The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 196

is 150 ppm and the behaviour of the actual and equilibrium concentration
as a function of time and place along the 10 km long canal is comparable
with the actual and equilibrium concentration of Case 2 (Section 7.4). The
actual and equilibrium concentrations are derived from the Ackers-White
predictor and the decrease in concentration after the start of the simulation
is very sharp, from 150 ppm to an equilibrium value of approximately
56 ppm. The decrease becomes gentler with time and moves in a
downstream direction. Figure 7.14 presents both the concentrations imme-
diately after the start of the simulation and after 30, 60 and 90 days. After
90 days the actual concentration is about 150 ppm near the gate and drops
gently to 56 ppm downstream of the headworks. Along the whole canal
the actual concentration is always slightly larger than the equilibrium con-
centration. Figure 7.15 presents the changes in bottom level in the canal,
mainly upstream of the gate at 2 500 m. The sedimentation moves from the
headworks in a downstream direction. After 90 days the increase in bed
level at the headworks is about 0.40 m and decreases in the downstream
direction towards the gate, where it is less than 0.30 m. At the gate location
no sedimentation occurs, but the sediment has proceeded downstream
of the gate where a layer of about 0.10 to 0.15 cm can be observed. The
sedimentation will move farther downstream with time.

160

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80

60

40

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0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Distance (m)

Figure 7.14. Changes in the
concentrations in the canal with
a gate at 2 500 m from the intake
after 30, 60 and 90 days.

C-eq at 30 days C-act at 30 days

C-eq at 60 days C-act at 60 days

C-eq at 90 days C-act at 90 days

Figure 7.16 shows the concentrations immediately after the start of the
simulation and after 30, 60 and 90 days for the case where a weir is placed
2 500 m from the headworks. The actual concentration behaves in the
same way as the concentration for the situation with a gate at 2 500 m. The
behaviour of the concentration changes when the sedimentation reaches
the weir after approximately 60 days. The bed load will be trapped by
the weir and the canal bottom level will increase upstream of the weir.