LEAKY WAVES OUT 





EDGE WAVE MODE 



BOUNDED LONG WAVES IN 



Figure 3. Schematic of infragravity wave types 



refractively trapped to the shoreline and leaky waves that escape out to deep 

 water upon reflection from the shoreline (Eckart, 1951). Edge and leaky waves 

 are free surface gravity waves, free to propagate away from the generating 

 source. Since forced waves are bound to wave groups and traveling at the group 

 velocity, they are generally referred to as bound long waves (BLW). 



A third type of low frequency motion was recently discovered (Oltman-Shay, 

 Howd, and Birkemeier 1989): shear instabilities, or sometimes referred to as 

 shear or far-infragravity waves. Tang and Dalrymple (1989) observed similar 

 low frequency motions during the Tony Pines field experiment. These waves 

 manifest as fluctuations in the horizontal current rather than as a surface 

 elevation displacement, and as such are actually a vorticity phenomena. Holman 

 and Bowen (1989) presented a theoretical basis for these motions being 

 generated by cross-shore shear in the mean longshore current. The significance 

 of finding these motions is the realization that low- frequency (0.001 to 0.03 Hz) 

 motion in the surf zone may not be composed entirely of surface gravity waves. 



Free infragravity waves 



Analytical edge wave solutions have been obtained for simplified nearshore 

 bathymetry. Eckart (1951) found edge wave solutions to the linear shallow- 

 water equations by assuming a planar beach with no alongshore currents. Ursell 

 (1952) derived a more general solution for edge waves using the full linear 



Chapter 2 Infragravity Wave Dynamics 



