Conveyance of Irrigation and Drainage Water

1. Conveyance of Irrigation and Drainage Water CHAPTER 3A

2. Diversion methods • Gravity diversions • Control structure • Turnout: inlet, conduit, a regulating and a measuring structure. • Pumping plants

3. Design of conveyance systems • Pipelines • A specified maximum allowable head loss along the pipe length. - To use a recommended design velocity (0.6 to 2.0 m/s)

4. Design of open channels • Non-erodible channels: • Kind of material to determine Manning’s n • Min velocity to avoid deposition • Channel bottom and side slopes • The free board: 5 to 30% of design depth • Best hydraulic section: most economic section

5. Best Hydraulic section • A = (b+Zy)y • P = b + 2y(1+Z2)0.5 • Consider A and Z are constants: • Minimizing P with respect to y requires: • Or: • Z= 1/3= tan 30 for trapezoid, b = 2y for rectangle

6. Example • Suppose we want to find the best hydraulic section to convey 400 cfs at a slope of 0.0016 and Manning n=0.025. • From Manning: R2/3 A = 167.79 • A=(b+Zy)y, R=y/2, b=2y( (1+Z2)0.5- Z), Z=1/3, then: y=6.61 ft and b=7.63 ft • For Z=2 , y=5.78 ft and b = 2.72 ft. • For b=20 ft and Z=2, y will be 3.36 ft.

7. Erodible channels • Disadvantages: • Excessive seepage losses. • Low velocities and therefore large cross-sectional areas. • Danger of breaks due to erosion and burrowing of animals. • Favorable conditions for growth of moss and aquatic weeds which retard the velocity and cause high annual maintenance.

8. Selection of side slopes for erodible channels

9. Permissible velocity

10. Example • Suppose a channel with noncolloidal coarse gravel. The discharge is 10 m3/s and a slope of 0.0016. Determine the cross section dimensions using Z=2?