The pressure response factor K, for the pressure at the bottom when
z = - d,
1
K*
cosh (27rd/L) ’
(2-31)
is tabulated as a function of d/Lo and d/L in Tables C-l and C-2 of
Appendix C.
It is often necessary to détermine 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)
~>
(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 hâve 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 complété discussion of the interprétation 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), Grâce (1970), and Esteva and Harris (1971).
************** EXAMPLE PROBLEM **************
GIVEN: An average maximum pressure of p = 2590 lbs/ft1 2 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.
1
1
— = ----------- œ 15 seconds ,
f
(0.0666)
Lo = 5.12 T2 = 5.12 (15)2 = 1152 feet ,
FIND: The height of the wave H assuming that linear theory applies and
the average frequency corresponds to the average wave amplitude.
SOLUTION:
d _ 40
l; " ÏÏ52
~ 0.0347 .
2-23