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Chapter 2 



Two Dimensional LFI Theory 



It is common to deploy a single measurement device to gather information about 

 the wave field at a given location. Measurement at a single point in space does not 

 give any information about the directionality of the sea state, but if the wave field 

 is assumed to be locally two dimensional, a reasonable description of the kinematics 

 can be established. This chapter outlines the LFI method as it is applied to the 

 interpretation of a time series from measurements taken at a single location in space, 

 such as recorded by a pressure gauge or wave staff. 



2.1 Problem Formulation 



The problem formulation for two dimensional irregular waves has much in common 

 with classical steady wave theory. The flow is assumed to be incompressible and 

 irrotational. The kinematics can therefore be represented by a potential function, 

 d>{x,z,t), where 



96 d(t> . 



u = ^- ^^^- (2.1) 



ax oz 



and u and w are the horizontal and vertical velocities, respectively. 



