(4) 
Z 2 
2t 
2 -1 
D y = vertical eddy diffusivity, cm sec 
Z = distance between relative point and measurement 
point (1/2 depth 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 limitations, 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 
388 
