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THE HONGKONG GOVERNMENT GAZETTE, 10TH JULY, 1875.
PUBLIC WORKS DepartmentT, SURVEYOR GENERAL'S OFFICE, HONGKONG, July, 1875.
SIR,-In continuation of previous correspondence upon the subject of the proposed Hongkong Government Water Works Extension, I have the honour to transmit you embodied in the accompanying twenty sheets of detailed drawings and estimates the results of the engineering surveys recently com- pleted. These surveys, as arranged with His Excellency Sir ARTHUR KENNEDY, have been confined to the investigation of two alternative projects, the first on a smaller scale than the second, and both of considerably less magnitude than the original scheme submitted by me in 1873.
It may be recollected that the drawbacks attending a supply of water from the drainage of the Aberdeen Valley or from triplicate reservoirs at Pokfoolum were found to be so great as to make it undesirable to look to either of those sources for help, it has therefore not been deemed necessary on this occasion to extend the detail studies beyond the Valley of Tytam, and it is to the discussion of the cheapest methods of turning to account this, the most generous watershed of the island, that the following pages are almost exclusively devoted.
*
In the original scheme it was proposed by means of a dam and reservoir, to catch and put by for use during the long droughts of winter, two hundred and fifty million gallons of rain. This volume was to be sent forward to Victoria by a conduit winding along the hillsides, intercepting in transitu the contents of twenty-five mountain streams which it was calculated would increase the daily dividend to thirteen and a half gallons per head of inhabitant in winter, and at least thirty gallons in summer. The project also included a service reservoir in town and a complete remodelling of the system of distribution through the streets. In short, every requirement pointed out by the best Hydraulic Practice at home was provided, but the whole entailed an outlay which the Government-however desirous to extend to the Colony the advantage of a liberal water provision-was unable to meet.
Accordingly it became necessary to devise means of obtaining a smaller volume of water.at less cost, and to this end certain reductions and modifications of the original scheme were submitted by me in a second Report bearing date 4th March, 1874. After a full discussion of this last Report the Colonial Legislature resolved to have detail surveys and estimates made of the proposed reduced projects and thus relieve the whole matter of the uncertainty resulting from an examination which had merely been preliminary, and consequently from an estimate which could only be approximative.
These detail surveys and estimates have now been effected in the searching manner which it was intended, and what they reveal is important in shewing that the cost of realizing the original scheme of 1873 had been over-estimated, and in removing beyond the region of doubt the favourable nature of the substrata in which we should have to work, tending of course to a corresponding diminution of the estimate, in the event of either one of the modifications of the original scheme, now submitted, being adopted.
These modifications, as already stated, are two, i.e. :-
1. A plan for bringing the Tytam stream into town from the 500 foot level, without the aid or cost of any dam or reservoir. This I have called the High Level Project.
2. A plan for a reservoir on the 400 foot level nearly upon the same site as that proposed in 1873, the water being conveyed direct into town by means of a tunnelled conduit under the dividing ridge of the island in lieu of by a surface conduit winding round the hills in search of tributary streams. This plan, for the sake of distinction from the foregoing, I have called the Low Level Project.
The High Level Project.—In designing efficient works for a city it is indispensable to place the point of delivery at a level sufficiently high, not only to disseminate the water over its highest districts with access to the upper stories of all houses situate in such districts, but also in case of fire, to maintain at all times such a pressure in the street mains as will force a jet of water over the roofs of the loftiest buildings without the aid of a fire engine. It is therefore unfortunate that all the feasible enterprises hitherto discussed by which Victoria could obtain an increased supply exhibit the awkward feature of an insufficiently high point of delivery. The Pokfoolum project with its three reservoirs was shown to be of no advantage to houses above the 446 foot contour, Tytam water under the original scheme, could not rise higher than 375 feet above the level of the sea, and the Aberdeen plan simply left one-third of the city above it unprovided for. In each of these three cases the question of levels as regards the point of delivery in town was governed by the height of the outlet works at the different reservoirs, the heights of the outlet works being in their turn determined by the positions of the dams which were to create the fountains of supply. The dams could only go in certain places, and these places happened to be lower than the upper parts of the city.
But we have now, for the first time in these investigations, to deal with a design which does not comprise an impounding reservoir or outlet works, and we are therefore free to select the height most convenient to ourselves at which the water may be made available in Victoria; accordingly the High Level Project has been arranged to deliver water at as great a height as the existing Albany Tanks. The plan does not aim at a provision of stored water for use during the dry season, but is limited to the diversion of the Tytam stream into a subterranean conduit leading direct to the head of the
* This allowance was to have been further increased by the Pokfoolum Subsidiary Works yielding nearly 5 gallons in winter, and in summer as much in excess as the conduit would carry.