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247
Dam. Drawing No. 5.
Drawing No. 6.
4
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 catchment 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.
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.
5
East Catchoater,
The East Catchwater would have a total length of 11,200 feet and would drain 314 acres. It would commence in the stream below Wongneichong Reservoir and follow the 100 feet contour to the gap North of Bennet's Hill where it would discharge through a tunnel 600 feet long into the reservoir.
The catchwater where in "soft" will be lined with cement concrete but will be unlined on the inner side where in rock. Necessary overflows, wash- outs and sand pits will be provided.
West Catch rater,
The West Catchwater would have a total length of 15,000 lin. ft. and would drain 496 acres. It would commence in the stream above Kellett Bay and contour the hill side at about the 380 feet level passing round and above the West side of the existing reservoir and discharge into the new reservoir at the North end of the Dam. It would be similar in construction to the East Catchwater.
Bennet's Hill Catchwater would have a total length of 4,200 ft. and drain 52 acres of the East and South sides of Bennet's Hill and would dis- charge into the Lower Reservoir.
Supply Mains.
East Catch- water.
West Catch- water,
Bennet's Hill Catchwater,
The supply main, which would be approximately 21,000 ft. in length and Supply Mains. 18" in diameter, would be either Cast Iron, or Steel, lined with concrete or bitumen. It would contour above the existing reservoir and join the Wanchai Gap-Aberdeen Road at the Gap west of the existing dam and follow the road as far as Aberdeen where the main Island Road is reached, and where a junction with a pipe from the existing reservoir would be made.
-The pipe would follow the Island Road as far as Pokfulam Hill then join Victoria Road and follow it as far as Telegraph Bay.
Telegraph Bay would be crossed by means of an inverted syphon thus avoiding the Villa Miramere Gap which is too high and Victoria Road would again be reached 400 yards east of Sassoon Road and be followed as far as the Pumping Station at the re-entrant on the road above Sandy Bay.
This pipe would be capable of delivering 3 millon gallons per day with a total loss of head of approximately 60 ft.
Pumping Station.
Station.
Sandy Bay Gap over which the water has to be pumped has an elevation Pumping of approximately +355 ft. O.D. or pipe line level of, say + 340 O.D. while the cope level of Elliott Filter Beds is approximately + 300 0,D.
Top Water Level of the Upper Reservoir is approximately +365 0.D. and the existing or Lower Reservoir + 261.85 0.D., so for calculation purposes the pumping level of the water in the reservoirs can be taken as + 340 and
+240 O.D. for the Upper and Lower Reservoirs respectively.
Necessary head to be pumped will therefore be (340-(340-60))-60 ft. and (340-(2-40-60)) · 160 ft. for the Upper and Lower Reservoirs respectively.
As the Pumping Station would be approximately 200 ft. above sea level and comparatively inaccessible, the cost of steam power would be high, owing to transport charges for coal, and I would therefore propose that electric energy be used. An electrical supply main is conveniently situated on Mt. Davis Road a few hundred feet distant.
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