260
7
6
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JAFFÉ ON HONG KONG WATERWORKS,
monthly distribution of the rainfall for the maximum and minimum years and the mean for the 30-year period is tabulated below:-
Wet Season
January
Rainfall recorded at Observatory.
Month.
1889.
1895.
Mean, 1884 to 1913.
Inches.
Inches.
Inches.
0.73
0.47
1.442
February
0-72
0.83
1.688
March
2.49
1.39
2-987
+
April
12.27
2.61
5-511
May.
48.84
5.64
11.713
June
9.72
4.97
15.681
July
4.57
18.87
12.555
August.
18.14
6.13
14.362
September.
11.80
3.96
9-668
October.
8.72
0.50
4.911
November
1-54
0.32
1.421
December
0.18
0.20
1.226
Total for
year
119-72
45.83
83.165
In 1910 the Author, who carried out the works previously detailed under the direction of Mr. William Chatham, C.M.G., M. Inst. C.E., Director of Public Works, was called upon to report on further extension. The position, was as follows :—
The aggregate storage-capacity of the reservoirs affording a gravitation supply amounted to 537 million gallons, and of that requiring pumping to 210 million gallons, making, in all, 747 million gallons, to which must be added the dry season discharge of the various streams feeding the reservoirs, about 170 million gallons,' thus giving, if all the reservoirs were filled at the begin- ning of the dry season, 917 million gallons to tide over the dry season, On a full supply the consumption was 5 million gallons per diem (20 gallons per head for a population of 240,000), or 1,060 million
This figure is obtained from the contents of the reservoirs less the quantities withdrawn for supply, and therefore includes the loss from evaporation.
Papers.] during the seven months of the dry season, and as a consequence even with the reservoirs fully charged at the end of the wet season -a state of affairs which was exceptional rather than otherwise— there was a deficit.
LOW-LEVEL RESERVOIRS AND WORKS FOR PUMPING.
Thus a much greater storage-capacity was necessary, especially if provision was to be made not only for the ordinary increase in population but also for the large increase which would follow on the completion of extensive reclamations then under consideration. The Author estimated at 482 million gallons the increase in storage which was absolutely necessary if full supply was to be maintained throughout the year for the population at that date, whatever the incidence of the rainfall might be, and he considered 558 million gallons the minimum provision which should be made for growth (normal and due to reclamation), or a total additional storage of 1,040 million gallons.
Gaugings in the main stream in the Taitam Valley below the Intermediate reservoir from 1906 to 1911 showed that a yearly average of 982 million gallons had flowed to waste; the returns of the reservoirs within the catchment-area of the gauges gave a yearly average supply for this period of 1,033 million gallons; the total average annual yield from the catchment area of 1,509 acres above the gauges was therefore 2,015 million gallons. The yield varied from 75 4 to 65.8 per cent. of the rainfall. Assuming that the effective yield of the catchment above the gauges holds good for that from gauges to sea-level (446 acres) the average quantity of water which flowed to waste into the sea for the years recorded by the gauges was 1,577 million gallons, the maximum being 2,016 million and the minimum 1,186 million gallons. The annual rainfall (Taitam gauge) over this period ranged from 96.51 inches to 72 64 inches, and, as the average rainfall is 83 inches, it was clear that a reservoir of large capacity would fill in years of average rainfall: further, it is possible to supplement the natural catchment area by catchwatering in adjacent gathering-grounds which are available to the extent of about 2,600 acres. The Author therefore felt safe in recommending a capacity of about 1,400 million gallons, making, with the Intermediate reservoir, a total capacity of fully 1,600 million gallons.
The works, which were designated "The Taitam Tuk Scheme- Second Section," comprise :-
(1) The construction of a dam at sea-level forming the Taitam Tuk Reservoir, with a capacity of 1,420 million gallons. (2) An extension of the existing pumping-station in Taitam
Bay.