32. For instance, if a 0.05 -Hz wave was incident at 10 deg from shore 

 normal, its longshore wavelength would be about 1 km. This means an array of 

 length 1,000 m would be needed to distinguish this wave from one that was 

 normally incident. Additionally, some sensor separations would have to be as 

 small as 10 m to avoid aliasing high-frequency, shorter waves. Hence, use of 

 Fourier analysis in directional estimation would require an unrealistic number 

 of sensors and an extremely challenging gage installation and maintenance 

 program. 



33. For this reason, alternate analysis methods know as high- resolution 

 estimators must be used to estimate longshore wave number and directional 

 content of signals at an incident wind wave frequency. These methods are 

 capable of resolving wavelengths that are approximately three times the array 

 length. This allows wave directional estimation to be performed with an array 

 of reasonable length. High- resolution methods are somewhat complicated 

 mathematically, and care is required when implementing them and interpreting 

 their estimates. Nonetheless, with proper use, these methods have proven 

 extremely effective in yielding reliable directional spectral estimates of 

 wind waves incident on a beach. Details of high- resolution estimators are not 

 given here but can be found in the literature. One of the classic approaches 

 is described by Davis and Regier (1977).* An improved adaptation of this is 

 derived by Pawka (1983) . A summary of both these methods is in the paper by 

 Oltman-Shay and Guza (1984). 



Linear Array with Nine Pressure Sensors 



34. The FRF linear array consists of nine bottom-mounted pressure 

 sensors placed along the 8-m, shoreline parallel contour. A tenth sensor was 

 placed 5 m offshore of the main array to form a slope (or Sxy) array in 

 combination with two of the linear array sensors. 



35. Historical records at the FRF field site indicate that peak wind 

 wave frequencies typically fall within the 0.06- to 0.35-Hz range (2.8- to 

 16-sec periods). To measure the high-frequency waves with bottom-mounted 

 pressure sensors, the sensors must be in rather shallow water because of the 



Sources of these works are given in the References of the main body of this 

 report. 



A13 



