1. Lecture 8: Design of Erodible Channels
25/03/2017
Lecture 8: Design of Erodible Channels
CEM001 Hydraulic Structures, Coastal and River Engineering River Engineering Section
Dr Md Rowshon Kamal
H/P: 1

2. Hydraulic Parameters used in Design of Unlined/Lined Channels
25/03/2017
Hydraulic Parameters used in Design of Unlined/Lined Channels
1. Channel must carry design flow/discharge (Qd).
2. Velocity in the channel must not be high to cause scour.
3. Velocity in the channel must not be low to cause deposition. 2

3. Minimum Permissible Velocity
25/03/2017
Minimum Permissible Velocity
This is the lowest velocity which prevents both sedimentation (deposition) and vegetation growth.
Recommendations by French (1985)
Prevent from sedimentation – 0.61~0.91m/s
Prevent from growth of vegetation – 0.76m/s 3

4. 1. Maximum Permissible Velocity Method
25/03/2017
1. Maximum Permissible Velocity Method
This is one of the methods we use to design a channel.
Special committee on Irrigation Hydraulics (ASCE) formed this method.
Design criteria:
Mean Flow Velocity < Max. Permissible Velocity 4

5. Maximum Permissible Velocity (m/s) Water carrying colloidal silts
25/03/2017
Maximum Permissible Velocities
Material
Maximum Permissible Velocity (m/s)
Clear Water
Water carrying colloidal silts
Fine sand, non colloidal
0.46
0.76
Alluvial silt, non colloidal
0.61
1.07
Stiff clay, very colloidal
1.14
1.52
Fine gravel
Coarse gravel
1.22
1.83 5

6. Maximum Permissible Velocities (con’t)
25/03/2017
Maximum Permissible Velocities (con’t)
Above values will be changed if;
1. Reduce values by 25% for sinuous (meandering) channels.
2. Increase by 0.15 m/s for depths greater than 0.91m.
3. Reduce by 0.15 m/s if channel carries abrasive material.
4. Increase by 0.3 – 0.6 m/s for channels with high silt load. 6

7. 25/03/2017
Example 02
Design a trapezoidal channel (side slope 1:2) to carry m3/s on a bed slope of Use Maximum Permissible Velocity Method.
Assume the following:-
Coarse gravel in water carrying colloidal silt
Depth to be greater than 1.0 m
Manning’s coefficient n = 0.025 7

8. Answer Ex-02 From Table in slide no 5:
25/03/2017
Answer Ex-02
From Table in slide no 5:
Corresponding allowable velocity = 1.83m/s
This may be increased by 0.15m/s because channel depth assumed to be greater than 1.0m,
Modified allowable velocity = 1.98m/s 8

9. Answer Ex-02 From Manning’s formula Cross sectional area
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Answer Ex-02 y b z
From Manning’s formula
Cross sectional area
Wetted perimeter
Cross sectional area
Wetted perimeter 9

10. Answer Ex-02 Substituting for the side slope, Solve for y and b
25/03/2017
Answer Ex-02
Substituting for the side slope,
Solve for y and b
Negative value is not possible
Design depth and width 10

11. 2. Permissible Tractive Force Method (Shear Stress Method)
25/03/2017
2. Permissible Tractive Force Method (Shear Stress Method)
Most rational and widely used method.
Based on the consideration of equilibrium of particle resting on the bed with drag and lifting forces balanced by the submerged weight of particle. 11

12. Permissible Tractive Forces by USBR
25/03/2017
Permissible Tractive Forces by USBR
USBR recommends the following values for boundary shear stresses:
Course non-cohesive material (D > 5.0mm)
Fine non-cohesive materials (Refer Example 2.4 – Next page please!)
Cohesive sediments – Not covered by USBR
D75 in mm 12

13. For Fine Cohesive Materials
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For Fine Cohesive Materials 13

14. Adjustment for Sinuosity
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Adjustment for Sinuosity
Degree of Sinuosity CS
Straight channels
1.00
Slightly sinuous
0.90
Moderately sinuous
0.75
Very sinuous
0.60 14

15. Allowable Shear Stresses for a Trapezoidal Channel
25/03/2017
Allowable Shear Stresses for a Trapezoidal Channel b 1 z y
s = ks gyS0
τs and τb - Max Bottom & Side Shear Stresses
ks and kb - Depend on y, b, z
b = kb gyS0 15

16. Allowable Shear Stresses for a Trapezoidal Channel (con’t)
25/03/2017
Allowable Shear Stresses for a Trapezoidal Channel (con’t)
ks and kb Factors If If
Tables need to be used
Design Conditions or
For bottom
For sides or 16

17. Bank Stability in a Trapezoidal Channel
25/03/2017
Bank Stability in a Trapezoidal Channel
Forces acting on the particle:
Drag force (FD)
Component from weight (Wsinθ)
Friction force opposing R
(Wcosθtanϕ) W
Wsinq q FD Q
Wcosq
Force normal to the side
Sand particle 17

18. Bank Stability in a Trapezoidal Channel (con’t)
25/03/2017
Bank Stability in a Trapezoidal Channel (con’t)
At incipient motion, resultant force, R will be equal to friction force.
For side
(1) 18

19. Bank Stability in a Trapezoidal Channel (con’t)
25/03/2017
Bank Stability in a Trapezoidal Channel (con’t)
For the bottom θ=0;
(2)
Combining (1) and (2) gives; 19

20. Bank Stability in a Trapezoidal Channel (con’t)
25/03/2017
Bank Stability in a Trapezoidal Channel (con’t)
For finer materials, θ=0;
i.e. Cohesive forces are much greater than the gravity force. 20

21. 25/03/2017
Example 03
Design a trapezoidal channel to carry 125.0m3/s on a bed slope of The channel is to be excavated in coarse alluvium, containing moderately angular stones with d75 of 50.0mm. The angle of friction for this material is 40º, which is also its angle of repose. 21

22. 25/03/2017
Question 04
(i) Define the terms maximum and minimum permissible velocities.
(ii) A river 30.0 m wide and 4.0 m deep and of a regular rectangular cross-section carries a discharge of m3/s through country with a bed slope of If the bed material is coarse alluvium having a D50 size of 10.0 mm and specific gravity s = 2.65, estimate the total transport load using the Ackers and White formula. 22 22

23. Thank You