AnnualReport-1910 — Page 408

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filtered, a channel has been constructed to intercept a portion of the flow from the Catchwater in addition to the waters of a natural stream and convey them into the main leading to the filter beds. This chan- nel extends from the Catchwater to the overflow dam, contouring the hills on the South side of the reservoir and just above top water level. It is 2,000 feet in length, has a sectional area of 24 square feet and a fall of 1 in 1,200 and is lined throughout with concrete, the bottom being of cement and the sides generally of lime concrete. Where spurs of the hill would cause any considerable detour, they have been cut through, 15′′ stoneware pipes being laid to conduct the water between the points where the channel ends. An intake has been formed in the bed of the Catchwater from which the water is con- veyed in 12" cast iron pipes into the channel and the waters of the stream already mentioned are intercepted in a similar manner. From the Overflow Dam, where the channel terminates, the water is con- veyed in an 8" cast iron pipe, which passes through the dam, crosses the Overflow Channel in a diagonal line and extends down the hill- side to the main gauge basin on the pipe-line leading to the Filter Beds.

(v.) Main from Storage Reservoir to Filter Beds. As already mentioned, the 10" stand-pipe, with which the draw-off pipes froni the reservoir are connected, is continued by a main of the same diameter through the culvert in the main dam, whence it extends down the gorge for a distance of 800 feet to an open gauge basin. From this point an 18" cast iron main is laid at a gradient of 1 in 1,000 to another gauge basin at the filter beds, a distance of 3,200 feet below the main dam. The main is carried down the right bank of the Lai Chi Kok stream for a distance of 1,400 feet and then, turning nearly at right angles, it crosses the stream on a bridge of steel girders and stone piers and passes through a spur of the hills in a tunnel 356 feet long constructed on the "cut and cover" system, finally discharging into the gauge basin at the filter beds. A branch on this main has been provided so that water may, when necessary, be passed to the Filter Beds at Lai Chi Kok from which the supply for the Shipping is drawn. The tunnel already mentioned is 6'0" high and 7' 6" wide, space being provided in it for another line of 18" pipes when found necessary. It is lined throughout with brick in cement 18" thick.

(vi.) Filter Beds. These are situated immediately below and to the North of the gap in the Kowloon range of hills, through which the Tai Po Road passes. A considerable amount of excavation had to be done in levelling the site for them. The beds are three in number, each 105′ 0′′ × 70' 0", giving a total area of 2,400 square yards. They are formed of lime concrete, finished with cement rendering round the sides and with a layer of cement concrete on the bottom. The filtering material consists of 3′ 6′′ of sand on 1 foot of broken stone. The water is drawn off through adjustable outlets, by means of which its level may be maintained constant, and passes through Venturi meters which record both the rate of flow and the filtering head. The levelling of the site, which will accommodate 3 more beds of the same size as those already described, entailed excavation to the extent of 83,000 cubic yards. The work included the erection of a small stone building to accommodate 3 coolies employed as caretakers.