RESTRICTaP 



17 



There are two inherent difficulties in applying these methods 

 in ijikini lagoon. First, the observed difierences are small, so 

 that the errors of ::ieasure:ient in any particular set of observa- 

 tions may be as riuch as 40-'^ of the total difference. Second, it 

 was not possible to- sauple the saine body of water at successive 

 intervals, v/hich introuuces ranaa-a ocean ographic variations. 



The errors are to a considerable extent eliiiiinatea by smooth- 

 ing the curves and by using several inaependent nethods of cori- 

 putation. i'hese raethods were: 



(a) The relationship between v/ind velocity and surface cur- 

 rent gives values of eddy viscosity, which in uniform v/ater 

 should be numerically equal to Ay. 



(b) The diffusion coefficient was computed from the diurnal 

 temperature variations previously aescribed, 



(c) At noon, and especially diiring low tide, the waiter flov/- 

 ing in over the eastern and northern reefs is appreciably warmea 

 during its passage, its salinity is raised, and it becomes rich 

 in oxygen. The mixing of this characteristic water from a "line 

 source" with the rest of the vrater in the lagoon provides another 

 iaethod of computing vertical dii'fusion which is of particular 

 interest because of its analogy with the surface contamination 

 expected in test Able, 



Method (a) and method (b),for the eastern station gave val- 

 ues of Ay around 200 to 250 cm^ per second, Llethod (b) for the 

 v/estern station gave values much higher than are reasonable, indi- 

 cating tliat processes other than vertical diffusion were active. 

 The result is believed to be due to sinking of surface water, 

 which effectively brings the surface temperature fluctuations 

 to greater depths by other means than turbulence. The sinking 

 is the result of the gradual slov;ing dovm of water which is being 

 driven against the v/estern reefs, and seems to be distributed 

 over a large part of the western lagoon. 



Method (c) leads to somev/hat smaller values of eddy diffu- 

 sivity near the reefs. This may be partly due to upwelling, 

 partly to the fact that the "line soTirce" is located near the 

 surface, v/here the scale of turbulence must be suppressed by the 

 existing boimdary. Hoxvever, theoretical considerations indicate 

 that Ay will increase to a value of about 25O at a depth of 2,5 m, 

 and probably changes little if any from that depth down to very 

 near the bottom. 



These figures will be used in the section that follows to 

 determine the rate of dilution of radioactive products by ver- 

 tical diffusion. 



