The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 206

the design of a flow control structure for a given water flow and sediment
characteristics.
This example will be based upon a canal with an upstream section of
2 500 m and a downstream section of 500 m. An overflow weir ( B

=

10, 12
and 14 m respectively) is situated at the end of the first section for a design
discharge of 25.75 m 3 /s and an upstream water depth of 2.38 m. The inflow
concentration is higher than the equilibrium concentration for this canal
in order to compare the results with some of the previous examples (Case
2 and Case 4). This high concentration means that the sedimentation in the
canal will be due to the combined effect of the higher concentration and
the backwater profile. Even without a weir deposition will occur in the
canal due to the high concentration. At the end of this section the effect
of a case with a concentration lower than the equilibrium concentration at
the design discharge will be discussed.
The design of the 3 broad-crested weirs, which will have the same
water level as the canal for the design flow, will be evaluated (see Figure
7.25). Downstream of the weir is a canal section with a bottom level
lower than the upstream bottom to guarantee a modular flow over the
weir; the upstream water level and the discharge over the structure are
independent of the downstream water level.

3.0

2.5

2.0

1.5

30

15 20 25

Q (m 3 /s)

Canal
B = 14 m
B = 12 m

Figure 7.25. Water depth in the
irrigation canal and upstream of
a weir with a width B of 10, 12
and 14 m respectively.

B = 10 m

For a wider weir the crest will be higher in order to maintain the set
point, and for all discharges smaller than the design discharge the flow
over all the weirs remains free and a backwater profile will develop in the
upstream canal section. For smaller weir crests the backwater effect will
be more pronounced than for wider ones. The width of the weir determines
the rise in water level for smaller discharges and therefore the sediment
deposition upstream of the weir.