Design of Open Channels

1. Design of Open Channels Non-erodible and Vegetated Channels

2. Nonerodible Channels • Solve using Manning’s equation. • Main consideration in design is to ensure adequate capacity. • Typically done by adding a freeboard or extra depth to the channel as a safety measure. • Generally, freeboard is around 20% of depth or 0.3 to 0.5 ft, whichever is greater is added to the channel depth.

3. Erodible Channels • When designing channels through erodible materials there are two major concerns. • Adequate capacity to carry flow. • Adequate stability to resist erosive action of flowing water. • Erodible channels can be either • vegetated • non-vegetated

4. Erodible Channels • Vegetated Channels • Protect the channel from erosion, permitting higher velocities. • Increase the roughness of the channel. • May decrease the capacity

5. Non-Vegetated Channels • Two main design procedures are used for ensuring stability of erodible channels. • Limiting Velocity Concept • Limiting Tractive Force (Boundary Shear) Concept

6. Limiting Velocity Concept • The channel is sized so that it has adequate capacity and so that the average velocity does not exceed the permissible velocity. • Permissible velocities for different types of erodible channel materials can be found in Table 4.2 (Haan et al., 1994).

7. Limiting Tractive Force Concept • Designing a channel that has adequate capacity and an average shear stress calculated by: that is less than the values for allowable tractive forces found in Table 4.2.

8. Vegetated Channels • Allowable velocities and tractive forces for non-vegetated, erodible channels are very small. • Wide, shallow ditches are the result. • If the channel can be protected, allowable velocities can be increased, resulting in deeper, narrower channels.

9. Vegetated Channels • Vegetation is an inexpensive and permanent form of protection. • Vegetation protects the channel material from erosive action and binds the channel material together. • Vegetated channels should be used to carry intermittent flows such as storm water runoff.

10. Vegetated Channels • Not recommended for channels that have sustained baseflow. • Somewhat more complex to design and require more care in their establishment. • Carry high flows at high velocities and require a minimum of maintenance. • An additional design consideration is the variation in roughness with the height and type of vegetation.

11. Vegetated Channels • Tall grass presents a great deal of flow resistance to shallow flow. • As the flow depth increases, the resistance may decrease. • Manning’s n can be related to the flow velocity and the hydraulic radius, vR. • Different grasses, have different n-vR relationships.

12. Vegetated Channels • Grasses have been divided into five retardance classes, designated by A, B, C, D and E. (Table 4.3). • If the grass will be mowed part of the time and long part of the time, both conditions of retardance must be considered. • Table 4.4 may be used to estimate vegetal retardance if specific information on the vegetation is not known.

13. Vegetated Channels • Maximum permissible velocities for vegetated channels are given in Table 4.5. • These velocities are for established sod in good condition. • If poor vegetation exists due to shade, climate, soils etc. the design velocity should be 50% of the values in Table 4.5. • Table 4.6 has permissible velocities for channels where specific vegetation and erosion characteristics are not known. • Figure 4.14 shows the n-vR relationship for the five retardance classes.

14. Design Procedure for Vegetated Channels • Select vegetation • Determine the retardance class • Determine the permissible velocity • Design the channel based on the curves of Fig. 4.14 or Figs.4.15 (a-e).

15. Design Procedure for Vegetated Channels • When two retardance classes are applicable, the channel should first be designed for stability based on the lower retardance and then additional depth added to the channel to accommodate the flow when retardance increases.

16. Design of Vegetated Channels • The graphs of Fig. 4.15 are solutions to Manning’s equation using the curves in Fig. 4.14. They can be used as a design aid for solving Manning’s n for all retardance classes.

17. Vegetated Channels • Temple et al. (1987) developed a relationship to approximate the n-vR curves. Where I is a value determined by the retardance class (A-E). A table of I values for is given on pg. 117.

18. Flexible Liners • Normann(1975) presents a uniform procedure for the design of open channels using flexible liners. • Liners considered are vegetation, temporary liners and riprap. • The procedure for vegetation is based on the procedure covered previously.

19. Flexible Liners • Results are presented in the form of equations describing the maximum permissible depth of flow for a stable design. Where d is in feet and S is in feet per foot.

20. Flexible Liners • The velocity equation is in the form of:

21. Flexible Liners • The values for m and n for vegetated channels are found in Table 4.7. • In comparing the Normann results and the results in the previous section indicate that better agreement is obtained if dmax is replaced by the hydraulic radius R. • For vegetation the velocity is determined from Figs. 4.15a-4.15e.

22. Temporary Channel Linings • Used to stabilize the channel during the period of vegetal establishment. • Should be constructed of materials that will deteriorate as vegetation emerges and will not interfere with growth. • A select group of linings is listed in Table 4.8.

23. Temporary Channel Linings • Values of m and n are in Table 4.9. • McWhorter et al. (1968) found that temporary linings acted much like vegetation, hence Manning’s n is not constant. • An equation with a form similar to that of Manning’s equation is used, but the exponents of R and S change with channel lining.

24. Temporary Channel Linings • Design procedure: • Design the channel for the permanent vegetation. • Select a temporary liner that will be stable in the channel. • A lower-return period storm might be selected for the design of the temporary lining since the exposure time is short.

25. Temporary Channel Linings • The decision to classify a soil as erodible or erosion resistant is somewhat subjective. • Normann(1975) suggests using the K factor from the USLE equation as an indicator of erosion resistance. • K=0.50 erodible • K=0.17 erosion resistant • For K values between 0.17 and 0.50, values would be interpolated between the values in Table 4.9. (K values are in Ch. 8)