Mixing 



In general, the upper and lower zone both become more saline to 

 seaward, from which it is reasoned that all the fresh water is dispersed 

 in the upper zone, and little or no fresh water is lost by downwards 

 transfer. This requires some explanation, since in any process of mixing 

 the volume of water transferred downwards must equal the volume 

 transferred upward, else the medium would become discontinuous. 



In the case of an upper zone carrying fresh water over a lower sea- 

 water zone which moves in the same direction with the same or greater 

 velocity, the exchanged water would be carried forward in both zones, 

 the upper zone becoming more saline and the lower zone less saline as 

 they approach homogeneity. However, this process would imply that 

 the upper zone flow was in the opposite direction and of lesser magnitude 

 than the movements in the lower zone. It does occur near the front 

 at the mouth of the inlet and results in modifications of the simple 

 system, which will be discussed later under " Complex Outflows." 



It is more usual to find the lower sea-water zone moving in an 

 opposing direction relative to the upper zone, in which case both layers 

 become more saline to seaward. The sea-water transferred upwards 

 is carried forward with increased velocity to be exchanged for more 

 saline water down-stream. The upper-zone water transferred down- 

 wards wiU lag behind the surface flow, or be carried backwards in a 

 boundary zone, in which there is a transition between the salinity of 

 the lower and upper zones, and be preferentially subject to further 

 mixing into the upper zone. Any fresh water accumulating in the lower 

 zone would be isostatically unstable and would tend to join with the 

 upper zone in the boundary layer. 



The nature of the mixing process is indicated by the vertical salinity 

 gradients. It may be assumed that there is a compensatory exchange 

 of water across the boundary, and then to the adjacent parts of the 

 zones. If the mixing at the boundary were more rapid than the mixing 

 in the adjacent parts of the zones, mixed water would accumulate, 

 and a homogeneous boundary layer would be created, as shown in 

 Fig. 3 {a). This may be observed in the vicinity of a front, as will be 

 discussed later. It is unstable since it implies a large turbulence function 

 confined to the boundary, and normally such a function also includes 

 the upper zone. 



If the rate of mixing in the boundary equalled the rate of mixing 

 adjacent to the boundary, the boundary zone would lose its identity 

 in the formation of a simple gradient from the surface to the lower zone 

 as shown in Fig. 3 (6). This is occasionally observed in Nature when 

 the stability of the boundary zone becomes small. 



Finally, if the mixing adjacent to the boundary were more rapid 

 than the mixing in the boundary, the salinity gradient would be expected 

 to be a maximum in the boundary which would be a distinct zone, as 

 shown in Fig. 3(c). This is the form of gradient commonly observed 

 in simple situations(l, 2, 3). 



The structure consequent on these processes is illustrated in Fig. 4 {a), 

 where a representative series of salinity gradients are shown in relation 

 to their position in the simple channel of Alberni Inlet(l). Figure (4) {b) 

 shows the same gradients plotted on a logarithmic depth scale and 

 superimposed on each other. 



272 



