Wave Pressure to Wave Height Transfer Function 



According to linear theory for gravity waves, a subsurface 

 wave pressure head fluctuation, Hp, can be related to the surface wave 

 height, H, by 



[1] H = H^ _£^!hJitL_ 

 P cosh k(h+z) 



where k is the wave number 2it/L, L is the wavelength in water depth, h, 

 and z is the depth of submergence of the pressure transducer, measured 

 upward from the still water level. The term (cosh k(h+z)/cosh kh) is 

 known as the pressure response factor, Kp. 



An empirical correction factor, N, has been introduced to 

 Equation 1 by many investigators: 



[2] H = N . H ^°2h kh 



P cosh k(h+z) 



A relation for N was determined from prototype scale tests conducted in 

 the 103 m long wind-wave flume of the National Water Research 

 Institute. These tests are reported elsewhere (Bishop 1984). Several 

 empirical relations for N versus |z|/L are shown in Figure 5. 



The measured pressure head fluctuations were analyzed using 

 fast Fourier transform techniques. Then the power spectral components, 

 Sg(f), of the water surface elevation were related to the power spec- 

 tral components of the subsurface pressure head variation, Sp(f). by 



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