2 Measurement Scheme 



Measurement Site 



The FRF is located on the Outer Banks of North Carolina, just north of the 

 village of Duck (Figure 1). The site is representative of many coastal sites 

 characterized by a broad continental shelf, relatively mild bottom slope, and 

 reasonably uniform longshore topography. The mild sloping bottom substan- 

 tially dissipates wind waves in the nearshore so that attention can be focused 

 on waves arriving from seaward. The uniform longshore topography allows 

 the use of a spatial array of sensors to detect wave direction without contami- 

 nation by a longshore inhomogeneity in the incident wave field. The facility, 

 described by Birkemeier et al. (1985), has a permanent staff and computing 

 facilities that are used to monitor, maintain, and error check the instrumenta- 

 tion on a day-to-day basis, and so minimize data losses from the myriad prob- 

 lems that can plague field data collection. 



Regional and local topography 



At the large environmental scale, the coastline in the vicinity of the FRF is 

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

 such that the shore-normal direction (facing seaward) is very nearly 70 deg 

 from true north (70 deg T). Waves and onshore winds can approach this site 

 along an easterly 180-deg arc from 340 to 160 deg T. The adjacent continen- 

 tal shelf is approximately 100 km wide, being narrower to the south where 

 Cape Hatteras juts eastward. The direction of nearest approach of the 100-m 

 isobath (the shelf break) is 10 deg to 15 deg south of east and is about 80 km 

 distant from the FRF. A characteristic bottom slope for the shelf is 1 m/km, 

 but, at a finer scale, the bathymetry is marked by numerous features of 1- to 

 10-km horizontal scales and 10-m vertical scales scattered irregularly across 

 the shelf. Waves with periods near 10 sec begin to shoal at depths of 100 m, 

 so that considerable refraction of wind waves propagating over the shelf is 

 expected (U.S. Army Engineer District, Wilmington 1980). 



Within a few kilometers of the FRF, the bathymetry 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 of the 600-m- 

 long research pier (Miller, Birkemeier, and DeWall 1983). Away from these 

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

 form at about 0.5 m/km and is reasonably dynamically stable, as indicated by 



Chapter 2 Measurement Scheme 



