2t 



Dy = vertical eddy diffusivity, cm sec 



Z = distance between relative point and measurement 



point (1/2 deptli in this case), cm 

 t = time for the concentration to asymptote, sec 



The average value of this parameter for Narragansett Bay has been computed 

 by Hess (1976) using a detailed numerical hydrodynamic model. 



The Horizontal Turbulent Field 



Determination of the horizontal turbulent component was more difficult. 

 At one extreme, we measured the time it took for small dye patches (0.5 ml of 

 1:100 dye in seawater) to disperse in the microcosms and in the bay under a 

 range of conditions. This approach was simple, rapid, and with enough 

 replication and a constant observer, it gave a good (low variance) relative 

 measure of horizontal mixing rates. Unfortunately, it is also a bit subjective 

 and qualitative and cannot be expressed directly as a standard hydrodynamic 

 parameter. 



In an attempt to overcome these hmitations, we have also obtained 

 measurements of neighbor diffusivity (Richardson, 1926; Stommel, 1948) and 

 the flux of turbulent energy along two arbitrary perpendicular coordinates 

 (Batchelor, 1950) using the relative motions of pairs of small floats with a 

 range of distances separating them. The measurements were made by releasing 

 several dozen floats and then photographing them from a fixed position at 

 short time intervals. The size of the floats used and the length of the time 

 interval were varied somewhat according to the scale of the turbulent eddies of 

 interest. In the West Passage of Narragansett Bay, larger scale mixing (1-25 m) 

 was studied using colored balloons filled with fresh water so that they floated 

 just beneath the surface. These floats were dispersed from a small boat and 

 photographed every few minutes from a high bridge. Smaller scale eddies 

 (1-200 cm) were studied in the field using small (~0.5 cm) colored plastic 

 beads that were released from the end of a pole off the stern of a small boat 

 and photographed every few seconds using a 16 mm movie camera operated 

 from the flying bridge of the boat. The beads and the movie camera were also 

 used in the microcosms. In all cases, floating rods of standard length were 

 included in each photograph to give an accurate scale. 



After they were developed, the films of the floats were put through a 

 microfiche reader for enlargement. Large numbers of pairs of floats were 



390 



