From Table C-1 o£ Appendix C, entering with d/L^, 



d 



hence 



and 



0.07712 

 L 



40 



L = ; = 519 feet , 



(0.07712) 



cosh I 1= 1.1197 



Therefore, from Equation 2-29 



cosh[27r(z + d)/L] cosh [27r(- 38 + 40)/519] 1.0003 

 K = . .„ .,^. = = = 0.8934 . 



z 



cosh(27rd/L) 1.1197 1.1197 



Since n = a = H/2 when the pressure is maximiom (under the wave crest) , 

 and N = 1.0 since linear theory is assumed valid. 



H N(p+pgz) 1.0 [2590 + (64.4) (-38)] 



- = ,, = = 2.47 feet 



2 pgK (64.4) (0.8934) 



Therefore, 



H = 2 (2.47) ^ 5 feet 



Note that the tabulated value of K in Appendix C, Table C-1, could not 

 be used since the pressure was not measured at the bottom. 



2.237 Velocity of a Wave Group . The speed with which a group of waves 

 or a wave train travels is generally not identical to the speed with which 

 individual waves within the group travel. The group speed is termed the 

 group velocity, C_; the individual wave speed is the phase velocity or 

 wave celerity given by Equations 2-2 or 2-3. For waves propagating in 

 deep or transitional water with gravity as the primary restoring force, 

 the group velocity will be less than the phase velocity. (For those waves 

 propagated primarily under the influence of surface tension, i.e., capil- 

 lary waves, the group velocity may exceed the velocity of an individual 

 wave . ) 



The concept of group velocity can be described by considering the 

 interaction of two sinusoidal wave trains moving in the same direction 



2-24 



