Environmental Engineering Reference
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a range of shear stresses between 1 N/m 2 and 4 N/m 2 , which is a range of
shear stress commonly used for the design of irrigation canals. In addition
it clearly shows that typical flow conditions in irrigation canals are large
enough to produce suspension of sediment particles. The sediment in
irrigation canals is transported in two modes: suspended load and bed load.
Initiation of motion
10 0
4
Typical values for irrigation canals
1
1.0
Bed and suspended load transport
Shields (motion)
Bed load transport
Van Rijn (suspension)
Minimum
Maximum
No motion
Figure 5.17. Initiation of
motion and suspension and
values of shear stress as
function of D .
0.1
0.10
0.20
0.30
0.40
0.50
Mediam diameter d 50 (mm)
In irrigation canals the sediment is transported both as bed load and as
suspended load. Therefore, any predictor to estimate the sediment trans-
port should be able to compute either implicitly the total transport (bed
load
suspended load) or the bed load and suspended load separately.
Only for very fine sediment ( d 50 < 0.1 mm) can a suspended sediment
transport predictor be used to estimate the sediment transport capacity of
irrigation canals.
+
5.3.3 Sediment transport in non-wide canals
In general the reliability of sediment transport predictors is low and at best
they can provide only rough estimates. A probable error in the range of
50-100% can be expected even under the most favourable circumstances
(Vanoni, 1975). The error is expected to increase further if the calculations
are based upon average values of flow and sediment parameters. Several
assessments of sediment transport formulas have been made (Brownlie,
1981b; Yang and Molinas, 1982; Van Rijn, 1984b; Yang and Wan, 1991)
and each provide different results. Woo andYu (2001) compared the results
assessed by different researchers and found that there is no universally
accepted formula for the prediction of sediment transport. Most of them
are based upon laboratory data of limited sediment and water flow ranges.
Hence they should be adjusted to make them compatible for the specific
purposes; otherwise the predicted result will be unrealistic.
The distribution of shear stress along the boundary is not constant,
in contrast to the general assumptions made in the calculations. Even
 
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