achieve environmentally acceptable standards. Before the effluent is dis-
charged into the sea, the grit is removed to avoid its deposition in the
outfall pipe and the suspended matter is reduced to small sizes (the
screens should pass a maximum particle size of 5–6 mm). If the suspended
load in the effluent is such that the marine treatment is not very effective
environmentally, then primary sedimentation may be incorporated before
discharging the effluent into the sea. If the effectiveness of the outfall falls
short of the acceptable bacterial content, then disinfection may be recom-
mended. It is worth noting that the sludge produced in primary sedimenta-
tion may be difficult to dispose of and that disinfection of the effluent can
be detrimental to the aquatic life.
In this concise text many aspects of sea outfalls could be touched on
only briefly; for a fuller treatment of the subject, the reader is referred to
Charlton (1985), Neville-Jones and Dorling (1986), Wood et al. (1995) and
Roberts (1996).
15.7.2 Site surveys
For effective design of outfalls, site surveys should be undertaken to assess
the quality of receiving water, tides and currents, and environmental con-
ditions (e.g. seaweed, animal life, bacteria, etc.). Charlton (1985) describes
the methods used to obtain a form of tidal atlas in which the current vari-
ation throughout the tidal cycle is presented on an hourly basis. The
overall performance of the outfall is predicted by tracer tests. These tests
are carried out usually in normal conditions of the sea by introducing
tracers at the site and the dispersion and dilution of the tracer are continu-
ously monitored mainly at a depth less than 1 m.
15.7.3 Initial dilution and secondary dispersion
The effluent passing through a port is lighter than the surrounding sea
water; the density of sea water is typically 1.026 kg m^3 and depends on the
ambient temperature and salt content. Thus a density gradient across
the depth may exist due to variations of temperature and salinity with the
density increasing in depth. The density stratification is likely to be more
pronounced in summer than in winter.
Consider that the effluent from the port exhausts in the vertical
direction into the sea of uniform density without ambient currents. The
resulting buoyant jet spreads owing to its initial momentum rate and the
buoyancy effects. Without the momentum rate, the effluent from the port
will spread as pure plume driven only by the buoyancy. As the jet rises to
654 COASTAL ENGINEERING
