Sessional_Paper_1927 — Page 188

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This in a minimum rain or critical rain period would give an average daily supply of 1.8 million gallons.

The existing reservoir would then be left with 145 acres of catch:nent area, the overflow from the upper reservoir, and a certain amount of overflow from the catchwaters, giving an average daily supply during a critical or minimum rain period of 0.32 million gallons.

The total supply from the two reservoirs and 1,460 acres of catchment area would therefore be 2.12 million gallons per day.

A pipe line would run from the upper reservoir through the gap West of the existing reservoir joining a pipe line from the existing reservoir on the main road round the Island. The pipe would then follow the main road to the bottom of Pokfulam Hill when it would join and follow the low or Victoria Road as far as the re-entrant below Mt. Davis and above Sandy Bay where the Pumping Station would be situated.

From the Pumping Station the water would be pumped through a rising main to Sandy Bay Gap immediately above, and then gravitate to Elliott Filter Beds which would require to be extended to deal with the additional water.

I would also propose to lay a 10" pipe connecting Pokfulam Reservoir with the main in Victoria Road and a byepass round the Pumping Station, so that Pokfulam Reservoir water could be gravitated direct to Elliott without pumping.

Dam. Drawing No. 5.

Drawing No. 6.

Dam.

The dam has a total depth of 100 feet from stream bed to overflow; a length of 340 feet and an overflow of practically two-thirds of its whole length. It would have a total drainage area of 1,315 acres composed of 505 acres direct, and 810 acres indirect by means of catchwaters, and would contain when full approximately 200 million gallons. It would be composed. of an inner lining of fine cement concrete backed by 7 to 1 and 8 to 1 cement concrete with displacers. A draw-off chamber is shown on the up- stream face to contain the inner draw-off and washout valves surmounted by a Valve House to contain the Valve Head-stocks etc.

The top water level would be + 365 O.D. and the Overflow, which would have a length of 192 feet, would be capable of passing 74 million gallons per hour with a rise of 3'0" on the crest.

As there does not appear to be any granite near the site suitable for ashlar, the front face would be either faced by. concrete blocks or concrete directly laid against forms and afterwards treated with a cement gun, while the back face would be faced with granite rubble to take the overflow.

At each end of the overflow, would be concrete and masonry floodwater channels to divert the water from the hill sides into the water cushion which would be formed at the base of the dam. They would also steady the flow and prevent erosion of the stream bed.

An outlet culvert to carry the supply pipes would connect the back of the Dam and the valve Well and while the dam was being constructed would take the floodwater.

Good rock is exposed in the stream bed and for some feet up on either side. It is not however expected from surface indications that the upper foundations will be very good, but this cannot be verified until excavations on the site are made.