4 Field Observations 



Huntley, Guza, and Thornton (1981) was the first to definitely show the 

 existence of low-mode (« < 2 ) edge waves using a longshore-aligned array of 

 current meters. Subsequent studies have produced abundant field observations 

 of both free and forced infragravity motions (e.g., Oltman-Shay and Guza 1987, 

 Elgar et al. 1992; Herbers, Elgar, and Guza 1995; Herbers et al. 1995). Few 

 studies have been done in laboratory wave tanks owing to the complexity of 

 infragravity motions and the problems associated with seiche actions in the 

 tanks. Laboratory studies are typically designed to examine a particular aspect 

 of infragravity wave generation, such as bound and leaky waves from normally 

 incident wave groups in two-dimensional wave tanks (e.g., Kostense 1984, Dally 

 1 987) or edge waves from obliquely incident wave groups in three-dimensional 

 wave basins (Bowen and Guza 1978). However, the bulk of measurements come 

 from field observations with bound, leaky, edge, and shear waves possibly 

 occurring simultaneously, and where interactions with themselves and the 

 incident waves further complicate data interpretations. 



Over the past two decades the most definitive observations of infragravity 

 waves have been made with longshore arrays of current meters. Much of these 

 data were obtained from the two southern California NSTS field sites, Torrey 

 Pines and Leadbetter Beaches, and from the SUPERDUCK, and DELILAH 

 experiments at the FRF (Figure 13). Instrument arrays in these experiments were 

 designed specifically for measuring infragravity waves. The shear wave 

 discovery was made with the SUPERDUCK data. The arrays for the subsequent 

 DELILAH experiment were designed with one cross-shore and two imbedded 

 longshore arrays to measure both infragravity and shear waves. Array spacing 

 was planned as a compromise for measuring both of these wave types, shear 

 waves having longshore wave numbers larger than free infragravity waves (same 

 frequency), thereby requiring shorter longshore gauge spacing in order to be 

 adequately resolved. 



An alternate method for measuring infragravity waves uses video image 

 processing to obtain swash spectra. The next section presents the methodology 

 for video analysis of runup, and a description of the photogrammetric methods is 

 presented in Appendix B. Strengths and weaknesses of the system are discussed, 

 along with suggestions for future improvements, some of which are being 



30 Chapter 4 Field Observations 



