Question 9.4: Design a submerged pipe outlet for a ditch to discharge 151 ......
Design a submerged pipe outlet for a ditch to discharge 151 {s}^{-1} from a supply canal, using the following data:
Pipe outlet: length of pipe = 30 m. Concrete pipes in multiples of 50 mm diameters with a k value of 0.1 mm are available.
| Supply canal | Field channel | |
| Bed level (m AOD) | 100.00 | 100.00 |
| FSL (m AOD) | 101.00 | 100.90 |
The working head, H = 101.00 – 100.90 = 0.10 m. The depth of flow in the supply canal, D = 101.00 – 100.00 = 1.00 m. Therefore the pipe setting, H/D = 0.1/1.0 = 0.1 < 0.3, and so the pipe outlet functions as a hyperproportional one with a flexibility of F = 0.3/0.1 = 3.
PIPE DESIGN
Combining Colebrook–White and Darcy–Weisbach equations (Featherstone and Nalluri, 1995),
{Q}=-2A(2g d S)^{1/2}\log[k/3.7d+2.51\nu/d(2g d S)^{1/2}]where d is the pipe diameter, A=\pi d^{2}/4 , k is the pipe roughness, and S is the friction gradient. Neglecting minor losses, S = 0.1/30 = 0.0033, and we obtain
d(mm) 100 150 200
Q(1 {s}^{-1}) 4.0 12.0 26.0
If minor losses are included (here they cannot be neglected), 200mm pipe gives 181 {s}^{-1} which is quite adequate.