Reynold showed that in a wide channel the incidence of turbulence 

 s a function of the average velocity and depth (of homogeneous layer), 

 as shown in Table I. 



Table I — Reynold Number 

 Flow is turbulent when — ■ 



VDp 



Nr = = 500 



V 



Nr Reynold number. 



V mean horizontal velocity. 

 D depth of channel. 



p density of fluid. 



V kinematic viscosity. 



Velocity Depth, 



cm. /sec. cms. 



7-65 3-05 



1-53 15-25 



0-77 30-5 



0-15 152-5 



In the vicinity of an estuary the mixing between fresh and sea- 

 water zones must be attributed to external forces such as the wind and 

 tide, because the isostatic flow is in general less than the threshold of 

 turbulence as defined in Reynold's experiments. Examining the dis- 

 charge of several rivers(l, 2, 3) it became evident that the flow due to 

 the dispersal of fresh water alone would cease to be turbulent at a short 

 distance from the outfall because of the widening front of flow in the 

 sea. 



The tidal velocities in most coastal regions are adequate to cause a 

 considerable degree of turbulence, and because they are rotary or 

 reversing their contribution to transport is small. In open seaways 

 mixing due to the acceleration of the wind must also be considered, and 

 may be assumed to be the predominant factor in the ocean, where 

 tidal movements are small. 



Obviously some degree of mixing would result from isostatic move- 

 ments, but in general the mixing function is superimposed on the 

 isostatic flow functions by extraneous forces. It is concluded that in a 

 simple channel mixing occurs primarily at the expense of the tidal rise 

 and faU, while the seaward transport of the water is due to the 

 displacement head. 



This was confirmed by observation in Alberni Inlet (1) where the tidal 

 velocities attained 150 cm. /sees., but the rate of net transport seaward 

 was of the order of 0-01 to 0-06 cm. /sec. 



It is evident that an isostatic movement would occur whether or 

 not there was mixing. Therefore the seaward movement of fresh 

 water is not consequent on the rate of mixing. However, it is implied 

 that the fresh water must move seaward at a rate equal to its supply, 

 wherefore the stream must accelerate or diverge in proportion to its 

 dilution with sea-water. This agrees with observations(l) which show 

 that the velocity and mass transport in the upper zone of such systems 

 increases to seaward, and that the dimensions of the zone is constant 

 within certain limits of variation. 



271 



