CHAPTER XIII.

THE DISCHARGE OF SEA OUTFALL SEWERS.


The head which governs the discharge of a sea outfall pipe is
measured from the surface of the sewage in the tank, sewer, or
reservoir at the head of the outfall to the level of the sea.
As the sewage is run off the level of its surface is lowered,
and at the same time the level of the sea is constantly varying
as the tide rises and falls, so that the head is a variable
factor, and consequently the rate of discharge varies. A curve
of discharge may be plotted from calculations according to
these varying conditions, but it is not necessary; and all
requirements will be met if the discharges under certain stated
conditions are ascertained. The most important condition,
because it is the worst, is that when the level of the sea is
at high water of equinoctial spring tides and the reservoir is
practically empty.

Sea water has a specific gravity of 1.027, and is usually taken
as weighing 64.14 lb per cubic foot, while sewage may be taken
as weighing 62.45 lb per cubic foot, which is the weight of
fresh water at its maximum density. Now the ratio of weight
between sewage and sea water is as 1 to 1.027, so that a column
of sea water l2 inches in height requires a column of fresh
water 12.324, or say 12-1/3 in, to balance it; therefore, in