PART III: MEASUREMENT SCHEME 



Measurement Site 



77. For the long-term climatology of directionally distributed waves to 

 be useful, the conditions under which data were collected must be known. This 

 section gives a brief description of the measurement site, the directional 

 gage, and some important aspects of data treatment. 



78. As shown in Figure 1, the coastline in the vicinity of the FRF is 

 nearly straight for several tens of kilometres north and south. It is 

 oriented such that the shore -normal direction (facing seaward) is very nearly 

 70 deg from true north. Waves and onshore winds can approach this site along 

 an easterly 180-deg arc from 340 to 160 deg relative to true north. 



79. The adjacent continental shelf is roughly 100 km wide, being 

 narrower to the south where Cape Hatteras , NC, juts eastward. The direction 

 of the nearest approach of the 100-m isobath (roughly, the shelf break) is 10 

 to 15 deg south of east and is about 80 km distant. A characteristic bottom 

 slope for the shelf is 1 m per kilometre. At a finer scale, the bathymetry is 

 marked by numerous features of 1- to 10-km horizontal scales and 10-m vertical 

 scales scattered in an irregular fashion across the shelf. Waves with periods 

 near 10 sec begin to be influenced by the bottom at depths of 100 m. Con- 

 siderable refraction of wind waves propagating over this shelf is expected 

 (see, e.g., US Army Engineer District, Wilmington 1980). 



80. Within a few kilometres of the FRF, the bathjmietry is more regular. 

 A complex bar system exists within about 300 m of the shore (Birkemeier 1984) , 

 and waves and currents have created some irregular bathymetry in the immediate 

 vicinity of the pier (Miller, Birkemeier, and DeWall 1983). Away from these 

 regions, isobaths are nearly shore parallel. The bottom slope is nearly 

 uniform at about 1 m per 500 m and is reasonably stable, as indicated by 

 repeated surveys (Howd and Birkemeier 1987). Figure 3 illustrates the 

 bathymetry over a region 600 m north to 600 m south of the pier and extending 

 about 900 m offshore. Figure 3 also shows the coordinate system used to 

 define wave direction and to which Equation 1 applies. 



81. The site is subject to a variety of climate which gives rise to a 

 broad diversity in directionally distributed wave energy. Typical wind and 

 wave climate can be roughly classified as four basic types (Leffler et al. 



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