2208 ORDINANCE No. 24 OF 1887 .
Public Health.
this must be carried off by the house-drains. A duplicate set of drains for such areas,
one for sewage and the other for rain-water, would be a costly complication and there
would be no security that each set would be used for its proper purpose only. Some
sewage would almost inevitably find its way into the rain-water drain, which, being of
large size, would be imperfectly flushed and therefore offensive.
L. Rain- water should be diverted from house-drains to the full extent that is
possible by the use of surface - channels. As a general rule, when surface- channels
cannot be used to divert rain-water, it must be admitted to the house-drain . Hence,
in the majority of cases, the amount of rain-water to be carried off determines the size
of the house-drain . The table, hereunto attached, gives the area from which 4" of rain,
falling in one hour, will be carried off by pipes of various sizes , laid at different inclina
tions. In providing for the removal of rain-fall, from a given surface, it is to be
remembered that it is not sufficient to provide a pipe of ample size to carry off the
rain-fall. Traps and gratings must be provided in sufficient numbers and of sizes to
admit the rain-water freely to the drain and the surface must have a fall to carry it to
the openings. In many cases neglect of these precautions has caused flooding. This
has been put down to the size of the pipes, whereas the inlets were really at fault.
Rain-water cannot be looked upon as a legitimate agent for cleansing house -drains
from deposit . Firstly, there should be no deposit to remove from well constructed and
properly managed drains ; and secondly, because there are long intervals in which no
rain falls so that this flushing agent fails when most wanted.
M. The irregularity and uncertainty of the flow of sewage, which precludes an
accurate determination of size, makes it impracticable, to calculate definitely the proper
inclination of house-drains. Experience shews that, under the normal conditions of use,
an inclination of one in thirty is desirable to ensure a self- cleansing drain ; especially when
the drain conveys the sewage from a single trap or inlet. An inclination of one in
thirty, therefore, should, as a rule, be given to house-drains. Drains laid at much
flatter gradients can be made to work perfectly, with care, and provided that the flow
of sewage is copious i. e. sufficient to fill the pipe nearly half full. The designer of
house-drains having no control over their subsequent usage, will do well to give them,
in every case, the greatest practicable fall.
N. The ordinary " trap " or syphon is not a perfect preventive against the escape
of sewage-tainted air from house drains , although it is the only appliance available for
the purpose. The water which seals the trap may evaporate during disuse, or a piece
of fibrous matter may act like a syphon and draw off the water sufficiently to leave a
free passage for the escape of sewage-tainted air. Again, if there be a series of inlets.
connected to a drain which is unprovided with any free outlet ; a sudden rush of water
through one trap will force the air contained in the drain through the other traps or
unseal them by drawing off their contents, and open a passage for the escape of sewage
tainted air into the dwelling, or into some confined space in its neighbourhood. Con
sequently, house-drains must be " ventilated," or rather provided with vents, to