Sessional_Paper_1905 — Page 753

No. 1905

31

HONGKONG.

REPORT ON THE EXISTING FLUSHING SYSTEM IN THE CITY OF VICTORIA AND PROPOSED SITES FOR NEW TANKS.

Laid before the Legislative Council by Command of His Excellency the Governor.

DRAINAGE BRANCH, PUBLIC WORKS DEPARTMENT,

HONGKONG, 21st June, 1905.

1. I herewith forward you a list of the existing Flushing Tanks in the City of Victoria, showing the state of the sewers flushed by them at the time of their inspection with other particulars (vide Appendix A.).

2. A general Plan is also attached with the positions of the Flushing Tanks marked on.

3. The flat gradients placed on the list are those that do not give a velocity of 24 feet per second when the pipe is half full. These gradients have been obtained from sections, in this office, of some of the sewers.

4. The whole of the Flushing Tanks have been measured, and their capacities and the sizes of the syphons have been revised.

5. The special points to be drawn attention to in the present flushing system are as follows:-

(a.) That the deposit that takes place in the sewers is chiefly sand and not sewage matter as mentioned by Mr. CHADWICK in his Report dated 10th April, 1902, para. 66.

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(b) That the automatic syphons now in existence are useless for flushing the sand deposits in the Low Level sewers on account of their small capacity, the long distance most of them are situated from where the flush is required to be effective, and if worked automatically they will not flush the Low Level sewers at the proper time, (ie., when the tides are low). I am of opinion that they may still be of some use in keeping down the smells arising during the dry season of the year, from the scum which is fre- quently left on the internal surfaces of sewers if not flushed. (c.) That a deterrent effect on the flushing of the existing sewers is caused by the presence of roots of trees, the flatness of some of the gradients of the Low Level sewers, inverted syphons, and the overflow plates at the outfalls.

6. In the case of (a.) I have found from experience here as well as elsewhere that it is very difficult to remove a sand deposit, which has been allowed to settle, and packs very closely. The tendency of the flush of water is to run over the top of the sand and leave it in the same position, except in the immediate neighbour- hood of the tank.

7. The first object should therefore be to eliminate the sand or prevent it from entering the sewers, and for this reason I think that the new type of gully should be extended to the High Level Districts from where I believe the greater part of the sand is brought down.