the measured distribution calculated from the wave observation data. 



Considering the errors inherent in the visual data collection method, in 



the data analyses techniques, and errors resulting from presenting the 



continuous distribution of wave approach angles as approach sectors, 

 Table 15 shows a favorable comparison. 



Table 15. Predicted and measured distribution of wave energy at 

 Wrightsville Beach. 



Sector 



Sector bisector 

 (rel. to North) 



Pet wave energy 



Predicted 



Measured 



1 

 2 

 3 

 4 

 5 



60° 

 103.5° 

 120° 

 137.5° 

 180° 



0.8 

 28.0 

 36.0 

 35.0 



0.2 



1.4 

 31.2 

 38.7 

 28.2 



0.8 



(b) Bathymetric Data. The wave refraction model requires knowledge 

 of the general bathymetry offshore from the study area to accurately 

 refract the approaching wave sets. The bathymetric data was provided on 

 a 150-meter (500-foot) square-grid spacing which extended from the MLW 

 position of the shoreline to a depth of approximatley 20 meters 



(65 feet), 15 kilometers (9.4 miles) offshore. The nearshore depths 

 were interpolated from the long beach profiles and the greater offshore 

 depths were measured from 1978 National Ocean Survey (NOS) nautical 

 charts . 



The offshore bathymetry of the study area is quite irregular and a 

 qualitative graphical representation of it is shown in Figure 34. This 

 figure is a three-dimensional line drawing display of the data generated 

 by a computer graphics program, and consequently the offshore 

 representation is quite accurate. However, the interpolation scheme 

 used by this program distorted the shoreline position, and a dot screen 

 pattern has been included to alleviate this visual distraction. 



(c) Wave Refraction Model. The numerical model used for the wave 

 refraction analysis is a modified version of the wave refraction model 

 developed by Dobson (1967). Dobson's model requires the wave ray to 

 originate in deep water, a condition which is not always practical (or 

 economical relative to computer costs) for long-period waves. There- 

 fore, a subroutine was added to account for the refraction and shoaling 

 of the wave ray which occurs in the deeper offshore regions. This 

 routine assumes that bathymetry in the offshore region has straight 

 and parallel contours. Snell's law is used to compute the refraction 

 coefficient and the change in the wave angle at an economically more 

 reasonable "offshore" boundary for the model. The partially refracted 

 wave ray is then used as the starting condition for Dobson's numerical 

 model which integrates the wave ray through shallower regions toward the 



67 



