Chinese water
from Muk Wu
48 IN.
48 IN.
DOT
Surge vessels
E
❤
*
ཐ་
Sluice valve
Butterfly valve
Electrically operated from control room Reflux valve
to Fabridam
154 IN.
54 IN.
48 IN.
E
48 IN.
To Tai Lam Chung
-
54 IN.
To Tau Pass
Fig. 3:2.
Air valve
Pressure relief valve
Magnetic flowmeter
WASHOUT.
36 IN.
3 IN. feed
3 IN.
66 IN.
E
Booster Pumps (Harland)
Chinese water
66 IN.
RIVER INDUS
Main Pumps
(Allen Gwynne)
River Indus pumping station schematic layout
livering 60 m.g.d. to Tau Pass were commissioned during 1965.
Allowance has also been made for improved future extensions should hydrological data in the future show this to be a worthwhile method of helping to meet the ever-increasing de- mand for water.
The overall cost of works directly associated with flood pumping from the River Indus is about HK$28 m.; this figure includes just over HK$13 m. for the structure of the pumping station, associated mechanical, elec- trical and control equipment and the Fabridam (see section 3:3).
Hydrology
The River Indus and its tributaries drain one of the largest catchments in the Colony. Unlike other water- supply catchments, a significant pro- portion of the area is agricultural land and there is also some urban develop- ment. The average rainfall over the catchment varies from about 90 in. on the hills to 83 in. on the lowland area. Crop irrigation results in the yield per unit area being rather less than elsewhere on the scheme although the amount by which actual crop water-use exceeds the evapo-transpira- tion of hillside vegetation (about 35 in.) is uncertain.
The flow from about 60% of the total area has been measured since 1960 but for a number of reasons the results are not very accurate. The main gauging point is an irrigation weir at Sek Pi Tau for which a stage- discharge relation has been establish- ed by the Waterworks Hydrological Section using weir formulae and cur- rent-meter observations.
Although this takes into account flow through irrigation off-takes and a natural secondary channel, flows be- low about 10 m.g.d. cannot be mea- sured directly as, under certain con- ditions, all the water by-passes the
54
36 IN
오오오
weir. Initially the head over the weir was read once daily but a level re- Low corder was installed in 1964. flows for 1965-66 have been estimated from current-meter readings but for earlier years use has been made of the quantity pumped to Tai Lam Chung from the temporary pumping station; however, this does not always reflect the true flow.
and The rough rainfall/run-off flow/duration curves shown in fig. 3:4 and 3:5 have been derived from the above data. An allowance has been made for the reduction in effective area that will occur when the Stage II tunnels are in operation, as the yield from part of the natural catch- ment will then be diverted direct to Plover Cove. Although high flows in excess of the tunnel intakes will con- tinue to pass the pumping station, no allowance for this was necessary as, under these circumstances, the flow from the remaining catchment will have already exceeded the pumping capacity.
As a comparison with the observed run-off data, a theortical line on fig. 3:4 indicates the runoff to be expected from a highland catchment with the same area and rainfall.
flow naturally give rise to a water of a lower quality than on other catch- ments. The average figure for total dissolved solids is about 150 p.p.m. compared to less than 50 p.p.m. for hillside streams. The chemical and bacteriological quality, although low, will be improved by dilution and storage in the Plover Cove reservoir. All water fed into the public supply system is purified and sterilised by chlorination at the Sha Tin treatment works, the efficacy of chlorination being continually monitored.
The water level at the pumping station rises appreciably at times of high flow, flooding over the Shung Shui plain. Flooding is aggravated as a result of backing up caused by high levels in the Sham Chun river; the flood level subsides slowly and long after the Indus flow has passed its peak.
The Sham Chun river, which forms the border with China, drains a catch- ment very much larger than that of the Indus. This ponding of flood water may have the advantage of al- lowing a greater proportion of the total flow to be pumped than would be the case if no ponding occurred.
Urban development, animal hus- bandry and a greater ground-water
Fabridam
In view of the serious flooding which occurs in the Sheung Shui and Tau Pass plains, it is essential that any method of controlling the intake ponds should not aggravate the pro- blem. After cost comparisons with orthodox flood-gates, collapsible Fab- ridams were adopted.
A Fabridam (see fig. 3:6) is a flexible neoprene bag, anchored to a horizontal concrete foundation and to vertical concrete abutments, which can be inflated with water to form a barrage. The Fabridam on the Indus is 107 ft. wide and can be inflated so that its crest level is up to 9 ft. above the foundation slab.
a
It is inflated by gravity feed from reinforced-concrete header-tank. which in turn is filled from tapping from each of the three delivery mains from the pumping station. The Tau Pass Fabridam is similar but only 65 ft. between abutments and the max- imum crest height is only 5 ft.
All operations are controlled auto- matically by "No-flote" electrodes, which are pre-set to give the required responses. As the pond level starts to rise when the flood flow in the river increases beyond the pumping capa- city, the Fabridam steadily deflates to maintain an almost constant pond level, until it is completely deflated and is lying flat on the foundation slab. As the flood flow recedes, the Fabridam is steadily reinflated to maintain the normal pond level.
Deflation occurs when the internal hydrostatic head exceeds a pre-set level, thereby causing internal water to be discharged over a small weir. If the pond continues to rise, an elec- trically-operated valve will open an additional discharge pipe, which can deflate the envelope completely in half an hour.
The water downstream of the Indus Fabridam is subject to tidal effects from the Sham Chun estuary in Deep Bay and can at times be higher than the normal operating crest level of +8 ft.P.D. In these circumstances the Fabridam is further inflated to a crest level of +10 ft.P.D., to prevent over-topping by saline water but re- verts to +8 ft.P.D. when the water recedes on the downstream side.
Small float - operated air - release valves are fitted in the quarter points of the crest and in the top of each abutment to prevent trapped air causing an uneven discharge over the
crest.
Far East Architect & Builder January, 1968
Page 40SONI
NTAIN
· PRESSURE RELIEF VALVES
MB TO TAU PALJ
14 IN TO DAU PADE
1 IN TO TU LAMI
CHUNG CATO-MATER