THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
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975
The whole of the sewage of this district now goes direct, into this bay or rather, all sewage that does not percolate into the subsoil, beneath the dwellings, which forms, it is to be fared, a very cor si lerable proportion of the whole.
The object of the present scheme is to discharge the sewage, int) relatively deep water, at a distance of one hundred feet or more, in advance of the existing Praya wall.
11. In accordance with these principles six out-falls have been selected.
No. 1 At East Point.
No. 2 Opposite Percival Street.
Observation Place.
No. 3
No. 4
39
Heard Street.
No. 5
No. 6
.,
Spring Gardens Lane.
Arsenal Street.
57
12. There is already the out-fall of a public sewer on the frontage of the East Point property. It is understood that the Government has the right to construct drains through this property. If it be decided to adopt mumerous out-falls, on the foreshore of the district, one of these, discharging the sewage of the immediately adjacent property, should pass through the East Point property. The actual line of the out-fall is of little moment, and might be arranged to suit the convenience
of the owners.
It may be safely asserted that an out-fall on the face of this property, would be of a lesser nuisance to them, than an out-fall on the shallow bay, on either side of their property.
13. From each of these out-falls, a pipe of a diameter sufficient to carry off the dry-weather flow of the sewage, should be laid, across the foreshore, and terminate at a depth of 2 feet below low-water spring-tides. This is a lesser depth than was recommended in the case of the out-falls of the Eastern and Central Districts but on the other hand the distance from the Praya wall is greater, so that in all probability no nuisance will occur in either case. At each out-fall, there should be a storm over-flow, to carry off the surplus, when the flow of the sewers, swelled by rain, is in excess of these the amount that the cast-iron out-fall pipe will carry.
14. The proposed main and branch-sewers will carry off about 1′′ of rain in 24 hours falling on the whole surface. By no means the whole of the surface of the district will be in connection with the sewers. All the existing appliances, for removing rain-water, will continue in operation, and storm over-flows to them will be provided, so that there will be no danger of flooding.
15. The proposed lines of sewers are shewn on the plans, and do not call for special description.
16. To effect the interception of the sewage proper, from the harbour frontage of this district, it must be concentrated at some point, whence it must be raised by pumping, to a height sufficient to permit of its free discharge to the sea, at any state of the tide.
17. The float experiments, made by Mr. COOPER, shew that North Point is the nearest satisfactory position for an out- fall.
Now the frontage of the Eastern district, from the westernmost direct out-fall, to the eastern boundary, is about 4,000 feet, whilst from the latter point to the out-fall at North Point is 5,000 fect; 9,000 feet in all.
To concentrate the whole sewage at a pumping station at North Point, a sewer 9,000 feet long would have to be con- structed. At its western extremity, the intercepting-sewer must be sufficiently low to receive the sewage of the most distant direct out-fall sewer. Its invert must therefore be at or about 2·00 on Ordnance Datum; or at the level of about low-water springs (3·7 OD.=Mean Sea). In order to obtain a self-cleansing velocity, this sewer should have a gradient of at least y or a total fall of 15 feet. At its out-let at the pumping station, the level of the invert sewer must be-13 below datum or 167 under mean-sea-level. The ground-level being about+11′OD., the invert at the level-13, would be 24 feet below ground. The construction of a large sewer, at such a depth, would be a costly operation. Being only partially filled, such a sewer would contain a large volume of sewage-tainted air, and must therefore be thoroughly ventilated. However well "managed, occasional nuisances might be anticipated from it.
18. The pumping station might be placed at or near to the eastern limit of the district, and the sewage forced thence to the out-fall, through a cast-iron pipe. This would reduce the length of the low level sewer to 4,000 feet; and the maximu depth to 466 below ordnance datum or about 15 feet under ground. The construction of a sewer, at this depth, would be a less formidable operation than in the case of the longer length, but still it would be costly. There is not a single contractor in the Colony that has had experience of such work, and from what I have seen, it appears that Chinese contractors have little knowledge of the proper manner of conducting deep cuttings; or of the use of timber, for supporting the sides of trenches. As the greater part of the sewer would be below low-water mark, it may be reasonably anticipated that water would be met with, in considerable quantities, which would have to be pumped out, in order to allow of the proper construction of the
sewer.
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