TM No. 377 



topography and wind conditions, together with tidal action- bring about 

 complex motions (see Shonting and Cook,, 3.966) <, Whatever the source of the 

 low frequency motion, its effect could be injurious to the assumptions of 

 stationari.ness discussed in chapter 111= These quasi-random horizontal 

 motions may be of an appropriate frequency to contribute unwanted spectral 

 energy to the wave motions during a 5= to 15<-minute period of observation. 

 Although it is not obvious how these motions ( -Ud ) interact with actual 

 wave motions ( 1&w ),, an. attempt should be made to establish the existence 

 or nonexistence of these low frequency components * 



The following sections present results of two series of long term 

 observations of the gross current structure at the BBELS., along with, the 

 simultaneous tide gauge records. This discussion of the gross environment 

 of BBELS is rounded out by a brief consideration of the seasonal sea tem- 

 peratures and meteorological measurements which supplemented the actual 

 wave measurements,, 



Tidal and Non^Tldal Current Observations — The most obvious motions 

 observed at the BBELS, aside from the waves, are currents generally associ- 

 ated with the semi-diurnal tides. During relatively calm seas,, surface 

 flew is observed from the platform in the form of wakes, extending down- 

 current from the platform legs, and in the- motion of seaweed and flotsam. 

 Also, during periods of maximum current 5 deflection can be observed in the 

 vertical cables supporting submerged anti-corrosion electrodes., which are 

 suspended from the catwalk underneath the platform* No quantitative data 

 are available regarding the tidal currents at the BBELS* The tidal ampli- 

 tude is recorded continuously on the Coast and Geodetic Survey tide gauge 

 system permanently installed in the BBELS laboratory. 



It was decided to make some long-term current records at the tower; but 

 to use a relatively short sampling interval* This was done to provide 

 information on current fluctuations having a tidal origin as well as those 

 having periods approaching the sampling intervals of the wave measurements „ 

 The specific aim was to determine . (1 ) r ne consistency of the pattern of 

 current speed and direction throughout the tidal cycle: (2) the relationship 

 of current speed and direction to the free- surface tide record}: and (3) the 

 energy contained in motions having periods smaller than the semi-diurnal 

 tide, expecially those approaching the sampling periods of the wave obser- 

 vations (i,e«, 5-10 minutes)* 



Two long-term current observations were made at the BBELS and provided 

 much information on these subjects., The sensor system used for both series 

 of measurements was a type-31o 5 self -recording,, histogram current meter 

 (shown in figure IV- 5) manufactured by the Braincon Corp* of Marion,, Mass* 

 The cylindrical housing is about 120 cm long and 18 cm in diameter. The meter 

 utilizes a Savonius rotor speed sensor enclosed in a bail at the end of the 

 cylinder housing* 



The instrument detects and records the following data as a photographic 

 analog on 16 mm film; (l) total number of rotations of the Savonius rotor.; 



77 



