145 



6.5 Field Measurements 



The following results are from data provided by Gary L. Howell of the US Army- 

 Corps of Engineers Waterways Experiment Station, Coastal Engineering Research 

 Center (CERC). The data were collected as part of CERC's Coastal Inlets Research 

 Program. They were collected near the Ponce de Leon inlet, Florida, by a DWG-1 

 with three absolute pressure gauges, data sampled at 5 Hz. The DWG-1 was resting 

 on the bottom, so that the pressure sensor diaphragms were positioned 0.21m from 

 the bed. 



6.5.1 Field Results 



Figure 6.8 is a segment of a record collected on August 20, 1996. The statistics of 

 the record, based on about one hour of record are: mean wave height = 1.05m, peak 

 period = 5.6s, peak direction = 83° from true north, in a depth of approximately 

 7.9m. 



The top plot is the dynamic pressure at the center of the array. The solid line 

 is the approximate measured pressure, computed by linear interpolation from the 

 three measured points in the array. The circles are the values predicted by the 

 LFI method. The other four plots are the predicted water surface, and the three 

 orthogonal velocities, as predicted by the LFI computation. The single wave method 

 was used, with the following parameters: third order [J = .3), four water surface 

 nodes and four samples on the pressure records (/ = M = 4), and a window width of 

 1.42s {0.3T;;), resulting in 20 equations in 11 unknowns. There were no data available 

 about the Eulerian current, so it is taken as zero. 



Window Width In the previous section, results on theoretically generated records 

 indicated the successful solutions were possible with quite narrow windows, gener- 

 ally one tenth of the zero crossing period. When working with this field record, the 

 optimization converged with a window width that narrow, but it resulted in wild fluc- 

 tuations in the predicted propagation direction from window to window. Figure 6.9 is 

 the same segment of the record as in figure 6.8, computed with the same parameters, 



