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8. The following are the leading considerations, which rule the selection of position for sewer out-falls. The sewage should be delivered into water, not less than six feet deep, at extreme low water, and at point where there is an uninter- rupted flow of tide; so that the effluent may be removed and diluted, as quickly as possible. For this reason, salient angles are preferable to re-entering as the position of out-falls.
9. The existing Praya affords a very considerable range of choice, as to the position of out-falls. When the new reclamations are complete and the extraordina- ry irregularities in alignment, which disfigure the existing Praya, have disappeared; one position will be very nearly as good as another. Therefore there will be no necessity for constructing costly intercepting-sewers, to conduct the sewage to one or two great out-falls. Indeed, unless the sewage is wholly intercepted and carried, say, to an out-fall to Sulphur Channel, opposite Green Island, or to North Point; no appreciable benefit will result from its concentration, to any one out-fall. Such complete interception cannot be effected, unless a considerable portion of the sewage is pumped; for the strip of land, between the Queen's Road and the Praya, is practically level; and no sewer, having a gradient sufficient to establish a self- cleansing velocity, could be constructed so as to convey the whole sewage, by gravity, to such a distant out-fall.
10. The complete interception of the sewage, from the harbour-frontage, would be exceedingly expensive, both as to first cost and maintenance. It would not effect any appreciable improvement in the sanitary condition of the city; nor, considering the mass of vessels which frequent the Praya, and the garbage which they throw overboard, would the amenity of the place be increased. The Praya is not a pro- menade; nor is Victoria a bathing place. The cost of complete interception 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 water- way, caused by the advance of the Praya, will most probably produce an improve- ment 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 berin 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 extension 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.