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23. The sewers, even in the level parts, will have gradients sufficient to establish self-cleansing velocities, with the normal flow of sewage that may fairly be counted upon. The dry-weather flow of the nullahs which traverse the town may be used to augment the volume of the sewage. It
It may therefore be anticipated that very little artificial flushing will be required, though provision for so doing, if necessary, will be made.
24. The natural gradients of the tributary-sewers are, in almost every case, more than sufficient to establish self-cleansing velocities; with the normal flow of sewage. To ensure proper cleansing, however, it will be well to establish, at the heads of some of the branch-sewers, where the flow of sewage is small, automatic. flush-tanks. These consist of a tank, containing from 30 to 100 gallons, according to circumstances; and provided with a specially-constructed syphon, which, when the contents of the tank reaches a certain level, comes into action, and discharges the same, in a sudden rush. These automatic flush-tanks may be conveniently erected, in connection with the public fountains, receiving the waste-water and slops from them. In this way, water, that would otherwise be wasted, will be used to cleanse the branch-sewers, without any additional demand on the water- works.
25. The size of the main sewers is sufficient to discharge about one inch rain in 24 hours, falling on the district, which drains to each.
26. With the exception of a few short branches, the minimum size of street sewer will be nine inches. As regards the conveyance of sewage only, a much smaller diameter would suffice. Experience has shewu, however, that long street- sewers, less than 9" in diameter, are inconveniently liable to casual obstruction. Moreover the rain-water from back-yards, and in some cases from the roofs of houses, must be admitted to house-drains, and thence to the branch-sewers. The excessive capacity which the use of 9" branch-sewers affords, will allow them to carry off the rain-fall, which must be admitted to the house-drains. But, at or near the points where the branch-sewers join the principal collecting sewers, storm- overflows will be provided, by which any water brought down by the branches, over and above the quantity which the collectors can carry, will escape to the storm-water channel. So, when the secondary collectors join the main-sewers, other storm-overflows will be provided. By these means, the main trunk may have a carrying capacity much less than the sum of those of the branches, and the evils attending large sewers may be avoided. In short, the system at its commencement, will be "combined," receiving both rain water and sewage, in the course of the collecting and out-fall sewers, more and more of the rain-water will escape by storm-overflows; till the iron outlet-pipe will carry the dry-weather sewage only.
27. The sewers will be provided with man-holes, at all junctions and changes of direction; so that there will be complete access to the sewer, for the purpose of inspection; and for the removal of casual obstructions.
28. In a report on the sewerage of the High-level District the principles of sewer-ventilation have been fully discussed. The same principles apply to the lower district also. Whenever practicable, it will be well to carry up ventilating pipes, above the roofs of the houses, care being taken that these elevated ventilators are as inconspicuous as possible. It must always be remembered that the real remedy for stenches from ventilating openings consists in self-cleansing sewers, good house-drainage, properly used, with a liberal water-supply; and not any elaborate or complicated system of ventilation.
29. The existing storm-water channels should be utilised, to the fullest extent, for carrying off rain-water. It is probable that the greater number of these will require little more than the usual repairs.
30. In the lower parts of the town, however, the storm-water channels will, in many cases, require renewal; on account of the new Praya works. When this is the case, care must be taken to raise the invert of the channel to the highest practicable level, consistent with sufficient discharging capacity. The object of raising the invert is two-fold: firstly to give a good gradient across the new recla- mation, and secondly to make the outlet as high as possible, so that the storm-water channel may be free from tide-water, for as long a daily period as possible. This is desirable, for the reasons set forth, when discussing the questions of sewer-outlets. An elevated invert also facilitates construction, at it increases the time that the foundation is uncovered by the tide. To give the necessary sectional area, to the