RESTO ICTUD 



16 



(b) 57f5 of the total outflov/ or 6.0 x 10 -"-^cn^ is of recent 

 oceanic origin by way of the southern passes and channels. It 

 has been in the lagoon only during one or a few tidal cycles and 

 has not had tirae to becoiae thoroughly :,iixod with the water in 

 the central part of the lagoon. The remaining k3^o or 4,6 x lu^^ 

 cm-' is lagoon water. 



(c) The total inflow can be allocated as follows: 



(1) 35?^ or 3.71 x lO-'-^ci!!^ per 12 hours cones in over 

 the northern reefs and Joins the main mass of 

 lagoon \7ater, 



(2) 65^^ or 6,85 x 10 ^^cm^ comes in tlirough the southern 

 passes and channels, of which 6.0 is transient 

 according to (b) above, and the re.iaining 0,85 is 

 transported into the central part of the lagoon. 



(d) Therefore of the total interchange of 10,6 x lo^^+cm^ 

 per 12 hours, 4.6 x 10-'-^cm3, or 1.6^-3 of the total volixie of the 

 lagoon, perforra a slow flushing of the lagoon as a whole, v/hile 

 the remainder rapidly flushes a small area in the south and 

 southwestern part of the lagoon. 



The estimates of total inflow and outflow, based on a mean 

 tide of 100 cm, , are adequate for determining the average rate 

 of flushing over a considerable period of time, inuring shorter 

 periods the rate will vary from about 50^S of the calculated val- 

 ues (neap tide) to 160% (spring tide). Still larger variations 

 are obtained in the relative amounts of water passing over 

 different parts of the periphery of the lagoon, 



3,4 Vertical diffusion 



In section 3.22 it was shoivn that vertical mixing is effect- 

 ive in maintaining a relatively uniform temperature in the la- 

 goon. Diurnal heating in the absence of v/ind v/ould increase the 

 temperatiire about 2^0 at the surface during the dav. 'fhe increase 

 at the bottom would be less them 0,01°, Actually, however, the 

 sxrrface increase was never more than about 0,2°, The rest of the 

 heat v/as transferred dovmward by vertical diffusion, and the 

 temperature change at lower levels v/as correspondingly'" increased. 

 The rate of vertical transfer is readily aetermined for any par- 

 ticular temperature distribution: the constant in the equation, 

 Icnown in oceanographic literature as the coefficient of eddy 

 dlffusivity and designated by the symbol A^, can then be used to 

 determine the rate of transfer of any property of the water v/ith 

 any assmned initial distribution. This method is essential in 

 determining the rate of dilution of contaminated water after the 

 blasts. 



