Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 128

1000

800

600

400

200

0

0.00 0.10 0.20 0.30 0.40 0.50

Sediment size d 50 (mm)

Figure 5.18. Effect of sediment
size (d 50 ) on the prediction of
the equilibrium concentration.

Engelund-Hansen
Brownlie
Ackers-White

The effect of the canal geometry and flow characteristics on the sedi-
ment transport has been analysed for the sediment size, velocity, B / y ration
and side slope (Paudel et al., 2013). The sediment size normally found
in an irrigation canal is 0.05 mm (50 m) to 0.5 mm. For the evaluation

of the effect of the sediment size on the predictability of the three above
mentioned sediment transport predictors the discharge, bed width, bed
slope, side slope and the roughness of a canal was kept constant and the
equilibrium concentration was computed for the three total load predictors
(see Figure 5.18). The Brownlie predictor shows an almost linear variation
for sediment sizes between 0.10 and 0.50 mm. Ackers and White predict
high values for sediment sizes below 0.15 mm. The predictability of the
three predictors for sediment sizes greater than 0.15 mm is comparable.
The sediment transport in relation to the change in velocity was eval-
uated for the same discharge, bed width and side slope, while the bottom
slope was increased to increase the velocity. The Froude number in all
cases was smaller than 0.5. The results are presented in Figure 5.19.
The influence of the velocity on the prediction of the transport capac-
ity is for the Brownlie and Engelund-Hansen predictors almost the same,
while the Ackers-White predictor is more sensitive to changes in the veloc-
ity. The change is more significant for higher velocities (greater than
0.70 m/s), which correspond to a Froude number of 0.24. The Froude
number in irrigation canals normally varies from 0.2 to 0.5.
To appraise the effect of a change in the B / y ratio and side slope m on
the sediment transport, the discharge, bed slope and the roughness were
kept constant. For each side slope, the bed width was changed to obtain
different B / y ratios. A change in bed width also changed the flow velocity.
The results for three different side slopes of 1:1, 1:1.5 and 1:2 are presented
in Figure 5.20. The Engelund-Hansen and Ackers-White methods show
similar prediction trends. For a small B / y ratio the transport capacity is

µ