151 



about 1/10 of a zero crossing wavelength would probably result in more stable results 

 with the smaller windows. 



6.6 Discussion 



The LFI method can be applied to interpretation of records from arrays of pressure 

 measurements. It is effective for records from primarily unimodal seas, as well as the 

 bimodal seas that are generated near reflecting surfaces. The result of the analysis is 

 a complete description of the kinematics of the waves throughout the water depth, in 

 the vicinity of the array. The method is local in that a separate potential function is 

 used to describe each small segment of the record, using information from only that 

 segment. This approach is rational in that it seeks to satisfy the governing equations 

 of gravity waves, including the full nonlinear free surface boundary conditions. 



Using a potential function representing a single progressive wave, the method 

 has been shown to be effective on theoretically generated records of steady waves in 

 both deep and shallow water. This formulation is appropriate in locations far from 

 reflecting surfaces, where the sea state is expected to be unimodal. When the method 

 is applied with a potential function representing two intersecting waves, it is effective 

 in accurately capturing the kinematics of standing and short crested waves, as result 

 from the reflection of steady waves from a reflecting surface, such as a sea wall. This 

 two wave formulation is appropriate near such reflecting surfaces, where the sea state 

 is likely to be bimodal. 



A number of parameters must be specified to establish a solution, including the 

 order, window width, number of water surface nodes defined, and number of samples 

 on the measured records. The examples in this chapter indicate that the criteria 

 used to define these parameters are similar to those developed in the analysis of point 

 pressure records and arrays of water surface measurements. Fairly low order solutions 

 (J = 2 or .3) are effective in deep water, while shallow water requires higher order 

 solutions (J = 5 or 6). 



Window widths of 1/10 the zero crossing period of the record are effective on 

 theoretical records. With field records, however, when the array used is small (less 



