116 



the DWG-1 pressure array that is used for the analysis of pressure records in Ch. 6. 

 A small array has the advantage of having a small distance to interpolate to the 

 center of the array, and is local in the sense that each of the sensors is in essentially 

 the same part of the wave. The disadvantage of a small array is that the differences 

 between the measured values at each of the nodes are small, and thus more sensitive 

 to measurement error. With the theoretical records used for the previous analysis, 

 the method was not sensitive to array size, giving equally accurate results with arrays 

 up to ten times the size of the DWG-1. The spacing between the gauges of an array 

 will determine the wavelengths to which the gauge is most sensitive. Window widths 

 of approximately one tenth a typical period have been found to be effective. This 

 indicates that a gauge spacing of about one tenth of the expected dominant wave 

 length might be an optimum spacing. 



5.5 Laboratory Measurements 



The results from the analysis of analytically derived records given in the previous 

 sections demonstrate the method's capabilities in a variety of conditions. They have 

 also been very useful in helping to determine the range of solution parameters that 

 must be adopted, including order of solution, window width, and the number of time 

 samples taken in each window. The question still remains, however, as to how well 

 the method works with actual irregular waves. 



5.5.1 Laboratory Experiment 



The following results are taken from a laboratory experiment performed together 

 with Dr. Steven A. Hughes at the Coastal Engineering Research Center, Army Corps 

 of Engineers Waterways Experiment Station in Vicksburg, MS, in June of 1995. The 

 waves were generated in a 0.46m wide flume by a programmable, hydraulically driven, 

 piston type wave board (see Fig. 5.36). At the other end of the flume was a beach 

 with 1:30 slope, and a single layer of wave-absorbing horsehair matting. This beach 

 has been shown to have bulk reflection coefficients that vary from about 5% for 



