At low tide the tidal forces vanish, and the only flow is the seaward 

 transport in the upper zone. As the tide rises the sea-water enters 

 the lower zone, since its density is greater than the upper zone, and 

 creates a flood current, which increases with the rate of tidal rise. The 

 slack water associated with low tide occurs when the opposing tidal 

 and displacement velocities are equal. As the tidal velocity continues 

 to increase, the upper zone is erased to seaward and retreats into the 

 channel. It becomes thicker, and fresh water accumulates, increasing 

 the isostatic forces until they equal the hydraulic forces of the tide. 

 In this situation a front separating the invading sea-water and the 

 upper zone is formed near the mouth of the channel (Fig. 1) which 

 progresses in-shore as the tide rises. 



Eventually the isostatic and tidal forces again attain equality, due 

 to increase of the former or decrease of the latter, and a second period 

 of slack water occurs, associated with high tide. Thereafter the 

 surface seaward displacement is resumed, while the flood movement 

 in the lower zone decreases to zero at high tide. 



The sea-water entering the lower zone during the flood, less the land 

 drainage, is the volume required to make up the tidal rise. The ebb 

 from both zones consists of the volume required by the fall of the tide, 

 plus the land drainage during this phase. 



Then during the steady state the fresh water transferred seaward 

 through any cross-section of the channel during a tide-cycle is equal to 

 the land drainage above the section, and equal volumes of sea-water 

 flood and ebb through the section in a cycle. If these consequences 

 did not obtain, fresh or sea water would accumulate over a series o"f 

 cycles and the channel would become totally fresh or sea water. 

 Obviously the considerable mixing during the process has no effect oh. 

 the quantities transferred. 



It is implied in this argument that surface sea-water submerges 

 below the upper zone at the front and enters the channel in the lower 

 zone during the flooding tide, some of it is transferred to the upper 

 zone in the channel, and sea-water escapes from the channel by both 

 zones during the ebb tide. It foflows that, aside from the fresh water 

 present, the movement of sea-water in the upper zone is biased in the 

 ebb direction, while that rn the lower zone is biased in the flood direction, 

 in compensation for the sea-water transferred upwards. 



Tttrbulence 



The progressive increase of salinity in the upper zone must be the 

 result of some mechanism of turbulence. Turbulent flow is usually 

 exemplified in Nature by a large number of more or less coherent eddies 

 having random compensatory motion within the fluid. This motion 

 is opposed by friction between the elements (eddy viscosity) resulting 

 in an exchange of fluid between the eddies and their surroundings 

 (mixing). 



In a density stratified medium any movement of the fluid elements 

 within a stratum is opposed by viscosity alone, but any movement of 

 the elements from one stratum to another is also opposed by isostatic 

 forces, which tend to return the element to its own level. From this 

 it would be expected that a stratified system would become more 

 homogeneous down-stream, and that lateral homogeneity would be 

 more readily attained than vertical homogeneity. This reasoning has 

 bsen confirmed by observation (1) and is indicated in Fig. 1. 



270 



