17 - 



The installation site of the DPG was proposed as the United 

 States Army Corps of Engineers Field Research Facility (FRF) at Duck, 

 North Carolina, operated by the Coastal Engineering Research Center 

 (CERC) . The instrument was to be placed near and hard-wired to the 

 Facility research pier. After examining the bathymetry near the pier 

 and considering the high cost of underwater cable, a nominal 

 deployment depth of 20 feet was selected. This indicated that the 

 instrument would be most sensitive to waves of about 5 second period, 

 (Figure II-2) . This was considered acceptable at the time since the 

 mean monthly nearshore wave periods for this site were reported 

 between A. 8 and 6.5 seconds (SPM, Figure 4-11, 1977). (It was 

 discovered much later that this estimate was poor. A better estimate 

 is given as about 7.5 seconds from Birkemeier, et.al., (1981).) A 

 design wave height of 16 feet was chosen and considered with a gage 

 length of five feet. This suggested the use of a +0.6 psid 

 transducer which was unavailable. A +0.5 psid transducer was 

 selected instead, mandating a gage length of 4.15 feet (1.27 meters) 

 using Equation 2.3. 



The overall dimensions of the final instrument, then, became 

 9 feet - 9 inches (2.97 m) along each axis (including protective caps 

 on the ends of each arm) and 40 inches (1 m) in height to accomodate 

 the electronics package. 



