1. Hydraulic Engineering
Eng. Osama Dawoud

2. Lecture 14
Open Channel Flow

3. Open Channel
Open channel hydraulics, a subject of great importance to civil engineers, deals with flows having a free surface in channels constructed for water supply, irrigation, drainage, and hydroelectric power generation; in sewers, culverts, and tunnels flowing partially full; and in natural streams and rivers.

6. Pipe system
Open Channel

7. Classification Steady Flow Unsteady Flow Typical situations
Not time dependent
Unsteady Flow
Is time dependent
Typical situations
Uniform flow
Gradually Varied Flow
Rapidly Varied Flow

10. Open channel section types

11. Uniform Flow

12. Chezy equation
C is the Chezy C, a dimensional factor which characterizes the resistance to flow

13. Manning equation

15. Example 1
open channel of width = 3m as shown, bed slope = 1:5000, d=1.5m find the flow rate using Manning equation, n=0.025.

16. Example 2
open channel as shown, bed slope = 69:1584, find the flow rate using Chezy equation, C=35.

18. Most Efficient Sections
During the design stages of an open channel, the channel cross-section, roughness and bottom slope are given.
The objective is to determine the flow velocity, depth and flow rate, given any one of them. The design of channels involves selecting the channel shape and bed slope to convey a given flow rate with a given flow depth. For a given discharge, slope and roughness, the designer aims to minimize the cross-sectional area A in order to reduce construction costs

19. Most Efficient Sections
The most ‘efficient’ cross-sectional shape is determined for uniform flow conditions. Considering
a given discharge Q, the velocity V is maximum for the minimum cross-section A. According to the Manning equation the hydraulic diameter is then maximum.
It can be shown that:
the wetted perimeter is also minimum,
the semi-circle section (semi-circle having its centre in the surface) is the best hydraulic section

20. Most Efficient Sections
Because the hydraulic radius is equal to the water cross section area divided by the wetted parameter, Channel section with the least wetted parameter is the best hydraulic section
Rectangular section

22. Trapezoidal section or k k

23. Other criteria for economic Trapezoidal sectionk
The best side slope for Trapezoidal section

24. Circular section Maximum Velocity using Manning or Chezy
Maximum Flow using Manning
Maximum Velocity using Manning or Chezy
Maximum Flow using Chezy

26. Example 3
Circular open channel as shown d=1.68m, bed slope = 1:5000, find the Max. flow rate & the Max. velocity using Chezy equation, C=70.
Max. flow rate

27. Max. Velocity

28. Variation of flow and velocity with depth in circular pipes

29. Example 4
Circular open channel as shown Q=10m3/s, velocity =1.5m/s, for most economic section. find wetted parameter, and the bed slope n=0.014.

30. To calculate bed Slope