order to ascertain the effective head producing discharge it
will be necessary to add on 1/3 in for every foot in depth of
the sea water over the centre of the outlet.

The sea outfall should be of such diameter that the contents of
the reservoir can be emptied in the specified time--say, three
hours--while the pumps are working to their greatest power in
pouring sewage into the reservoir during the whole of the
period; so that when the valves are closed the reservoir will
be empty, and its entire capacity available for storage until
the valves are again opened.

To take a concrete example, assume that the reservoir and
outfall are constructed as shown in Fig. 34, and that it is
required to know the diameter of outfall pipe when the
reservoir holds 1,000,000 gallons and the whole of the pumps
together, including any that may be laid down to cope with any
increase of the population in the future, can deliver 600,000
gallons per hour. When the reservoir is full the top water
level will be 43.00 O.D., but in order to have a margin for
contingencies and to allow for the loss in head due to entry of
sewage into the pipe, for friction in passing around bends, and
for a slight reduction in discharging capacity of the pipe by
reason of incrustation, it will be desirable to take the
reservoir as full, but assume that the sewage is at the level
31.00. The head of water in the sea measured above the centre
of the pipe will be 21 ft, so that

[*Math: $21 \times 1/3$],