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 inclinations. 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 shows 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. Consequently, house-drains must be "ventilated," or rather provided with vents, to