SEA OUTFALLS 655
the surface, entrainment of the surrounding water takes place with the
maximum velocity and concentration along the centreline of the jet decay-
ing in the streamwise direction. Instead of concentration, the term dilution
is commonly used. If, say, 99 ml of pure salt water mixes with 1 ml of the
effluent, then the dilution is 100:1 or simply 100. On reaching the surface
of the sea the effluent tends to spread horizontally.
The density of the effluent increases through dilution. If the receiv-
ing water is density stratified, then the effluent may not even reach the
surface as, when it reaches a value at some depth equal to the density of
the surrounding water, the plume is arrested and spreads horizontally.
The next phase of the effluent transport is the advection by ocean
currents and turbulent diffusion. The phase, called the secondary disper-
sion, depends on the current and the turbulence structure of the ocean.
Ports generally discharge effluents horizontally to achieve the best
dilution (Fig. 15.12). Figure 15.12 also shows the spread of the jet to be dis-
cussed further in Section 15.7.6. For horizontally discharged effluent,
about 20% to 50% greater dilutions are obtained at the water surface than
for vertical discharge.
Fig. 15.12 The spread of horizontal jets in still water (after Charlton,
1985)