The pressure response factor K, for the pressure at the bottom when 

 z = - d, 



1 



K, = K = — 1 .^ ,,.-> , (2-31) 



2 cosh (ZTrd/L) ' 



is tabulated as a function of d/L^ and d/L in Tables C-1 and C-2 of 

 Appendix C. 



It is often necessary to determine the height of surface waves based 

 on subsurface measurements of pressure. For this purpose it is convenient 

 to rewrite Equation 2-30 as, 



_ N(p+pgz) 

 r? - , (2-32) 



where z is the depth below the SWL of the pressure gage, and N is a 

 correction factor equal to unity if the linear theory applies. Several 

 empirical studies have found N to be a function of period, depth, wave 

 amplitude and other factors. In general, N decreases with decreasing 

 period, being greater than 1.0 for long-period waves and less than 1.0 for 

 short-period waves. 



A complete discussion of the interpretation of pressure gage wave 

 records is beyond the scope of this Manual. For a more detailed discussion 

 of the variation of N with wave parameters, the reader is referred to 

 Draper (1957), Grace (1970), and Esteva and Harris (1971). 



************** EXAMPLE PROBLEM ************** 



GIVEN : An average maximum pressure of p = 2590 lbs/ft^ is measured by a 

 subsurface pressure gage located 2 feet above the bed in water at d = 40 

 feet. The average frequency f = 0.0666 cycles per second. 



FIND : The height of the wave H assuming that linear theory applies and 

 the average frequency corresponds to the average wave amplitude. 



SOLUTION: 



1 1 



T = — = ^ _^^^ ~ 15 seconds 



f (0.0666) 



L^ = 5.12 T^ = 5.12(15)^ = 1152 feet 



K 1152 



40 



0.0347 



2-23 



