Shear waves 



Oltman-Shay, Howd, and Birkemeier (1989) examined data from the along- 

 shore aligned array of bidirectional current meters deployed during the 

 SUPERDUCK experiment (Crowson et al. 1988) and found an energetic, 

 longshore progressive wave in shallow water (lm depth) that could not be 

 gravity waves. The most energetic frequencies for these waves, observed to 

 date, fall into the lower end of the infragravity band (0.001 </< 0.01 Hz); 

 hence, they were initially given the term "far-infragravity" waves (Figure 6). 

 These motions were shown to be consistent with the Bowen and Holman (1989) 

 model of vorticity waves or instabilities generated by cross-shore shear in the 

 mean longshore current. This suggests that the term "shear instability" or "shear 

 wave" is more appropriate than "far-infragravity wave" since these motions are 

 observed as fluctuations in the mean longshore current and not as vertical 

 displacements of the water surface. 



Oltman-Shay, Howd, and Birkemeier (1989) noted that shear waves were 

 only observed in the presence of a mean longshore current and propagate in the 

 same direction (Figure 6). Furthermore, their alongshore wave numbers are too 

 large to be surface gravity waves, exceeding the wave number of a mode edge 

 wave (largest free wave k 's) by more than an order of magnitude. 



Figure 7 demonstrates the rapid spin-up of shear waves with an increase of 

 longshore current. Oscillation periods were near 400 s in the middle of the 

 record and decreased to 200 s toward the end. Horizontal root mean square 

 velocities exceeded 30 cm/s, alongshore wavelengths were typically of the order 

 10 2 m, and celerity was of the order 1 m/s. These observations were consistent 

 with the shear instability model of Bowen and Holman (1989) which predicts a 

 celerity of approximately V max /3, where V max is the peak longshore current. 



The observed waves were found to have a characteristic longshore wavelength of 

 approximately twice the width of the longshore current and to be highly coherent 

 and homogeneous over an alongshore distance of two wavelengths. 



An alternative hypothesis for the generation of shear instabilities was 

 presented by Fowler and Dalrymple (1991) where interaction of two different 

 wave trains produce longshore variations in energy (i.e., a longshore interference 

 pattern, Figure 8). Tang and Dalrymple (1989) analyzed data from the Torry 

 Pines field experiment for very low frequency (< 0.0024 Hz) motions. They 

 confirmed the presence of significant energy at very low frequencies (O.0015 

 Hz) that was easily distinguished from edge wave dispersion curves. These 

 motions were found to have wavelengths similar to observed slowly migrating 

 rip current cells. They suggested that incident wave groups forced a nearshore 

 circulation in the form of migrating rip currents. Fowler and Dalrymple (1991) 

 validated their proposed mechanism in a laboratory experiment using a 

 directional wave basin. 



Chapter 2 Infragravity Wave Dynamics i ' 



