CO129-598-2 Tai Lam Chung Reservoir Project 24-6-1948 - 3-3-1949 — Page 40

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Another feature of this scheme was the decision to remove the silt which had accumulated above the old dam, and a total of 129,032 cu. yds. was eventually removed, representing an increase of 21.77 M.G. of storage capacity. This also gives a useful illustration of the amount of silting which is possible, for 21.77 M.G. represents about 20% of the storage capacity of the completed reservoir in a period of only 40 years, or 0.5% per annum. No doubt the vegetation on the hillsides was not so good when the reservoir was first built, and a considerable amount of material was undoubtedly washed down from the Peak during the construction of roads and houses, so that the figure of 20% may be regarded as unusually high.

Those reservoirs are not high enough for an entirely gravity supply so the pumping scheme is designed to enable the lower draw- offs to be used to deliver to the filters whose level was fixed beforehand, as the new rapid gravity filters were sited in one bed of the Elliot Slow Sand Filters. When the reservoirs are full the upper reservoir can discharge by gravity to the filters, but as the water level drops and the discharge of the pipe line falls, the lower pumping station situated in Aberdeen, can be brought into action to boost the supply. The upper pumping station situated below the lower dam, pumps water from the lower reservoir into the upper reservoir or direct into the delivery main to the filters.

Tytam Valley. This practically undeveloped valley on the South- East side of the Island was an obvious choice for a waterworks conser- vation scheme, with its direct catchment area of 1580 acres. Its obvious drawbacks from the technical standpoint are two, the low level of the lower reservoirs and the lack of direct access to the City. The driving of the tunnel and the construction of the conduit along Bowen Road was in 1889 a bold and very satisfactory solution. Water from the two upper reservoirs can be fed directly to the gauge basin at the tunnel entrance and so to the city entirely by gravity. The lower reservoirs are fed to the same gauge basin by means of the Tytam Tuk Pumping Station. The scheme is simple but requires a fine judgement in operation. It is necessary to balance the economics of pumping costs. against the desirability of keeping the number of days storage in the different areas approximately equal, and also of maintaining a reserve of gravity water in case of pumping station breakdown, a contingency not to be lightly dismissed in these days of strikes which might produce power shut down or delays in fuel deliveries.

The raw water conduit along Bowen Road has served well in the past, but is now nearing the end of its useful life. A rather unusual feature is the string of filters, necessitated primarily by the impos- sibility of finding a site big enough to hold all the necessary slow sand filters. With the development of rapid gravity filters this difficulty can be overcome, and today work is proceeding on an eleven million-gallons-a-day plant on a site, at the Eastern Filters, approxi-

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