Environmental Engineering Reference
In-Depth Information
of 0.80% of the critical shear stress is recommended:
τ
design
=
D
75
(
D
75
in mm). The allowable shear stress is a function of the mean diameter
and the sediment concentration of the water (see Table 4.3).
0
.
75
∗
Table 4.3. Recommended critical shear stress (N/m
2
) for fine,
non-cohesive sediment (Dahmen, 1994).
D
50
(mm)
Clear water
Light load
Heavy load
0.1
1.20
2.40
3.60
0.2
1.25
2.49
3.74
0.5
1.44
2.64
3.98
Dahmen (1994) suggested as
a rule of thumb
for many irrigation
engineers, that the maximum boundary shear stress in 'normal soil', for
a 'normal canal' and under 'normal conditions' can be between 3 and
5 N/m
2
. When the shear stress is too high, the most sensitive factor to
reduce this factor is a gentler bottom slope. Table 4.3 shows some recom-
mended values for the critical shear stress (N/m
2
) along the boundary for
fine, non-cohesive sediment.
For cohesive soils no definite criteria exist, but based on field data,
the USBR gives the following values (see Table 4.4).
Limiting boundary shear stress (N/m
2
) in cohesive material.
Table 4.4.
Compaction
Description
Loose
Fair
Well
Very well
Void ratio
2.0-1.2
1.2-0.6
0.6-0.3
0.3-0.2
Cohesive bed material
Limiting boundary shear stress (N/m
2
)
Sandy clays
1.91
7.52
15.7
30.2
Heavy clay soils
1.48
6.75
14.6
27.0
Clays
1.15
5.94
13.5
25.4
Lean clayey soils
0.957
4.60
10.2
16.9
The shear stress on the side slopes is only considered when the bed and
sides are covered with coarse, non-cohesive material. For this situation the
shear stress together with the angle of repose has to be taken into account.
For fine, non-cohesive material and for cohesive material the component
of the weight is small and can be ignored.
The values of the critical boundary shear stress are established for
the design of straight canals and they should be reduced for sinuous
canals; Table 4.5 presents some reduction values (in %) for non-straight
canals.
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