Equation (2-27) can be written as 



co sh[2Tr(z + d )/L] ,„ „„^ 



P = P^^ cosh(2.d/L) - PS^ <2-28) 



since 



The ratio 



cosh[2Tr(z + d)/L] 

 z cosh(2Trd/L) ^ ^ 



is termed the pressure response factor. Hence, equation (2-28) can be written 

 as 



p = pg(nK^ - z) (2-30) 



The pressure response factor K for the pressure at the bottom when z = -d, 



z cosh(2Tid/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(^_f_pgz), 



PgK 

 z 



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

 tion 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 great- 

 er 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 5*************** 



GIVEN ; An average maximum pressure p = 124 kilonewtons per square meter is 

 measured by a subsurface pressure gage located in salt water 0.6 meter (1.97 

 feet) above the bed in water depth d = 12 meters (39 feet). The average 

 frequency f = 0.0666 cycles per second (hertz). 



2-22 



