Environmental Engineering Reference
In-Depth Information
Q
s
Q
s
deposition
Q
se
Q
sa
Q
sa
erosion
Q
se
X
X
Case I
Case II a
Q
s
Q
s
sediment transport rate
Q
se
Q
sa
actual sediment transport rate
Q
se
equilibrium sediment transport capacity
Q
sa
erosion prevented
X
distance
Figure 5.35. Sediment transport
for uniform flow.
X
Case II b
Figure 5.36 shows the effect of a backwater profile on the sediment
transport in a canal.
•
For
Q
s
larger than
Q
se
deposition will occur to adapt to the equi-
librium
Q
s
). From that point
onwards a continuous deposition in the downstream direction may be
expected.
sediment
transport
capacity
(
Q
se
=
•
For
Q
s
smaller than
Q
se
, the actual sediment transport can either remain
constant along the canal (
θ<θ
cr
) or increase up to
Q
se
(
θ>θ
cr
). The
flow will attempt to attain equilibrium.
The effect for a drawdown profile is shown in Figure 5.36.
•
In a drawdown flow and for
Q
s
larger than
Q
se
deposition will occur up to
the equilibrium condition (
Q
se
=
Q
s
); the water will pick up sediment
from the bed. From that point onwards a continuous entrainment is
expected.
•
When
Q
s
is smaller than
Q
se
the former either remains constant (
θ<θ
cr
)
or increases to reach
Q
se
(
θ>θ
cr
). From that point onwards a continuous
entrainment in the downstream direction will occur.
Figure 5.36 shows some of the steps in the sediment behaviour for
the two gradually varied flow situations in an irrigation canal. They also
illustrate the procedure to compute the sediment mass balance after one
time step.
The sediment movement in irrigation canals is not simple once flow
control structures are introduced. The standard design practices that
include the sediment transport aspect are not capable of predicting the
sediment transport behaviour, including erosion and sedimentation. The
design assumes a steady and uniform flow condition and the design dis-
charge is generally the maximum discharge, which is not released during
most of the time. Once the flow is less than the design discharge the
sediment transport capacity will decrease even for the new uniform flow
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