TM No. 377 



Preliminary Wave Measurements in Narragansett Bay 



Two series of preliminary wave measurements were made with the OMDUM I 

 wave meter from a Navy pier in Narragansett Bay during May and August of 

 1963. These measurements were conducted primarily to determine the feasi- 

 bility of using the orthogonally mounted ducted meter ( OMDUM) design to 

 study surface waves. 



The experiments were conducted from the seaward (southwest) end of the 

 pier at the NUWS still basin, which is located at Coddington Cove in the east 

 passage of Narragansett Bay (see figure 17-1 ). Wave records could only be 

 made when the wind was from the SSW to SWW directions. Otherwise, the wind- 

 driven waves would have been grossly affected by refraction and defraction 

 caused by the breakwater to the west of the still basin and the Navy piers 

 to the south (figure 17-1). 



The first experiment was conducted on 20 May 19t>3» r ^ ie OMDUM I wave 

 meter (shown in figure II-3) was supported by a vertical pipe 8 cm in diameter. 

 •This pipe was securely fastened to the cap log of the pier, which was about 2 

 meters above the mean free water surface. Since the supporting pipe was 

 fastened at only one point, it displayed a slight horizontal oscillation (about 

 1-2 cm) in response to the waves moving past the meter. At the time of the 

 wave measurements, estimates of the wind speed and direction, wave length, 

 height, and direction were obtained. Since it was desired to measure wave 

 particle motions in the vertical plane normal to the wave crests, care was 

 taken to aim the horiztonal flow sensing component (the u meter) directly 

 into the oncoming waves radiating from the southwest, and to place the 

 vertical motion sensor (the w meter) in the exact vertical position. 



The power supplies required for the electronics of OMDUM I and the two- 

 channel Sanborn recorder were housed in the oceanography laboratory located 

 about 100 meters from the end of the pier. The power and signal were relayed 

 between the pier and the laboratory via a shielded, multi-conductor, weather- 

 proof power cable. 



This first experiment ended in failure. Shortly after recording commenced, 

 the signal leads of the two meters shorted. The resulting electrical leakage 

 caused cross-talk from one channel to the other, giving a garbled record oa 

 both the u and w channels. The brief two-channel record made prior to the 

 failure clearly displayed wavelike oscillations associated with the components 

 (similar to the upper trace of figure II-7)° When the electrical short was 

 repaired, the voltage supply was mistakenly plugged to the amplifier-oscil= 

 lator circuit (see figure 11-4), a 11 * 3 - the transistorized amplifier circuit was 

 damaged beyond repair. 



After electrical repairs were made to the OMDUM I system, wave measurements 

 were resumed on Ik August 1963 at the still basin. The second experiment yielded 

 results which were encouraging, and a brief resume of the preliminary results was 

 published (Shonting, I96I+ and 1965). 



72 



