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direct areas, for there should be no loss from floods. Actually there is usually a small loss to cultivation areas for irrigation. The 13,650 acres of catchment, therefore should produce about 8.0 M.G.D.
If the reservoirs, having been full at the end of September, still contain 1300 M.G. in storage at the end of April, the draw-off will have been the maximum possible, namely 4670 M.G. This in 213 days represents about 22 M.G.D., which puts the total resources at 30 M.G.D., or allowing for a small amount for losses, say a possible maximum of only 29 M.G.D. in an average year.
In a dry year the catchments could probably not produce more than 5.0 M.G.D., and with storage say, 1000 M.G. down at the end of September, this would only supply 17 M.G.D., a combined total of only 22 M.G.D. In official estimates in the past the minimum resources have been estimated at 27.5 M.G.D. for a twelve month period, but it is the author's opinion that the dry season figure gives a more accurate basis for design, as representing the true minimum available resources.
TRUNK MAINS
Connecting the various storage reservoirs with their respective distribution areas are the trunk mains. The obvious ones in Hong Kong have already been referred to, namely:
(a) The Pokfulam Conduit,
(b) The Tytam tunnel and Bowen Road Conduit,
(c) The Aberdeen main,
(d) The Shing Mun-Kowloon-Hong Kong mains.
That these are vulnerable points in the supply scheme was only too obviously shown in 1941 during the Japanese attack, when one after the other was damaged to a greater or lesser extent. Their vulner- ability is also connected with the service reservoir capacity available at the delivery end, as obviously the smaller the storage at the town end, the more urgent do repairs to a damaged main become.
As might be expected in the Hong Kong terrain, none of these mains are of a uniform section throughout their length, and the maximum use has been made of the various forms of construction in order to avoid the need to import large pipes.
There is no doubt that the brick conduits have given excellent service, but their capacity is limited to the free discharge on very flat gradients, for being covered only with loose stone slabs they do not permit of any pressure head. The Bowen Road conduit, at present a raw water trunk main, will be converted to a filtered water main with
the concentration of filters at Eastern, referred to below, with the result that it will have to be reconstructed and the scheme proposed is to lay a 24 ins. reinforced concrete pipe alongside the existing conduit, but which will be capable of taking a small pressure head to enable a greater discharge to be achieved.
The main feature of the Shing Mun system is, of course, the Cross Harbour connection. The raw water from Shing Mun is con- veyed in conduit or tunnel to the filtration point at Shek Lai Pui, and unfortunately the North Conduit is not very satisfactory. It has been cut into the steep hillsides on the south of the Gorge and in one or two places it cannot be regarded as very safe-there was in fact one serious break caused by a slip, during the occupation period-and consequently its replacement is under consideration, but the favoured solution of a high pressure steel main down the gorge, rising to the entrance of the first tunnel, will have to await delivery of steel pipes from England which cannot be promised in less than two years.
From Shek Lai Pui the filtered water is fed to the town by pipe lines, as is the output from the adjacent slow sand beds. With the exhaustion of the Island resources, however, it became necessary as long ago as 1923 to investigate the possibility of bringing over water from the Mainland.
Investigations were commenced in 1924 into the nature and levels of the harbour bed between Kowloon Point and Statue Square and in 1925 plans for two 18 ins. cast iron mains were drawn up by Con- sulting Engineers in England, but were not proceeded with, and when work did eventually commence later, in 1929, it was on a scheme prepared by the Waterworks Engineer, Mr. R. M. Henderson, for a 12 ins. steel main.
This main consisted of lap welded steel pipes 12.265 ins. internal diameter and ins. thick, which were welded into lengths of 100 ft. with Albion loose flanges, which on lowering to the harbour bed could be bolted by divers with ten 1 in. diameter bolts. At intervals of 300 ft., ball and socket joints were inserted with 18° or 25° play and expansion joints of the Vulcan type with 10 ins. travel at every 600 The pipe was laid generally on the harbour bed but soft areas near the shore were first dredged and backfilled with rubble. Anchor blocks in deep water and piles in shallow water on either side of the pipe prevented lateral movements.
ft.
For protection against corrosion the pipe was coated inside and out with bitumen, then doubly wrapped in bitumastic impregnated hessian. In all, 62 sections were laid having an overall length of 5914 ft. The work of laying was completed in 57 working days.
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