THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890. 971
of the place be increased. The Praya is not a promenade; nor is Victoria a bathing place. The cost of complete inter- ception cannot, therefore, be justified by any corresponding advantage, and should not, therefore, be incurred; until, at least, experience has demonstrated the inaccuracy of this conclusion.
11. Were the existing Praya the permanent sea-frontage of the city, it would be necessary to carry out the sewage out- falls by means of iron pipes, laid at the bottom of the sea into deep water, and to a distance of from 50 to 100 yards from the shore, so as to be clear of tiers of vessels which throng the wharf; and check the flow of the tide. But it has been decided that the Praya is to be advanced by about 280 feet, and the rubble-bank on which the new wall will stand will be in 15 to 30 feet of water. It is also intended that the wall itself shall be carried down, vertically, to a depth of about three feet below low-water-mark spring-tides. The rubble-bank will always be covered with water. The increased depth, the improved alignment of the new wall, together with the constriction of the waterway, caused by the advance of the Praya, will most probably produce an improvement in the tidal current, along its front. When the new works are complete, it will be sufficient to carry the out-fall, to the edge of the berm of rubble, on which the wall is founded, or so far beyond it as to reach a depth of six feet at low-water spring-tides. By using cast-iron pipes with ball-and-socket joints, a further ex- tension may at any time be made.
12. During the progress of the Reclamation Works, the sewage should be carried out to the new line of sea-wall, by means of temporary wooden shoots, supported on piles. In arranging the levels of the sewers, the position of the out-fall; and the level of the sewers, has been selected, in accordance with the proposed alignment of sea-wall.
13. The Harbour being accepted as the destination of the sewage, the only reasons, for limiting the number of out-falls, are to save cost in their construction; and to make sure that sufficient sewage will flow through each, to keep it clear of deposit.
14. The invert of the main-sewers should be laid not lower than 2,75 feet above Ordnance-Datum, at the point where the sewer passes through the new Praya-wall. Now mean-sea-level is 3.69 feet above Ordnance-Datum; so that the invert of the sewer will be 0.94 below mean-sea-level. As the out-fall sewers will be not less than 1′ 6′′ in diameter; when the water in the harbour stands at mean-sea-level the middle of the sewer-pipe will be only about two inches under water. Now mean-sea-level means the average level of the sea, as obtained by adding together the height thereof recorded, every hour, for a whole year or more, and dividing by the number of observation. By thus fixing the centre-line of the main-sewer, at or near to mean-sea-level, it is certain that the pipe will be free from sea-water, for nearly one-half of every day, month or year, and that the sewage will flow freely through the sewer, and cleanse it from deposit.
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.
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 house-drains to discharge into them.
19. It is further desirable that the sewage out-falls should not be in the immediate 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.