A highway 6 m wide is provided alongside the canal by dividing the flume
into two compartments by a 0.3 m thick partition. The entire trough (flume
section) can be designed as a monolithic concrete structure. Provide side
walls and a bottom slab of about 0.4 m (to be fixed by the usual structural
design methods).
SIPHON BARRELS
Thirteen barrels, each 7 m wide and 2.75 m high, are provided; assume that
the effective roughness, k0.6 mm (concrete). The length of the barrel,
L12.500.30 2 0.4013.60 m. The head loss through the barrel,
hf(1.5L/4R)V^2 /2g. The velocity through the barrel, V500/(13
7 2.75)1.998 m s^1. The hydraulic radius, R 7 2.75/{2(72.75)}
0.987 m. Therefore the Reynolds number 4 VR/v 8 106 andk/4R
1.5 10 4. Hence, from Moody’s chart, the friction factor 0.015, giving
hf0.316 m. Therefore, the upstream HFL200.5000.316200.816 m
AOD.
The uplift pressures on the roof of the barrel are as follows. The RL
of the bottom of the trough200.00 0.40199.60 m AOD. The entry
loss at the barrel0.5V^2 /2g0.102 m. Therefore the pressure head inside
the barrel just downstream of its entry200.816 0.102 199.600
1.114 m11 kN m^2.
The most critical situation arises when the canal is empty and the
siphon barrels are full. The weight of the roof slab0.42.49.81
9.42 kN m^2 (assuming the relative density of concrete to be 2.4). Hence
the roof slab needs additional reinforcement at its top to resist the unbal-
anced pressure forces (uplift pressures).
The total weight of the trough (when empty) needs to be checked
against the total upward force and suitable anchorages to piers provided, if
necessary. Equally, the trough floor slab has to be checked when it is car-
rying water at FSL and the level in the drainage is low, i.e. barrels running
part full.
The uplift on the floor of the barrel (assuming the barrel floor thick-
ness to be 1 m initially) is as follows:
RL of the bottom of the barrel199.60 2.75 1.00195.85 m
AOD;RL of the drainage bed198.00 m AOD.Therefore the static uplift on the floor198.00 195.852.15 m (the
worst condition with the water table as the drain bed level). The seep-
age head (a maximum when the canal is at FSL and the drainage is
empty)202.00 198.004.00 m.
In spite of the three-dimensional seepage flow pattern, Bligh’s creep