21 



Chapter 3 



LFI Method for a Point Pressure 

 Record 



Pressure gauges are relatively inexpensive and easy to deploy, and as a result are 

 frequently used in the field to measure waves, particularly in shallow water. The 

 simplest instrument consists of a single subsurface pressure gauge. While a single 

 point measurement provides no directional information, a reasonable estimate for the 

 kinematics of the measured waves can be obtained. This chapter describes a technique 

 for applying the two dimensional LFI method presented in the previous chapter to 

 the analysis of a point subsurface pressure trace. 



3.1 Formulation of Solution 



The flow is taken to be two dimensional, with the governing equations described 

 in Chapter 2. These include the potential function, 



J 



cosh. jk{h + 

 cosh jkh 



the modified kinematic free surface boundary condition (/^), 



X, z,t) = Ux + y 4- -^.^ ,, ^ sm j{kx - ut) (3.1) 



^-^ cosh ikn 



