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mately the same as that required for slow sand beds designed to filter onlyM.G.D. The old filter sites will then be used for additional service reservoirs, which will be referred to below.
There is little of note about the dams, except the vicissitudes of the original Tytam Dam. The raising of this dam by 9 ft. in 1897 has already been referred to, but in 1936 due to leakage having developed through the main dam its safety was suspect and it was decided to lower the cill of the overflow dam by 2′4′′. The use of the extra boards had been discontinued for the same reasons even before 1929. The water is not lost however as most of the leakage is collected and led into the gauge basin, and the remainder of course finds its way into the lower reservoir, though in this case it has to be pumped back. So long as the stability of the dam is not impaired therefore the leakage here is not serious.
An important feature of the Tytam Valley Scheme is the exten- siveness of the catchwaters. It will be seen from Table No. 1 that the indirect catchment totals 2418 acres which is brought in by means of ten sections of catchwaters with a total length of 94,444 ft., or nearly 18 miles. Most of this vast work was undertaken as a result of the 1929 shortage.
One result of this extensive catchwater system having been planned after the dams were built, was the necessity of providing additional overflow provision. This was accomplished in two ways; firstly by lowering the cills of Tytam Tuk spillways 6 inches by dressing down the masonry and incidentally sacrificing 13 M.G. storage and secondly by providing the extensive Tytam Tuk East Catchwater with sluice gates which allow the discharge to be directed into the sea when the reservoir is already full.
Kowloon Reservoirs. The steep valleys in the hills around Lai Chi Kok, provide obvious sites for reservoirs, though as usual in Hong Kong the direct catchment areas are small, and again catchwaters have been resorted to, notably the long catchwater on the north side of the hills around the Shatin Valley, the Beacon Hill Catchwater.
The four reservoirs situated close together are linked in a very interesting manner. The original reservoir was the Kowloon Reser- voir and the Shek Lai Pui Reservoir was next, the overflow from which flows into the Kowloon Reservoir. When the Shing Mun scheme was developed advantage was taken of the proximity of these reservoirs to create useful cross connections. Shek Lai Pui water can be fed into the Reception Reservoir, and the Reception in turn overflows into the Byewash which also receives the overflow from Kowloon Reservoir. These cross-connections enable the diversion of limited quantities of water to one or other of the two filters the Rapid Gravity Filters which normally handle the Shing
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Mun water from the Reception Reservoir and the slow sand filters which treat the draw off from the other three reservoirs.
Shing Mun Valley. The Shing Mun Valley scheme which comprises the Jubilee Reservoir connected via two conduits and two tunnels to the Reception Reservoir, is the largest of the Reservoirs to date. The reservoir is formed by means of two dams, the main Gorge Dam and a subsidiary dam at Pineapple Pass. The Gorge Dam has been fully described in a paper presented to the Institution of Civil Engineers by Messrs. W.J.E. Binnie & H.J.F. Gourley M.M.INST. C.E., in 1939 and in a summary of the nature of this paper only a brief description of the more interesting features is possible. It may not be generally known that this reservoir replaces two out of nine reservoirs proposed in Mr. R M. Henderson's original proposals, which included five more sites for possible future reservoirs, which would however be at low levels and require pumping, together with two high level reservoirs which were to act as flood storage reservoirs to equalise the flow in the long Tai Mo Shan Catchwater.
The Jubilee Reservoir is not entirely a gravity reservoir as the level of the draw off conduit is 488.25 ft. A.P.D. or 131.25 ft. above the bottom of the reservoir and the dead-water could if needed, be pumped up to the conduit from a pump house at the foot of the dam, though the pumps have not as yet been installed.
The catchwater around the southern slopes of Tai Mo Shan is the most ambitious of the many catchwaters so far built, but has neces- sarily been brought through very unstable hillsides and slips are unfortunately only too common. The answer to this problem has not yet been found and the yearly bill for repairs to the catchwater due to slips following typhoon rainstorms is still considerable.
The Dam itself is still listed as the highest in the Colonies, and is by any standards a fine achievement. It is a most ingenious com- bination of reinforced concrete shell, mass gravity, and rockfill dams. The water is retained by the articulated skin of reinforced concrete, any leakage through which can be detected in inspection galleries. This is supported by a thin concrete gravity section, known as the thrust block, which transmits part of the water pressure to the rockfill, with a sand wedge between the two to equalise the pressures. When the reservoir empties the thrust block acts as a retaining wall to support the rockfill.
The secondary dam at Pineapple Pass is a rockfill dam with concrete core, and originally had a sand wedge similar to the Gorge Dam. In this case, however, the sand wedge did not work and pre- sumably after deflection the core was left unsupported with the result that cracks developed. This sand was therefore removed and the wedge backfilled with concrete. No further movement appears to have taken place.
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