Work in progress on diversion canal outfall
The Bukit Timah flood scheme, as proposed by the Public Works Depart- ment of Singapore, was approved by the Government in 1966. The first phase project was put in hand towards the end of 1966 but the second phase is still subject to approval, although the land required for the project has been safe-guarded.
Assessment of run-off
The drainage design run-off from Bukit Timah catchment was derived by the use of the rational formula and the reliability of the results so derived was checked with field measurements. The recurrence interval of rainfall in- tensity adopted for this design, as for the rest, is once in five years.
The rainfall intensity for a given duration is based on N.H. Taylor's rainfall study made in 1932. The formula given by Taylor for once in five years storm is:
] =
300 T +35
when I =
intensity of rain- fall in inches per hour
when T = duration of rain-
fall in minutes.
Completed U-shaped drain
degree of semi-urban type develop ment. The design run-off for the first diversion is 3,180 cusecs, while the design run-off for the second diversion is 4,020 cusecs.
Hydraulic consideration for first diversion
Taylor's works, done over 35 years ago, were based on pluviograph records of five stations over a period of eight years (1925 to 1932) and three gauges The first diversion channel has a over a period of three years (1930 to total length of 10,402 ft. and connects 1932). Their reliability was recently the existing Bukit Timah Canal at 6 checked in the public works depart- m.s. Bukit Timah Road with the exist- ment by a fresh effort at rainfall analy-ing Ulu Pandan River at Clementi Road. sis based on much larger rainfall records.
he recent study confirms that Tay- lor's formulae relating rainfall frequen- cy, intensity and duration are satisfac- tory. The run-off coefficient used for the diversion canal is 0.60 which is considered to be sufficient for a high
It is made up of three lengths of twin- tunnels (total length 1,505 ft.), 695 ft. of box culverts, 2,095 ft. of 'U' shaped concrete lined canal, and 6,107 ft. of concrete lined trapezoidal open channel.
The difference in levels between the
rivers at the connecting points with the diversion canal is 11.21 ft., i.e. the hydraulic head available is 11.21 ft.
It is not economical to distribute the head evenly in order to obtain a uniform gradient and, as the tunnel work is the most expensive, maximum head is alloted to the tunnel sections. However since it is desirable to keep the flow in the tunnels below the cri- tical velocity, an upper limit of the head to be alloted to the tunnel sec- tions was imposed.
The maximum velocity in the open channel is around 8 ft. per second and this velocity is accelerated to about 14 ft. per second by the time it flows past the transition into the tunnels. To avoid undue loss of energy, an outlet transition is similarly designed to de-
Far East BUILDER, March 1969
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