Fundamentals of Engineering Design
Table 5.8 Permissible Canal Velocities (Fortier and Scobey, 1926)
Mean velocity, after aging of canals with flow depths # 3 ft
Water transporting
Clear water, no
Water transporting
noncolloidal silts, sands,
detritus
colloidal silts
gravels or rock fragments
Original material excavated
for canals
(ft/sec)
(m/sec)
(ft/sec)
(m/sec)
(ft/sec)
(m/sec)
1. Fine sand (noncolloidal)
1.5
0.46
2.5
0.76
1.5
0.46
2. Sandy loam (noncolloidal)
1.75
0.53
2.5
0.76
2.0
0.61
3. Silt loam (noncolloidal)
2.0
0.61
3.0
0.91
2.0
0.61
4. Alluvial silt (noncolloidal)
2.0
0.61
3.5
1.07
2.0
0.61
5. Ordinary firm loam
2.5
0.76
3.5
1.07
2.25
0.69
6. Volcanic ash
2.5
0.76
3.5
1.07
2.0
0.61
7. Fine gravel
2.5
0.76
5.0
1.52
3.75
1.14
8. Stiff clay
3.75
1.14
5.0
1.52
3.0
0.91
9. Graded, loam to cobbles (noncolloidal)
3.75
1.14
5.0
1.52
5.0
1.52
10. Alluvial silt (colloidal)
3.75
1.14
5.0
1.52
3.0
0.91
11. Graded, silt to cobbles (colloidal)
4.0
1.22
5.5
1.68
5.0
1.52
12. Coarse gravel (noncolloidal)
4.0
1.22
6.0
1.83
6.5
1.98
13. Cobbles and shingles
5.0
1.52
5.5
1.68
6.5
1.98
14. Shales and hard pans
6.0
1.83
6.0
1.83
5.0
1.52
2. Determine the soil properties of the bed and banks of the design reach and of the channel
upstream.
3. Determine sediment yield for the reach and compute sediment concentration for design flow.
4. Check to see if the allowable velocity procedure is applicable using the Channel Evaluation
Procedural Guide, Figure 5.27.
5. Determine the basic channel velocities from Figure 5.28a and multiply them by the appropriate
correction factors as found in Figure 5.28b. Compare the design velocities with the allowable
velocities determined from Figures 5.28a and 5.28b.
6. If the allowable velocities are greater than the design velocities, the design is satisfactory.
Otherwise, if the allowable velocities are less than design velocities, it may be necessary to
consider a mobile boundary condition and evaluate the channel using appropriate sediment
transport theory and programs.
5.3.4 TRACTIVE FORCE DESIGN
Lane (1953a,b) developed an analytical design approach for shear distribution in trapezoidal
channels. The tractive force, or shear force, is the force which the water exerts on the wetted perimeter
of a channel due to the motion of the water.
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