wave generation occurs after a frontal passage when strong winds arise in the 

 northeast quadrant. A third type of storm is an extratropical depression that 

 typically forms south of the FRF along the south Atlantic coast. These storms 

 tend to migrate northward along the coast, creating high winds and waves 

 from the east to northeast. Less frequent but more intense wave generators 

 are hurricanes, the fourth type of disturbance. These originate in the equatori- 

 al Atlantic, migrate west to northwest, and can make landfall on the gulf and 

 Atlantic coasts with very destructive winds and waves. No hurricanes made 

 landfall at the FRF during the course of this study, but several passed by in 

 the deeper Atlantic, creating a distinctive local field of high, long-period 

 waves narrowly distributed in frequency and direction. 



It is noted that the regional topography and climate limit the generalization 

 of results obtained at the FRF to sites having similar characteristics. For 

 example, one would not expect nearshore dynamics of sites on the Great 

 Lakes, gulf, or west coasts of the United States to behave in the same way be- 

 cause their climates and bafhymetries are different. Nonetheless, fundamental 

 physical processes such as nonlinear wave behavior, wave breaking, and air- 

 sea interaction, for which insights are gained by studies at the FRF, will be 

 more universally applicable; the limitations are not too restrictive, and gener- 

 alizations are justified. 



Instrumentation 



Data from two gauges are presented in this report. One gauge consists of 

 a set of near-bottom-mounted pressure gauges, which together constitute a 

 high-resolution directional wave gauge. This gauge provides estimates of the 

 frequency-direction spectra of wind waves which are the basis of this report. 

 The other gauge is an anemometer that gives a measure of local wind speed 

 and direction. It provides a characterization of the larger wind field that 

 generates some of the wave fields measured with the directional wave gauge. 

 These gauges are described here briefly for completeness and because their 

 properties affect the way data are interpreted. 



High-resolution directional wave gauge 



The directional wave gauge consists of two fundamental parts: (a) an array 

 of gauges that sense sea surface displacement, and (b) an analysis algorithm 

 that converts the data to an estimate of S{f,6), the frequency-direction spec- 

 trum, where / is cyclic wave frequency and 8 is wave direction. 1 The sen- 

 sors are a set of nine near-bottom-mounted pressure gauges, laid out in a 

 straight line along the 8-m isobath as shown in Figure 2. Spacing between the 

 gauges follows the guidance given by Davis and Regier (1977) for the design 

 of wave direction detectors. Pressure time series data from each gauge are 

 converted to units of equivalent seawater depth, Fourier transformed, and 

 converted to sea surface displacement Fourier transforms using linear wave 



' For convenience, symbols and abbreviations are listed in the Notation (Appendix B). 



Chapter 2 Measurement Scheme 



