surf zone, and the released free wave to reflect from the beach and travel 

 back to the wave recorder. Munk referred to his observations as "surf beat." 



More recent studies have measured current velocity and water elevation 

 variances in the surf zone and found that a significant fraction of the total energy 

 is in the infragravity band (Guza and Thornton 1985; Sallenger and Holman 

 1987; Wright, Guza, and Short 1982; Bowen and Huntley 1984). Swash motions 

 on steep-sloped (reflective) beaches, where incident waves are not fully 

 dissipated, will have a major portion of the energy in the wind wave band 

 (Wright et al. 1986). However, on shallow-sloped (dissipative) beaches, 

 infragravity swash motions dominate the energy spectrum (Holman 1981, 

 Holman and Sallenger 1985, Holman 1983). Holman and Bowen (1984) 

 observed swash motions on an extremely dissipative beach and found 99.9 

 percent of the total variance in the infragravity band. 



In the inner surf zone, energy spectra are often dominated by infragravity 

 waves during high energy storm events, whereas incident wave energy will be 

 depth limited due to dissipation in the surf zone (Figure 1). The surf zone is 

 called "saturated" when an increase in offshore incident wave energy only serves 

 to widen the surf zone, and local wind wave energy remains constant. However, 

 infragravity waves do not exhibit saturation, but continue to increase in 

 amplitude with an increase in the offshore incident wave height (Guza and 

 Thornton 1982; Sallenger and Holman 1984; Holman and Sallenger 1985). 

 Furthermore, beaches normally regarded as reflective can become highly 

 dissipative during high energy storms (Wright et al. 1986). 



A principal interest in infragravity waves has been to determine their 

 importance to sediment transport and large scale morphology response (Bowen 

 and Inman 1971, Short 1975, Holman and Bowen 1982). This interest developed 

 in part from the observation that infragravity waves have length scales similar to 

 typical nearshore morphologic scaling (O(10 2 - 10 3 m)) (Wright and Short 1984, 

 Lippmann and Holman 1990). Linear bar formation has been proposed to occur 

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

 1973; Bowen 1980; Sallenger, Holman, and Birkemeier 1985). More complex 



Chapter 1 Introduction 



