the real distribution of wave energy in shallow coastal waters. The primary 

 reasons for this are (a) difficulties in high-resolution directional wave 

 gaging, and (b) difficulties in obtaining observations at any one site over a 

 time span long enough that the statistical character (mean conditions, extreme 

 conditions, conditions during storms, etc.) of natural, shallow-water seas can 

 be identified. 



21. There is a variety of gages that sense wave direction, ranging from 

 the human eye to elaborate arrays of sea surface displacement gages and on to 

 satellite imagery. These gages vary markedly in their resolution of direc- 

 tionally distributed seas. The human eye is not well calibrated, gets tired, 

 and gives results that change from human to human. Images from satellite 

 overpasses tend to give data at one point in time (a snapshot) and not the 

 time series required to determine mean properties of the directional distribu- 

 tion. Furthermore, the large extent of their horizontal sampling, often 

 averaging over tens of kilometres, make them inappropriate for detailed 

 (typical scale of a few hundred metres) nearshore studies. Radar devices have 

 potential but have yet to demonstrate that they can yield high- resolution 

 frequency-direction spectra. 



22. Some commonly used wave direction sensors which obtain time series 

 at a point in space are of low resolution in that they tend to overestimate 

 directional spread and can only resolve wave trains of the same frequency 

 which are separated by 90 deg or more in direction. They are of three general 

 types : 



a. The PUV gage (named for the pressure and two horizontal 

 components of water velocity measured) relies on the unique 

 phase difference between pressure and the horizontal velocity 

 vector in a wave train running at a given direction. 



b. The heave-pitch-roll buoy relies on the phase difference 

 between sea surface displacement (the heave axis) and com- 

 ponents of sea surface tilt (or slope) determined from pitch- 

 and-roll measurements in a wave train. 



c. The Sxy or slope array gage is like the heave-pitch-roll buoy 

 except surface displacement and surface tilt components are 

 found from pressure gages mounted in the water column. 

 Pressure records are converted to surface elevation by way of 

 linear wave theory. 



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