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15. It has to be remembered that the normal dry-weather flow of sewage, will only fill the sewer to a depth of perhaps one-fourth or one-third of its diameter. At high water, the sea rises above the top of the sewer, and fills it completely. The stream of sewage then occupies the whole area of the sewer, and its velocity is reduced; so that deposit may take place. If the sewer were laid lower, say at low-water spring-tides then it would only be free from sea-water, and thus in a position to establish a proper velocity, for a few hours each month; and thus the probability of deposit would be much greater.
16. A good example of the manner in which in the absence of special flushing arrangement, deposit accumulates in a channel, to which the tide has access, may be seen at the mouth of the Albany Nullah. This nullah, above the tidal action, keeps itself quite free from deposit, the stream, running down its centre, having ample velocity. But in the part to which the tide has access, this velocity is checked, when the tide is high, and deposit takes place, which the stream cannot remove, when the tide falls. Had the invert of this nullah been made higher, as it might have been; this deposit would not have taken place.
17. The centre-line of the main-sewer at the face of the Praya-wall should, therefore, be at or near mean-sea-level. From the wall, the sewage should be carried out, to deep water, by a cast-iron pipe, of smaller size than the sewer; the diameter of this pipe should be so calculated, that the ordinary dry-weather-flow of sewage will establish a sufficient current in it, to keep it clear of deposit. This pipe may be prolonged, to any desired distance, into the harbour. At the Praya-wall, a storm- overflow will be provided, which will allow the sewage to escape direct, in rainy weather, when the sewer brings down more water than the iron outlet-pipe can carry. When this takes place, the sewage will be so diluted with rain-water, that the direct escape will cause no inconvenience.
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18. The level of the outlet, thus fixed, determines the distance at which the out-falls must be placed from each other. The main sewers must have a certain fall; and their upper extremities must be at a sufficient depth below the ground, to allow the bouse-drains to discharge into them.
19. It is further desirable that the sewage out-falls should not be in the im- mediate vicinity of the main storm-water outlets; especially those that discharge storm-water, from the hills, above the city. The silt which these storm-water channels bring down, will, in time, form deltas at their mouths, which will have to be removed by dredging. The cast-iron outlet-pipes will impede this operation; or be liable to injury in its execution. It is desirable also to keep the delta of silt and sand, free from sewage contamination.
20. In accordance with these principles, six out-falls have been selected, in the following positions:-
1. Opposite Queen Victoria Street,
2.
Hillier Street,
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3.
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Wing Lok Street,
4.
Eastern Street,
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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 than 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.