972
THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
20. In accordance with these principles, six out-falls have been selected, in the following positions :-
1. Opposite Queen Victoria Street,
2.
19
Hillier Street,
3.
*
Wing Lok Street,
4.
"
Eastern Street,
5.
"
Western Street,
6. at Slaughter-house Point.
21. In the present condition of the Praya, the position of the out-fall opposite Hillier Street, in a bay, is not all that could be desired. The Praya Reclamation will, however, remedy any defect in this respect; and after careful examination, it has been found impracticable to drain the whole of the flat district, near Bonham Strand, to the Wing Lok Street out-fall, without the use of gradients so flat as to endanger the self-cleansing properties of the main sewer; and thus give rise to evils far greater thau any that can be caused by the out-fall, even were it to remain as it is at present.
22. The area draining to the several out-falls is shewn by the coloured lines on the plan. In the arrangement of the tributary-sewers, care has been taken to concentrate the sewage flow of each district, in the upper ends of the intercepting sewers, along the Praya, which have, necessarily, flat gradients; compared with those in the upper parts of the town. This will ensure a sufficient flow through them, whenever the tide is down, to set up a self-cleansing velocity through them; and thus obviate, to a great extent, if not altogether, the necessity for artificial flushing.
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 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, contain- ing from 30 to 100 gallons, according to circumstances; and provided with a specially-constructed syphon, which, when the contents of the tank reach 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 raiu 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 shewn, 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 commence- ment, 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 com- plete 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 clevated 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 honse-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. bable that the greater number of these will require little more than the usual repairs.
It is pro-
30. In the lower parts of the town, however, the storm-water chaunels will, in many cases, require renewal; on account of the new Praya works. When this is the ease, care must be taken to raise the invert of the channel to the highest praticable level, consistent with sufficient discharging capacity. The object of raising the invert is two-fold; firstly to give a good gradient across the new reclamation, 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
No comments yet.
Private notes are available after approval.