The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 207

The differences in sediment deposition, which are determined using the
Ackers-White sediment predictor, are not very significant for the various
widths. For the design discharge the relative sedimentation is about 0.6
and for the smallest width the relative sedimentation is about 0.85 (see
Figure 7.26). This figure also gives the sediment deposition for a gate
(without a weir) at the same location, which maintains the identical set
point for the design discharge and smaller discharges as the design flow
over the weirs.

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.0

Figure 7.26. Relative deposition
in the canal upstream of a weir
with a width of 10, 12 and 14 m
and of a gate respectively.

Relative discharge

B = 10 m
B = 12 m

B = 14 m
B = Gate flow

For the given sediment characteristics ( d 50 = 0.15 mm) and using the

Ackers-White sediment predictor, the equilibrium concentration for the
design discharge is approximately 56 ppm. When the incoming concen-
tration is less than 56 ppm, only the structure will affect the sediment
deposition in the canal. For the design discharge and for an incoming
sediment concentration less than the equilibrium concentration there will
be no deposition, which is valid for all weir widths that are designed for
the same set point. For discharges smaller than the design discharge a
backwater profile will develop that determines the sediment deposition in
the canal.
The next example has an inflow concentration equal to the equilibrium
concentration for the design discharge (55 ppm). The same incoming con-
centration is maintained for the simulation with smaller discharges to
show the effect of mainly the backwater. The relative sediment deposition
in the canal is for all weir widths smaller than in the previous example.
The results for various weir widths are presented in Figure 7.27. Weirs
with a small width result in relatively small relative deposition (15-25%)
and the effect for wider weirs is slightly higher but still comparable
(around 40%).
In addition the effect of the crest height on the sediment deposition
for the design discharge has been evaluated for a weir width of 10 m.