Worked Example 9.4
Design a submerged pipe outlet for a ditch to discharge 15 l s^1 from a
supply canal, using the following data:
Supply canal Field channel
Bed level (m AOD) 100.00 100.00
FSL (m AOD) 101.00 100.90
Pipe outlet: length of pipe30 m. Concrete pipes in multiples of
50 mm diameters with a kvalue of 0.1 mm are available.
Solution
The working head, H101.00 100.900.10 m. The depth of flow in the
supply canal, D101.00 100.001.00 m. Therefore the pipe setting,
H/D0.1/1.00.10.3, and so the pipe outlet functions as a hyperpro-
portional one with a flexibility of F0.3/0.13.
PIPE DESIGN
Combining Colebrook–White and Darcy–Weisbach equations (Feather-
stone and Nalluri, 1995),
Q 2 A(2gdS)1/2log[k/3.7d2.51v/d(2gdS)1/2]
wheredis the pipe diameter, Aπd^2 /4,kis the pipe roughness, and Sis
the friction gradient. Neglecting minor losses, S0.1/300.0033, and we
obtain
d(mm) 100 150 200
Q(l s^1 ) 4.0 12.0 26.0
If minor losses are included (here they cannot be neglected), 200 mm pipe
gives 18 l s^1 which is quite adequate.
408 DIVERSION WORKS
