Sessional_Paper_1902 — Page 879

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The sub-joined sketch shows a section of the 12′′ main sewer in Queen's Road W'est to its outfall :-

GROUND LEVEL

7 MEAN HIGH WATER

Methods of preventing

MEAN SEA LEVEL

12" SEWER

} IN

130

The pipe, 12" in diameter, has a gradient of 17. Running half full, with a free outlet, it would discharge 101 cubic feet per minute with a velocity of 246 feet per minute, or more than is necessary to prevent deposit.

When, however, the same volume of water fills the whole pipe the velocity will be reduced to 123 feet per minute, or barely sufficient to prevent deposit. A flow of only 3.6 cubic feet per minute filling the pipe to a depth of about 14 inch would give a velocity of 120 feet per minute. But the velocity of this stream would, when in consequence of the outlet being tide-locked it filled the whole pipe, be reduced to 10 feet per minute, which is wholly non-self-cleansing. Hence this sewer which receives a large quantity of sewage, would be self-cleansing, all the day through, if it discharged above high-water mark, but as it is, it is a sewer of deposit for the greater part of the day though, doubtless, it cleanses itself when the tide is below the level of the outfall.

12. The level of the Praya is so low that it is impracticable to place all outlets tide-locking. above high-water mark. The sewers would have to be so near to the surface that they would be liable to damage by traffic, and they would be too high to receive the sewage discharged from the house-sewers of tenements on the Praya.

The sewers on the Praya must be, for a great part of their length, below even mean sea level. The only way to prevent tide-locking is to exclude the sea water altogether, and to do so and at the same time prevent stagnation, will involve pumping. In many towns tanks are constructed to collect the sewage which is then discharged at or near the hour of low-water. This plan, always objectionable as it involves stagnation, cannot be adopted here on account of the great variation in the range of the tide at neaps and springs, and of the diurnal variation in range. At certain periods one tide in the day almost disappears. The sudden discharge of the sewage stored during say 18 hours out of the 24 could not fail to produce an insufferable nuisance. Pumping must therefore be resorted to. If there is to be pumping then it will be well to pump to some distant outfall, and to intercept all sewage from the harbour frontage.

It would be most expensive and almost impracticable to conduct all sewage to one pumping station. There must, therefore, be two intercepting sewers, with their respective heads near to Murray Road, one flowing eastward and one flowing westward to two pumping stations. The ultimate outfall at which the sewage from the Eastern District should be discharged should be at North Point and that from the Western District in Sulphur Channel facing Green Island. I have already suggested in a Report of 1890 such an arrangement for the Eastern District. The syphon arrangement therein described would be most economical, but an ordinary sewer, flowing partly full, would be most satisfactory though probably considerably more expensive. I am leaving on record a plan and section of the Western intercepting sewer in order to show how interception might be effected, if

any time it is considered desirable to carry it out.

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The intercepting sewer when below mean sea level should be of cast iron. This is really the most economical material, for the difficulty of making a water- tight sewer of brick or stone-ware in wet ground is very great.

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